The paleartic region with mediterranean climate (southern Europe and northern Africa; the Mediterranean Basin; Fig. 1) is a hotspot of biodiversity, a hotspot of climate change (warming of the region is above global average), and a hotspot of human population (a highly populated area and a top tourist and retirement destination). In addition, the Mediterranean Sea is the world’s largest inland sea, and climatic disruptions in the region have consequences in the large catchment area that includes central-eastern Europe (Fig. 1). That is, environmental changes and disruptions of the water cycling in the Mediterranean region have consequences affecting a large human population [1].
Fig. 1. Area with mediterranean climate (green) and limits of the Mediterranean catchment (red). The European catchment limit based on Cortambert (1870). From [1]; shape files available here.
The region, as all the planet, is subject to global warming. In addition there are three main local processes (not directly related to global warming) that are very important in understanding dynamic changes in the region [1]:
a) Rural abandonment in an environment depauperate of native herbivores; this increases wildlands (greening) but also the abundance and continuity of fuels that feed wildfires [2]
b) Increasing the wildland-urban interface; this increases biodiversity degradation (e.g., alien species), fire ignitions, and the vulnerability of the society to fires
c) Coastal degradation enhances drought (browning) through negative feedback processes; that is, the desiccation of coastal marshes, the deforestation for agriculture, and more recently, the explosive coastal urbanization, have drastically reduced the original ecosystems and thus the water available for the sea breeze that was once feeding the rain in the upper part of the mountains [1].
All these mechanisms act in different directions (greening, browning), and the current balance is still towards greening, as land abandonment is buffering the browning drivers; however, it is likely to switch with global warming. The challenge is to mitigate the browning processes. The good news is that the importance of small-scale drivers suggests that local policies and actionscan make a difference in reducing overall impact on the landscape and society.
Mechanisms acting at a fine scale, together with global drivers (CO2 enrichment and climatic warming) interact and drive current vegetation changes in Mediterranean landscapes. Any model aiming to predict the future of our vegetation and climate must consider these local mechanisms; and failing to consider them at an appropriate scale is likely to produce inconclusive predictions.
Fig. 2. The disruption of the natural fire and drought regimes in Mediterranean landscapes is driven by global and local drivers. Increased fire activity is a response to the fuel amount and landscape homogeneity generated by rural abandonment (fire hazard) in an environment depauperated of herbivores and with increasing human ignitions (fire risk) and droughts (fire weather). The increased dry conditions are the consequence of global warming, but also of storm losses caused by the disruption of the water cycle generated by the coastal degradation. WUI: wildland-urban interface. From [1].
References
[1] Pausas J.G. & Millán M.M. 2019. Greening and browning in a climate change hotspot: the Mediterranean Basin. BioScience 69: 143-151 [doi | OUP | pdf | map (shape file)]
[2] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf]
“Every time you build anything, a highway, a school, whatever, you are altering the precipitation regime somewhere downwind” Millán M. Millán
Los incendios forestales constituyen procesos complejos, a muchas escalas. Los físicos aún tienen enormes dificultades para modelar y predecir el comportamiento del fuego a escalas temporales y espaciales pequeñas, debido a la complejidad de la vegetación y las interacciones con los parámetros meteorológicos. A escalas mayores, en paisajes heterogéneos, con historia, y con personas asentadas en él, la complejidad es enorme. Prueba de ello es que el llamado ‘problema’ de los incendios forestales no está resuelto ni en nuestro país ni en ningún otro. Parte de la dificultad probablemente resida en un enfoque poco apropiado para abordar el tema. Lo más probable es que se necesite un cambio de paradigma en cuanto a cómo entendemos los incendios forestales en nuestros paisajes. Para este cambio de paradigma se requiere acumular evidencias, cultivar el pensamiento crítico, y romper algunos mitos. Aquí estamos para contribuir a ello.
Régimen de incendios
El clima mediterráneo es propenso al fuego porque tiene una vegetación densa, una estación seca, y tormentas secas (que provocan igniciones). Por lo tanto, como mínimo, hay incendios desde que tenemos clima mediterráneos (hace unos pocos millones de años), aunque hay evidencias de carbones fósiles producto de incendios desde que las plantas colonizaron el medio terrestre. A lo largo de la historia geológica el régimen de incendios ha ido cambiando, es decir, han ido cambiando características tales como la frecuencia, la intensidad, la estacionalidad, el tamaño, o el tipo de propagación. Y estos cambios han sido debidos a las variaciones a escala geológica de la concentración de oxígeno en la atmósfera, a cambios en el clima y en la vegetación, y a variaciones en la abundancia y tipo de herbívoros.
A una escala temporal más reciente (humana), los regímenes de incendios se han ido modificando según el uso del fuego, los cambios antropogénicos del paisaje, de la ganadería/pastoreo, de las políticas de gestión y, finalmente, los cambios climáticos recientes (antropogénicos). Todos estos cambios han ido modificando el régimen de incendios en diferentes direcciones, a veces de manera abrupta. Por ejemplo, el aumento de la población está asociado a un mayor número de igniciones; el incremento de la agricultura está asociado a la disminución de los incendios; y el reciente abandono rural conllevó un incremento de estos, especialmente de su tamaño. De manera que la dinámica del régimen de incendios está fuertemente ligada a cambios socio-económicos.
Adaptaciones
La larga historia de incendios ha conllevado que muchas plantas mediterráneas tengan rasgos o estrategias que les confieren supervivencia y capacidad de reproducción en ambientes con incendios recurrentes (rasgos adaptativos a los incendios). Existe una variedad de rasgos de este tipo; por ejemplo, muchas plantas rebrotan muy bien aunque se quemen totalmente, gracias a tener yemas en estructuras subterráneas protegidas por el suelo, o protegidas debajo de cortezas gruesas. Otras especies germinan de manera abundante después de los incendios, y así aprovechan la elevada disponibilidad de recursos tras el paso del fuego; y en muchos casos, estas especies aumentan su tamaño poblacional respecto al tamaño previo al incendio. Otras especies florecen rápida y abundantemente después de un fuego, aprovechando las condiciones favorables. Todo ello genera una frenética actividad de plantas y animales durante la primavera después de los incendios. La gran mayoría de plantas mediterráneas presenta alguna de estas estrategias para vivir en zonas con incendios recurrentes (ver aquí), aunque cambios bruscos en el régimen pueden conllevar problemas ecológicos.
Los bosques no son la única alternativa
Aunque la vegetación potencial de muchos ecosistemas pueda ser el bosque cerrado, la larga historia de incendios ha producido la apertura en los bosques de manera tan recurrente que han evolucionado especies típicas de matorrales abiertos que ni siquiera pueden vivir dentro del bosque. De hecho no es raro encontrar matorrales más diversos que bosques. Por lo tanto, la sociedad debería poner más en valor los ecosistemas que no son bosques, tales como los matorrales, las dehesas y los pastizales; todos ellos con una elevada diversidad en nuestro territorio. El fuego genera espacios abiertos, y da oportunidad a especies heliófilas, tanto de plantas como de animales. Sin incendios recurrentes nuestro paisajes serían más pobres en especies.
Incendios en el antropoceno
A pesar del carácter natural y antiguo de los incendios, el incremento de igniciones (aumento de la población urbana en ambientes semi-rurales), la elevada continuidad de la vegetación (por abandono rural y falta de herbívoros), y el cambio climático, junto con la política de exclusión del máximo número de incendios, nos lleva en conjunto muy a menudo a regímenes de incendios fuera del rango histórico, donde la mayor frecuencia de incendios de grandes dimensiones podría generar problemas ecológicos y sociales. En un mundo con climas y paisajes cambiantes, la gestión de los incendios (que no la exclusión o extinción), resulta de suma importancia.
La gestión debe partir del conocimiento de que los incendios son propios de los ecosistemas mediterráneos. La política de tolerancia cero a los incendios no ha funcionado en ningún país del mundo. El reto de la gestión no debería ser eliminar los incendios, sino crear paisajes que generen regímenes de incendios sostenibles tanto ecológica como socialmente. Eliminarlos es imposible, antinatural y ecológicamente insostenible. Para generar esos paisajes resilientes se precisan acciones a distintos niveles, tales como aceptar abiertamente un cierto régimen de incendios, crear discontinuidades en paisajes forestales homogéneos (por ejemplo, mosaicos agrícola-forestales), o reducir el combustible (pastoreo y quemas prescritas) en zonas estratégicas o próximas a viviendas. También implica decisiones tan conflictivas como limitar la interfaz urbano-forestal, es decir, limitar la expansión de urbanizaciones y polígonos industriales en zonas rurales y naturales. A los efectos ambientales que supone la expansión de estas zonas de interfaz (por ejemplo, en biodiversidad, especies invasoras, contaminación lumínica y visual, etc.), hay que añadir que constituyen una gran fuente de igniciones, y que convierten en catastróficos (socialmente) incluso a regímenes de incendios ecológicamente sostenibles.
En conclusión
Se están acumulando evidencias que sugieren que hasta ahora teníamos una visión muy incompleta y sesgada de los incendios forestales, relegándolos a un factor externo que destruye ecosistemas y genera problemas ecológicos y sociales. Los estudios realizados en los últimos años sugieren un cambio de visión, donde los incendios constituyen una característica interna de los sistemas socio-ecológicos, una perturbación natural en muchos ecosistemas y una herramienta de gestión para moldear los regímenes de incendios futuros. Por ello se requiere que aprendamos a convivir con los incendios. Este cambio de paradigma se hace más urgente con el cambio climático, ya que la actividad de incendios está aumentando, y solo se puede abordar si se integra los incendios y el fuego dentro de nuestros sistemas socio-ecológicos.
Pausas J.G. & Paula S. 2012. Fuel shapes the fire-climate relationship: evidence from Mediterranean ecosystems. Global Ecol. & Biogeogr. 21: 1074-1082. [doi | pdf | supp]
Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | springer | pdf]
Chergui B., Fahd S., Santos X., Pausas J.G. 2018. Socioeconomic factors drive fire-regime variability in the Mediterranean Basin. Ecosystems 21: 619–628. [doi | pdf | post]
Pausas J.G. 2018. Incendios forestales, encrucijada natural y social. En: Ecología de la regeneración de zonas incendiadas (García Novo F., Casal M., Pausas J.G.). Academia de Ciencias Sociales y del Medio Ambiente de Andalucía. pp. 9-14. [pdf | libro]
Pausas J.G. & Millán M.M. 2019. Greening and browning in a climate change hotspot: the Mediterranean Basin. BioScience. [doi | pdf]
Después de un incendio, lo peor que puede ocurrir es que haya pérdida de suelo. Por ello se aconseja visitar pronto las zonas afectadas con el fin de detectar puntos donde es posible que esto ocurra, y en su caso, poder aplicar medidas para evitarlo. En la zona mediterránea las zonas con posible pérdida de suelo suelen ser puntuales dentro del área afectada por el incendio. Estas zonas sensibles suelen estar asociadas a pendientes pronunciadas, suelos poco pedregosos, texturas arenosas, bancales abandonados, etc.. Entre las medidas para evitar la pérdida del suelo se incluye cubrir la parte sensible con paja o similar (empajado o mulching, que evita el contacto directo de las gotas de agua con el suelo), la siembra de hierbas de crecimiento rápido, o la colocación de troncos en fajas para reducir los movimientos de suelo.
En el pasado agosto ardieron unas 3200 ha en el incendio de Llutxent, afectando principalmente a los municipios de Llutxent, Gandia y Pinet (provincia de Valencia). Poco después del incendio se iniciaron acciones para proteger el suelo de la erosión, tales como la corta de pinos muertos y su colocación en el suelo en fajas. Sin embargo, estas acciones se están aplicando de forma generalizada, incluyendo zonas muy pedregosas y con poca pendiente (ver fotografías), o donde la regeneración de la vegetación es buena. En estas zonas no es previsible que haya erosión, y por lo tanto esta acción parece ecológica y económicamente innecesaria. En algunos casos, podría ser perjudicial si las acciones realizadas afectan a la regeneración natural. De hecho, con las lluvias de las semana pasadas, ya se podría evaluar si realmente ha servido para algo estas acciones y si vale la pena continuarlas.
Acciones que se están realizando en la zona afectada por el incendio de Lutxent (fotos tomadas en septiembre y octubre, 2018, por R. Badenes y J.G. Pausas).
Actualización, 27 Nov 2018: Hace unas semanas en Llutxent llovió entre 300-600 mm en pocos días, y no se ha observado erosión en sitios pedregosos y poco inclinados. A pesar de esto, se sigue aumentando las fajas para frenar la erosión … (pincha en la foto para ampliar)
One of the unifying approaches in ecology is to search for common strategies, that is, to group species sharing mechanisms and responses to environmental factors and disturbances. Plant strategies to persist in fire-prone ecosystems (and the traits involved) are now quite well known [1]. However, less is known about fire strategies in animals, despite many fire-prone ecosystems harbor a very rich fauna [2]. This difference in knowledge is probably due to the intrinsic differences between plants (immobile) and animals (mobile) [2]. However, there is a demand for unifying plant an animals paradigms in order to better asses biodiversity in fire-prone ecosystems [3]. In a recent paper [4] I am providing an unifying framework by emphasizing the similarities between plants and animals in relation to the mechanisms for living in fire-prone ecosystems. To do so, I propose a very simple fire strategy scheme that should be valid for both plants and animals (Table 1). The advantage of having a unified framework of fire strategies include: (1) we can learn how species respond to fire from a great diversity of life forms; (2) animal and plant ecologists can benefit from shared expertise in fire responses (some common strategies in plants may be overlooked in animals, or vice-versa); (3) we could better predict changes in plant-animal interactions with fire regime changes, and (4) we could better assess and generalize the effects of fire on biodiversity. I hope this framework would facilitate finding knowledge gaps and directing future research for gaining a better understanding of the role of fire on biodiversity.
Table 1. Generalized mechanisms of species response to fire (strategy), their fire dynamics and persistence scale, and the prevalence for animals and plants in fire prone ecosystems (low, moderate, and high). The last column refers to the fire characteristics where this strategy is most likely to occur (‘high’ and ‘low’ refers to fire intensity). [4]
Fig. 1. The rhea (Rhea americana) has a cryptic coloration in postfire environments, when sitting in the ground, the neck cannot be differentiated from a burned stem. Photo: JG Pausas
Fig. 2. Charaxes jasius colonizing the middle of a burnt area 10 days after the wildfire that burned with very high intensity in NE Spain (note that only thick branches remained). Photo: JG Pausas.
References
[1] Keeley J.E., Bond W.J., Bradstock R.A., Pausas J.G. & Rundel P.W. 2012. Fire in Mediterranean Ecosystems: Ecology, Evolution and Management. Cambridge University Press. [the book]
[2] Pausas J.G., Parr C.L. 2018. Towards an understanding of the evolutionary role of fire in animals. Evolutionary Ecology 32: 113–125. [doi | pdf]
[3] Kelly L.T., Brotons L, Giljohann K.M., McCarthy M.A., Pausas J.G., Smith A.L. 2018. Bridging the divide: integrating animal and plant paradigms to secure the future of biodiversity in fire-prone ecosystems. Fire 1(2): 29. [doi | mdpi | pdf]
[4] Pausas J.G. 2019. Generalized fire response strategies in plants and animals. Oikos 128:147-153 [doi | wiley | pdf | oikosblog]
In early August a wildfire ignited by a lightning burned about 3200 ha, affecting mainly the municipalities of Lutxent, Gandia and Pinet (in Valencia, Spain). One month later I visited the area, and below are the main plant species that were already resprouting. There were also two species already flowering, both geophytes: Urginea (Drimia) maritima and Scilla autumnalis; they showed flowers but not the leaves (they are protanthous: flowering before the foliage appears [1]). There were also many seedling germination from the seedbank, but they were too small to identify.
The area affected by the fire include a small marginal population of Quercus suber (cork oak; el surar de Pinet) that we had studied few years ago [2]. This oak was also resprouting (epicormically).
(click to the photo to enlarge)
Urginea maritima, 1 month
Scilla autumnalis, 1 month
Smilax aspera
Daphne gnidium
Quercus coccifera
Pistacia lentiscus
Globularia alypum
Phillyrea angustifolia
Erica scoparia
Pteridium aquilinum
Quercus suber
Fraxinus ornus
Chamaerops humils
Lonicera implexa
Thymus piperella
Notes and references
[1] The terminology of the flower/leaf phenology is a bit confusing; here is my understanding following Simpson (Plant Systematics, 2011) and Lamont & Downes (2011, Pl. Ecol. 212):
· Synanthous (syn= same time): flowers and leaves develop at the same time
· Hysteranthous: flowering occurring out of phase with leafing
· Protanthous (pro= early): flowers develop before the leaves
· Seranthous (ser= delayed): flowers develop after the leaves
[2] Pausas J.G., Ribeiro E., Dias S.G., Pons J. & Beseler C. 2006. Regeneration of a marginal Cork oak (Quercus suber) forest in the eastern Iberian Peninsula. Journal of Vegetation Science 17: 729-738. [pdf | doi | wiley]
Fa poc, la revista Mètode me va fer la següent pregunta: Per què hi ha plantes que necessiten el foc per a germinar? Ací la meua resposta: català | castellano. També la copio aquí sota, amb alguna lleugera modificació:
La finalitat de tot ésser viu és reproduir-se, i en el cas de les plantes, açò inclou que les llavors germinin en un ambient favorable per al creixement. És per açò que les plantes han desenvolupat estratègies per dipositar les seues llavors en espais oberts que permetin la germinació i el creixement de la descendència. En moltes plantes, aquestes estratègies es basen en la dispersió de les llavors per animals o pel vent. Però hi ha altres que aprofiten els espais oberts dels incendis!
En els ecosistemes mediterranis els incendis són relativament freqüents de manera natural, i generen grans espais oberts ideals per a la germinació i el creixement de moltes plantes mediterrànies (molta llum, poca competència, i elevada disponibilitat de nutrients). Conseqüentment, en moltes d’estes espècies han evolucionat estratègies per optimitzar la germinació just després del foc. Per aconseguir sincronitzar la germinació amb el moment dels espais oberts, les plantes acumulen les llavors (formen un banc de llavors) i el foc els hi fa de senyal de quan poden germinar. Hi ha dos estratègies. Una és que les plantes acumulen les llavors al sòl (banc de llavors al sòl), i la calor o el fum dels incendis fa de senyal i estimula la germinació. Un exemple ben conegut d’aquesta primera estratègia ho constitueixen les espècies d’estepes del gènere Cistus així com molts arbust de la família de les lleguminoses (argelagues, etc.). Una segona estratègia és acumular les llavors a la capçada dels arbres (banc de llavors a la capçada), dins d’estructures que resisteixen be els incendis (les pinyes). Amb la calor dels incendis, les pinyes s’obren, i les llavors cauen a l’espai obert creat per l’incendi i germinen. Este és el cas del del pi blanc (Pinus halepensis), tant abundant al nostre territori. En tots aquests casos, la reproducció està fortament lligada als incendis, cosa que se considera una adaptació al foc.
Exemples de germinció massiva poc després d’incendi a Odena 2015, Doñana 2017 , i Chile 2017, respectivament (clicar per a ampliar):
Ejemplos de germinción masiva poco después de incendio, en Odena 2015, Doñana 2017, y Chile 2017, respectivamente (clicar para ampliar).
Hace poco, la revista Mètode me hizo la siguiente pregunta: ¿Por qué hay plantas que necesitan el fuego para germinar? Aquí mi respuesta: català | castellano. También la copio aquí debajo, con alguna ligera modificación:
La finalidad de todo ser vio es reproducirse, y en el caso de las plantas esto incluye que las semillas germinen en un ambiente favorable para su crecimiento. Es por esto que las plantas han desarrollado estrategias para depositar sus semillas en espacios abiertos que permitan la germinación y el crecimiento de la descendencia. En muchas plantas, estas estrategias se basan en la dispersión de las semillas por animales o por el viento. Pero hay otras que aprovechan los espacios abiertos por incendios!
En los ecosistemas mediterráneos los incendios son relativamente frecuentes de manera natural y generan grandes espacios abiertos ideales para la germinación y el crecimiento de muchas plantas mediterráneas (mucha luz, poca competencia y elevada disponibilidad de nutrientes). Como consecuencia, muchas de estas especies han evolucionado para optimizar la germinación justo después del fuego. Para conseguir sincronizar la germinación con el momento del incendio, las plantas acumulan las semillas en el suelo (banco de semillas en el suelo) y las elevadas temperaturas o el humo de los incendios actúa de señal y estimula la germinación. Un ejemplo bien conocido de esta primera estrategia lo constituyen las especies de jara del género Cistus, así como muchos arbustos de la familia de las leguminosas (aliagas, etc.). Una segunda estrategia es acumular las semillas en la copa de los árboles (banco de semillas de copa), dentro de estructuras que resistan bien los incendios (piñas). Con las elevadas temperaturas del fuego, las piñas de abren y las semillas caen al espacio abierto creado por el incendio y germinan. Este es el caso del pino carrasco (Pinus halepensis), tan abundante en nuestro territorio. En todos estos casos, la reproducción esta fuertemente ligada a los incendios, hecho este que se considera una adaptación al fuego.
For two years we sampled invertebrates after two large wildfires in eastern Spain and demonstrate that two flower beetle species, Protaetia morio and P. oblonga (Cetoniidae; Fig. 1 & 2 below), show a pyrophilous behaviour [1]. These beetles were much more numerous after the fires than in unburnt plots around the fire perimeter; in addition, these species tended to increase in number with the distance from the fire perimeter and with fire recurrence (Fig. 3 below). These results suggest that local populations survived the fire as eggs or larvae protected in the soil, and then they were favoured postfire (i.e., population size increased, compared with unburnt zones). We propose that this could be driven by the reduction of their predator populations, as vertebrates that feed on these beetles are disfavored by fire. That is, the results suggest that these flower beetle species benefit from fire because fire disrupts antagonistic interactions with their predators (predation release hypothesis).
Fig. 1. Protaetia morio: eggs, larva, pupal, and adult (photos: S. Montagud); pitfall trap full of Protaetia beetles (bottom left).
Fig. 2: Protaetia morio (male and famale) and Protaetia oblonga (male and female)
Fig. 3: Abundance (number of individuals) of Protaetia morio one and two years after fire (from two fires that occurred in 2012). Green: Unburnt; Yellow: Burnt edge (< 700 m from the fire perimeter); Orange: Burnt center (> 1.3 km from the fire perimeter). P. oblonga showed a similar pattern. For details, see [1].
Reference
[1] Pausas, J.G., Belliure, J., Mínguez, E. & Montagud, S. (2018) Fire benefits flower beetles in a mediterranean ecosystem. PLoS ONE, 13: e0198951. [doi | plos | pdf]
Recientemente, Greenpeace hizo un par de preguntas a varias personas que trabajan en diferentes ámbitos relacionados con incendios forestales (noticia | documento). Aquí copio mis respuestas.
¿En qué han cambiado los incendios?
En las últimas décadas se ha observado un cambio brusco en el régimen de incendios, aumentando la frecuencia y, especialmente, el tamaño de los incendios. Los incendios requieren de tres factores: combustible (vegetación densa y continua), igniciones, y condiciones propensas a la propagación del fuego (sequedad, viento). Estos tres factores se han visto favorecidos en los últimos años y de manera simultánea:
(1) ha aumentado la cobertura, continuidad y densidad de la vegetación en el paisaje, incrementando la biomasa y el combustible disponible para los incendios;
(2) ha cambiado el clima hacia veranos más cálidos, más secos, y más largos, por lo que se dan mejores condiciones para la propagación del fuego y durante más tiempo; y
(3) ha aumentado la población urbana en las zonas de interfaz urbano-forestal (ver foto), lo que conlleva un mayor número de igniciones, tanto accidentales como provocadas.
En definitiva, actualmente hay más incendios porque tenemos paisajes y climas más propensos a la propagación del fuego, y más igniciones.
De estos tres factores, el que más ha influido en el cambio del régimen de incendios es el aumento de vegetación (biomasa) en nuestros paisajes. Este aumento se debe principalmente al abandono de las actividades rurales (tales como la agricultura, el pastoreo, la extracción de leña, o la gestión de las plantaciones forestales), y a las políticas de exclusión total de los incendios, sin una sustitución por otros sistemas que controlen la vegetación, tales como los herbívoros silvestres, o las quemas y el pastoreo prescritos.
¿Como mitigar el impacto de los incendios?
La política de tolerancia cero a los incendios no ha funcionado en ningún país del mundo. El reto de la gestión no debería ser eliminar los incendios, sino crear paisajes que generen regímenes de incendios sostenibles tanto ecológica como socialmente. Para ello se precisan acciones a distintos niveles, tales como aceptar abiertamente un cierto régimen de incendios (especialmente en zonas poco pobladas y en ecosistemas con adaptaciones al fuego), crear discontinuidades en paisajes forestales homogéneos (por ejemplo, mosaicos agrícola-forestales), o reducir el combustible en zonas estratégicas o próximas a viviendas. También implica decisiones tan conflictivas como limitar la interfaz urbano-forestal, es decir, limitar la expansión de urbanizaciones y polígonos industriales en zonas rurales y naturales. A los efectos ambientales que supone la expansión de estas zonas de interfaz (por ejemplo, en biodiversidad, especies invasoras, contaminación lumínica y visual, etc.), hay que añadir que constituyen una gran fuente de igniciones, y que convierten en catastróficos (socialmente) incluso a regímenes de incendios ecológicamente sostenibles. Los mecanismos para limitar estas zonas pueden ser diversos, incluyendo la recalificación de terrenos (a no urbanizables), o la implementación de tasas (disuasorias) por construir en áreas con alto riesgo de incendios, entre otros. Además, y con carácter más general, incrementar las medidas que frenan el cambio climático contribuiría a reducir los cambios no deseados en el régimen de incendios.
Foto: Ejemplo de interfaz urbano-forestal (Port d’Andratx, Mallorca; foto: @xarxaforestal). Con este modelo de urbanismo, además de aumentar la probabilidad de igniciones, convertimos en catastróficos incluso a regímenes de incendios ecológicamente sostenibles.
Fire is a key ecological factor in many Mediterranean shrublands [1]. But there is another shrubland, the Florida scrub, that share many characteristics with mediterranean ecosystems. Fire is frequent in the Florida scrub, and most plant strategies to deal with fire are the same to those found in mediterranean ecosystems, despite the species are different (a likely case of convergent evolution). The Florida scrub occurs on sandy soils of the Florida Peninsula (USA), under a subtropical climate.
Eric Menges, a fire ecologist at Archbold Biological Station in Florida, explains in this 16-minute video the main adaptive traits of plants to live in the Florida scrub. In his words “the lack of fire is a bigger disturbance than the fire”. All strategies explained in the video also occur in most mediterranean regions, including the Mediterranean Basin (i.e., from Portugal to Syria).
Video “Surviving fire in the Florida scrub”, also available in youtube.
[Versión en español]
El fuego es un factor ecológico clave en muchos matorrales mediterráneos [1,2]. Pero hay otro matorral, el matorral de la Florida, que comparte muchas características con los ecosistemas mediterráneos. El fuego también es frecuente en este matorral, y la mayoría de las estrategias de las plantas para persistir después de incendio son las mismas que las que se encuentran en los ecosistemas mediterráneos, a pesar de que las especies son diferentes (con ejemplos de evolución convergente). El matorral de Florida aparece en suelos arenosos en la península de la Florida (EEUU), en clima subtropical.
Eric Menges, ecólogo en la Estación Biológica Archbold en Florida, explica en este video de 16 minutos los principales estrategias adaptativas de las plantas para vivir en el matorral de Florida. En sus palabras, “la falta de fuego es una perturbación más importante que el fuego”.Todas las estrategias explicadas en el video también ocurren en la mayoría de las regiones mediterráneas, incluida la Cuenca Mediterránea (de Portugal a Siria, pasando por España, claro).
References
[1] Keeley J.E., Bond W.J., Bradstock R.A., Pausas J.G. & Rundel P.W. 2012. Fire in Mediterranean Ecosystems: Ecology, Evolution and Management. Cambridge University Press. [the book]
[2] Pausas J.G. 2012. Incendios forestales. Una visión desde la ecología. Catarata-CSIC. [Libro]
‘Marjal del Moro’ is a small coastal wetland located in the municipalities of Puçol and Sagunt (Valencia, Spain). It is a Special Area of Conservation (SAC; ZEC in Spanish) and a Special Protected Area for the conservation of birds (SPA; ZEPA in Spanish), of the European Union. In January the 4th a wildfire burned 320ha which is ca. 80% of the wetland. Here are some dynamics after the fire.
1 month after the fire (18th February 2018):
At this time, Tamarix trees (salt ceder) were not resprouting, but some other plants already started to resprout (click to enlarge):
Dittrichia viscosa
Schoenus nigricans
Foeniculum vulgare
cf Suaeda
4 months after fire (1st May 2018):
Note that the two trees (salt ceders, Tamarix) are the same ones from the picture above (taken in February). At this time, Tamarix species were already resprouting (click to enlarge):
Tamarix africana
Tamarix cf canariensis
Thanks to E. Laguna for his help on the species identification.
Plants show a plethora of adaptive traits for persisting under recurrent fires [1]. However, fire-prone ecosystems also harbor a rich fauna, and little is know about their adaptive traits for fire survival. In a recent paper [2] we review this issue and suggest that many animals are adapted to the open habitats generated by fire; yet although they require fires for survival (fire-dependent animals), they do not necessarily show any specific morphological adaptation to fire. However, these species would become very rare or even extinct in the absence of fires generating their habitat. In addition, in some cases, animals from these fire-prone ecosystems show specific fire adaptation (fire-adapted animals). Currently, there are few examples of morphological adaptations to fire in the animal kingdom (reviewed in [2]). In part this may simply reflect the low number of studies that have attempted to look for fire adaptations. We propose that there remains significant scope for research on fire adaptations in animals, and especially in relation to the rich behavioral traits that allow persistence in fire-prone ecosystems. This is because, in contrast to plants, most animals are unitary organisms with reduced survival when directly burnt by fire, but are mobile and can move away from the fire. That is, behavioral traits are poorly explored under the framework of the evolutionary fire ecology and may provide a rich source for fire adaptations. Developing this understanding is critical to better understand the role of fire in determining the biodiversity of our landscapes.
Photo 1: An owl hunting in the fire front (fire-foraging) at Aransas National Wildlife Refuge in Texas (Photo: Jeffrey Adams/USFWS; from www.fws.gov).
Photo 2: The rhea (Rhea americana) is a flightless bird living in Brazilian savannas; it has a cryptic colors in postfire environments, when it sits in the ground in cannot be differentiated from burn stems (Photo: JG Pausas, 2009).
References
[1] Keeley J.E., Bond W.J., Bradstock R.A., Pausas J.G. & Rundel P.W. 2012. Fire in Mediterranean Ecosystems: Ecology, Evolution and Management. Cambridge University Press. [the book]
[2] Pausas J.G., Parr C.L. 2018. Towards an understanding of the evolutionary role of fire in animals. Evolutionary Ecology. [doi | pdf]
Many plants are able to survive recurrent disturbance by resprouting from a bud bank. In fire prone ecosystems, plants must protect their buds from fire heat or perish. One way to protect them is by growing a thick insulating bark or sink the buds in the stem [1,2]. Another way is to locate the buds below ground, as soil is an excellent heat insulator (belowground bud bank or BBB). In fire-prone ecosystems, there is a diversity of ways by which plants successfully conceal their buds below ground that enable them to survive and resprout vigorously after fire [3]. There are at least six locations where belowground buds are stored [3]: roots, root crown, rhizomes, woody burls, fleshy swellings and belowground caudexes. These support many morphologically distinct organs (figure below). Considering their history and function, these organs may be divided into three groups:
(a) Those that originated in the early history of plants and that currently are widespread; they act as a resprouting source after a range of disturbances, not just fire. These include bud-bearing roots and root crowns.
(b) Those that also originated early and have spread mainly among ferns and monocots; they are often tied to seasonal stresses and have been highly successful under recurrent fire regimes. Theses include non-woody rhizomes and a wide range of fleshy underground swellings. They are characteristic of the geophyte growth forms occurring in many ecosystems, often tied to seasonal stresses; they have been highly successful under recurrent fire regimes.
(c) And those that originated later in history and are strongly tied to fire-prone ecosystems. These are woody rhizomes, lignotubers and xylopodia. They are characteristic of the geoxyle growth form.
Recognizing the diversity of BBBs is the starting point for understanding the many evolutionary pathways available for responding to severe recurrent disturbances.
Figure: Stylized diagrams of 16 belowground bud bank structures that enable plants to resprout postfire (highlighted in red). Broken horizontal line indicates position of soil surface. Structures characterized by woody tissues, in pink; fleshy tissues, in blue; and neither woody nor fleshy, in orange (usually highly sclerified primary tissues, fibrous or ‘wiry’). Shoots highlighted in apple green: stems with leaves, branched; leaves only, unbranched. Roots highlighted in olive green: triangular-shaped roots indicate a primary system, those arising directly from the bud-storing structures are adventitious. Drawings from [3]. From top left to bottom right:
· Xylopodium (in red) joined to tuberous root (in blue); Lignotuber; Root Crown; Woody Rhizome, here arising from a burl
· Bud-bearing lateral Root arising (here) from a burl (the root is not necessarily woody); Taproot Tuber; Bulb; Corm, with previous year’s corm still present
· Stem Tuber; Non-woody Fleshy Rhizome; Rhizophore (note buds are only supported by the oldest rhizophores); Adventitious Root Tuber
· Non-woody fibrous Rhizome with a monopodial arrangement leading to expansive clone; Non-woody fibrous Rhizome with sympodial arrangement leading to a caespitose habit; Stolons that produce new ramets postfire (note that it is not a BBB); Belowground Caudex
For details and a full description of each structure, see reference [3].
References
[1] Pausas J.G. 2017. Bark thickness and fire regime: another twist. New Phytologist 213: 13-15. [doi | wiley | pdf | post1, post2]
[2] Pausas J.G. & Keeley J.E. 2017. Epicormic resprouting in fire-prone ecosystems. Trends in Plant Science 22: 1008-1015. [doi | pdf | post ]
[3] Pausas J.G., Lamont B.B., Paula S., Appezzato-da-Glória B., Fidelis A. 2018. Unearthing belowground bud banks in fire-prone ecosystems. New Phytologist [doi | pdf | suppl | BBB database]
We often tend to think that the main driver of fires is drought, but in many cases wind is extremely important; and the perfect combination is a strong wind after a long drought period. Foehn-type (adiabatic) winds are especially important for fires as they are fast-moving hot and dry winds that quickly dry out the vegetation, and thus they spread the fire very easily. In the recent months, we have witnessed several very destructive wind-driven fires, affecting many infrastructures and lives. Wind-driven fires are common in Southern California where the Santa Ana winds blow after the summer. They typically occur in October, although this year they came later, in December. For instance, the Thomas fire started at the early December, and has now become the largest wildfire in California history (> 110,000 ha, Fig. 1) with more than 1000 houses destroyed, more than 100,000 residents evacuated, and several fatalities. This fire has been largely driven by Santa Ana winds.
Both Santa Ana and Diablo winds are Foehn winds going down from the mountains (inland) to the coast (Santa Ana and Diablo winds at a glance). Wind-driven fires are natural in California and have been generating large fires since long ago, but the increasing population living in the wildland-urban interface is making these fire more destructive than ever. In addition, climate change is extending the fire season into the late fall and winter, increasing the probability of large fires.
Wind-driven fires have also occurred this year in north-western Iberia (Spain and Portugal), caused by the hurricane Ophelia. Typically, tropical hurricanes do not get to Europe, but this year the Ophelia touched western Europe (probably due to the warming of the ocean) and spread massive fires in Portugal and Spain that were fuelled with large poor-managed forest plantations; the ashes from these fires reached England and Ireland (Fig. 2 below).
In conclusion, global change is likely modifying wind patterns, and thus to understand new fire regimes we need to predict wind regime; however, predicting future wind regimes is more difficult than predicting temperature changes.
Figure 1. The beginning of the Thomas fire (started in Ventura, Southern California) was clearly driven by Santa Ana winds (Image: NASA / MODIS, December 5, 2017). This fire has grown and become the largest wildfire in California history (> 110,000 ha).
Figure 2. Massive October wildfires in NW Iberia were fueled by the hurricane Ophelia; smoke and ashes from these fires reached England and Ireland (Image: NASA Terra / MODIS, October 16, 2017); see also Severe Weather Europe. You can see an animation from NASA Earth here.
The cover of the December issue of Trends in Plant Science(22:12) is a picture of Pinus canariensis resprouting epicormically (from stem buds) 3 months after a fire in Tenerife (Canary Islands, Nov 2012). It features our review paper on this type of resprouting [1]. Many plants resprout from basal buds after disturbance, however epicormic resprouting is globally far less common, and the Canary Island pine is a very good example; it resprouts in this way even after intense crown fires.
[1] Pausas J.G. & Keeley J.E. 2017. Epicormic resprouting in fire-prone ecosystems. Trends in Plant Science 22: 1008-1015. [doi | sciencedirect | pdf | post ]
Some trees species, like many Eucalyptus, resprout from a lignotuber (a basal burl [1]) when young, and from epicormic (stem) buds [2] at the adult stage. This seems also the case for Juniperus deppeana (alligator juniper), at least the ones from the Trans-Pecos region, Far West Texas, USA. Big trees can survive surface fires (Fig. 1a below) thanks to their relatively thick bark (Fig. 1b). In the upper part of the Guadalupe mountains, a fire in May 2016 spread throughout the surface, crowning in some specific spots. In these areas, smaller trees were resprouting from lignotubers (Fig. 1c) while large trees were resprouting from epicormic buds (Fig. 1d). In this dry forest in Guadalupe, Juniperus deppeana is abundant; in addition, two other conifers relatively rare in Texas are also common: Pinus ponderosa and Pseudotsuga menziesii (Douglas fir); many of the large individuals of the latter species were dead from a recent drought previous to the fire. The forest also included some oaks, both tree and shrub oak species, and an understory with grasses, Agave and Dasylirion species.
Figure 1. Photos of Juniperus deppeana (alligator juniper). a) A very large juniper with fire scars from surface fires (and Dylan Schwilk, Texas Tech University, in front of it). b) Detail of the bark. c) Basal stem excavated to show that postfire resprouts originates from a below-ground bud bank, a lignotuber. d) Postfire epicormic resprouting. Photos a) and b) from Davis Mountains, c) and d) from Guadalupe Mountains (1.5 years after a fire), Trans-Pecos region, Texas, November 2017.
References
[1] Paula S., Naulin P.I., Arce C., Galaz C. & Pausas J.G. 2016. Lignotubers in Mediterranean basin plants. Plant Ecology 217: 661-676. [doi | pdf | suppl.]
Many plants resprout from basal buds after disturbance, and this is common in shrublands subjected to high-intensity fires [1]. However, resprouting after fire from epicormic (stem) buds is globally far less common. In a recent paper we review the ecology and evolution of this mechanism [2]. Many plants can generate epicormic shoots after light disturbances (e.g., browsing, drought, low intensity fires, insect defoliation, strong winds), but this does not mean they generally resprout epicormically after fire, as the heat of a fire may kill epicormic buds if they are not well protected (e.g., by a thick bark). The most well-known examples of epicormic resprouting are many species of eucalypts (Fig. 1A below), the cork oak (Quercus suber [3], Fig. 1B below), and Pinus canariensis ([4], Fig. 1C, D below). There are other pines and oaks that also resprout epicormically, and many species from savannas, especially those from the Brazilian savannas (cerrado) where many trees have a thick corky bark [5].
Epicormic resprouting has appeared in different lineages and on different continents and thus it is an example of convergent evolution in fire-prone ecosystems. It is an adaptation to a regime of frequent fires that affect tree crowns. It has probably been favoured where productivity is sufficient to maintain an arborescent growth form, fire intensity is sufficient to defoliate the tree canopy crown, and fire frequency is high (in conifers, too high for serotiny to be reliable) [2]. Given the high resilience of forest and woodlands dominated by epicormic resprouters, these species are good candidates for reforestation projects in fire-prone ecosystems [3].
Figure: Examples of postfire epicormic resprouting after a crown fire from very different lineages: (A) Eucalyptus diversicolor 18 months after fire in Western Australia. (B) Quercus suber woodland 1.5 years postfire in southern Portugal. (C) Pinus canariensis woodland a few years after fire; (D) epicormic resprouts of P. canariensis 3 months postfire. Photos by G. Wardell-Johnson (A); F.X. Catry (B) and J.G. Pausas (C, D), from [2].
References
[1] Pausas, J.G., Pratt, R.B., Keeley, J.E., Jacobsen, A.L., Ramirez, A.R., Vilagrosa, A., Paula, S., Kaneakua-Pia, I.N. & Davis, S.D. 2016. Towards understanding resprouting at the global scale. New Phytologist 209: 945-954. [doi | wiley | pdf | Notes S1-S4]
[3] Aronson J., Pereira J.S., Pausas J.G. (eds). 2009. Cork Oak Woodlands on the Edge: conservation, adaptive management, and restoration. Island Press, Washington DC. 315 pp. [The book]
[5] Dantas V. & Pausas J.G. 2013. The lanky and the corky: fire-escape strategies in savanna woody species. Journal of Ecology 101 (5): 1265-1272. [doi | pdf | suppl.]
During the 2016/17 fire season in central Chile, wildfires burned about 600,000 ha, a record for the region (most of the area burned between 18-Jan and 5-Feb, 2017). Two factors are considered the main responsible of such a large area burned: (1) an intense drought with strong head waves (January was the hottest month in record), and (2) the fact that the region is covered by large and dense tree plantations that create a continuous fuel bed. The tree planted are two alien species: Pinus radiata and Eucalyptus sp., from California and Australia, respectively. Most burned area (+60%) were plantations, and if we standardize the area burned in relation to the area with each landuse in the region (plantations, native forest, grasslands, agriculture) we see that the plantations were more affected by fire than expected by their area in each region; and this contrast with the other landuses (Figure 1, [1]). That is, tree plantations were an important driver for the large area burned (highly flammable).
Interesting is that the two species planted not only are highly flammable, they also have very good (although very different) postfire regeneration mechanisms, because both are originally from fire-prone ecosystems and have adapted to coupe with crown fires. Pinus radiata have serotinous cones (closed cones that open with fire) and showed an extraordinary “natural” seedling regeneration postfire (Figure 2 top), while those eucalytps planted show epicormic (stem) resprouting that allows a quick canopy recovery (even young trees, Figure 2 bottom). All suggest that these plantations were born to burn!
Figure 1: Analysis of the areas affected by fires according to types of use (forest plantations, native forest, Scrubland + pastures, and agricultural areas), in relation to what is available in each of the 4 regions that have burned the most (V, RM, VI, VII are: Valparaiso, Metropolitana, O’Higgins, and Maule). Positive data means that fire has positively selected this type of use (it has burned more than expected by the area it occupies); the negative data indicate that fire tends to avoid such landuse. There is a strong tendency for plantations to burn more than expected according to their abundance in the landscape (positive values), while native forests, scrub, or agricultural areas are burned similar or less than expected according to their abundance (negative values). The region VII (Maule) is the most extreme in positive selection of plantations and negative of other uses. Elaborated on the basis of official SIDCO-CONAF data (Chile) [1].
Figure. 2. Postfire regeneration of tree plantations. Top: Extraordinary postfire seedlings regeneration of Pinus radiata (adult trees are dead). Bottom: epicormic resprouting of eucalypts (mixed with dead pines). Photos from early September (ca. 7 months after fire), in the Nilahue Barahona fire (O’Higgins region, Chile).
Recently, a colleague asked me about the difference between fire hazard, fire risk and fire danger. I’m not an expert in these concepts, but here is my understanding of these and other related terms. In short, fire hazard is related to fuel (forestry), fire risk is often used for mapping the probability of ignitions (geography), and fire danger is typically associated with weather conditions (meteorology); below is a longer answer (see Fig. 1). Feel free to improve or qualify these definitions by leaving a comment (see top right).
Fire weather: Weather conditions that influence fire ignition, behaviour, and suppression. E.g., extreme (or severe) fire weather refers to very low moisture, high temperatures (heat waves) and strong winds. Fire weather indices (FWI) can provide information for estimating the fire danger (see below).
Fire hazard: the degree of ease to fire ignition and propagation, and the resistance to control (given an ignition source). It depends on the quantity and continuity of the vegetation (fuel) and it is independent of the weather (in contrast to fire danger). It reflects the potential fire behaviour associated with static properties of fuel (fire hazard doesn’t change from one day to the next, in contrast to fire danger). Fire hazard reduction treatments refer to fuel treatments.
Fire risk: typically it refers to the probability of ignition, i.e., the chance that a fire might start (risk of ignition). It can be split into lightning fire risk and human fire risk; the latter typically decreases with distance to roads and increases with population density in rural or semi-rural areas. Other authors define fire risk as potential damage (or degradation risk), and thus they include fire hazard and fire vulnerability in the concept of fire risk. Fire risk is relatively static (in contrast to fire danger; e.g., a zone with high fire risk), and often used to produce maps (fire risk mapping).
Fire danger: sum of the factors affecting the initiation, spread, and resistance to control in a given area; it is typically expressed as a semi-quantitative index (e.g., from very high to very low). Very often it largely depends on weather (i.e., moisture; sometimes also lightning activity) and is reported by meteorological agencies (weather-based fire danger). Because it considers the weather, fire danger is very dynamic (e.g. fire danger today; daily fire danger forecast). Note that if fire danger is based on weather only: (1) the fire danger may be very high in areas where the likelihood of having a fire is very low due to their low fuel (i.e., overestimation in arid ecosystems; Fig. 2 below); (2) weather-based fire danger may fail to capture short-term increases in dead fuel due to strong droughts (underestimation); and consequently, (3) predictions of fire danger for the future under climatic warning may be questionable. A good prediction of fire danger should consider fire risk, fire weather and fire hazard (including fuel dynamics).
Fire damage: detrimental changes in value after a fire (e.g., ecological fire damage, social fire damage); i.e., it refers to negative fire effects. Note that fire may damage some species and favour others; also it depends on the temporal scale, as some short-term effects may be different from mid- or long-term effects.
Fire vulnerability: probability of fire damage; potential effects of fire on values. It is often presented as fire vulnerability maps. Ecological fire vulnerability is typically computed from the type of vegetation, soil and topography, to estimate postfire erosion risk and regeneration capacity. Societal vulnerability is typically associated with exposure, sensitivity, and adaptive capacity; these characteristics depend on socio-economic level and cultural experience.
Fig. 1. Schema of the different concepts related to fire danger and fire vulnerability. Sometimes, fire danger + fire vulnerability is called (overall) fire risk.
Fig. 2. Map of the Fire Danger Forecast for the Mediterranean region on 5 Aug 2017 from the Global Wildfire Information System (GWIS, EFFIS-Copernicus). Darker heat colours indicate higher fire danger (green: very low). In fact, this is based on fire weather; note that it is not predicted for arid areas (white, in Africa) where the low biomass may produce extremely unrealistic results (it should probably be green). So it looks more like a heat index than a fire danger index. I would also say that the palette of colors seems a bit too contrasted.
In the matorral (chaparral-type vegetation) of Central Chile, natural fires are assumed to have been much less frequent (during the Quaternary) than in the other Mediterranean-type ecosystems (MTEs) of the world [1]. Thus, plant adaptive responses to fire are expected to be uncommon. Resprouting is a relatively widespread trait in Chilean woody species, although this traits is not really an indicator of the fire history as resprouters occur in many environments, not only in fire-prone ones [1,2]. Fire-stimulated germination (i.e., the increased seed germination after a heat shock or after the smoke produced by a fire) is a trait more specifically tied to fire [1,3]. A recent study [4] demonstrates that fire-stimulated germination is not as common in the Chilean woody flora as in other MTEs; i.e., negative seed responses to fire cues were more frequent than positive responses. Some seeds were damaged by fire, but many species were able to resist the heat shock although without an increase on germination. In few species, germination was stimulated (by heat or smoke), but the magnitude of the stimulation was relatively low. The overall effect is that fire-stimulated germination is poorly represented in the Chilean matorral. These results support the idea that this matorral had a history of lower fire activity than other mediterranean-climate regions, despite having a fire-prone climate. This low fire activity has been attributed to the effect of the Andes blocking many summer thunderstorms in central Chile, and thus reducing lightning and natural ignitions [1]. Lightning fires do occur in Chile, but typically further south; most current fires in central (mediterranean) Chile are of anthropogenic origin.
Two views of the Chilean matorral; left: La Campana National Park (photos: S. Gómez-González).
References
[1] Keeley JE, Bond WJ, Bradstock RA, Pausas JG, Rundel PW. 2012. Fire in Mediterranean ecosystems: ecology, evolution and management. Cambridge University Press. [the book]
[2] Pausas, J.G., Pratt, R.B., Keeley, J.E., Jacobsen, A.L., Ramirez, A.R., Vilagrosa, A., Paula, S., Kaneakua-Pia, I.N. & Davis, S.D. 2016. Towards understanding resprouting at the global scale. New Phytol. 209: 945-954. [doi | wiley | pdf | Notes S1-S4 | Table S1]
[3] Moreira B. & Pausas J.G. 2012. Tanned or burned: The role of fire in shaping physical seed dormancy. PLoS ONE 7: e51523. [doi | plos | pdf]
[4] Gómez-González S., Paula S., Cavieres L.A. & Pausas J.G. 2017. Postfire responses of the woody flora of Central Chile: insights from a germination experiment. PloS ONE 12: e0180661. [doi | plos | pdf] New!
A menudo me preguntan, ¿pero tu realmente crees que los incendios forestales es un fenómeno natural? Aquí intento responder a esta pregunta. Este texto apareció primero en 20minutos (Ciencia para llevar); aquí incluyo la primera versión que escribí, un poco más larga que la publicada; la principal diferencia está en el último párrafo, que por razones de espacio se recortó en la versión final en 20minutos.
Para que se produzca un incendio forestal se requieren tres condiciones: una ignición que inicie el fuego, un combustible continuo e inflamable, y unas condiciones de propagación adecuadas. ¿Se dan estas tres circunstancias en nuestros ecosistemas?
Empecemos por el final, las condiciones de propagación. Una de las principales características del clima mediterráneo es que la estación más seca coincide con la más cálida (el verano), cosa que no se da en la mayoría de los climas del mundo. En verano se genera un periodo relativamente largo con unas condiciones de elevadas temperaturas y baja o nula precipitación, que son ideales para que, si hay un incendio, se propague fácilmente. Además no es raro tener días de viento relativamente fuerte, seco y cálido (por ejemplo, los ponientes en la costa valenciana) que aún facilitan más los grandes incendios.
La siguiente condición es la existencia de un combustible continuo e inflamable. En la mayoría de los ecosistemas ibéricos, la vegetación es suficiente densa y continua que permite, si hay un incendio en verano, que este se pueda extender a grandes superficies. Esto es aplicable tanto a los bosques como a la gran diversidad de matorrales que encontramos en nuestro territorio. De manera que la vegetación mediterránea forma lo que a menudo se llama el combustible de los incendios forestales. No hay que olvidar que este ‘combustible’ está compuesto por una gran diversidad de seres vivos que tienen detrás una larga historia evolutiva; son parte de nuestra biodiversidad. Esta continuidad en la vegetación era especialmente evidente antes de que los humanos realizará esa gran fragmentación que se observa actualmente en nuestros paisajes, principalmente debida a la agricultura, pero también a las abundantes vías y zonas urbanas y periurbanas.
Pero con una vegetación inflamable y unos veranos secos no es suficiente para que haya incendios, se requiere una ignición inicial. Hoy en día, la mayoría de igniciones son generadas por personas, ya sea de manera voluntaria o accidental. Pero, ¿Hay igniciones naturales? La respuesta es sí. A menudo tenemos tormentas secas en verano, cuando las condiciones de propagación son óptimas, de manera que los rayos generados por estas tormentas pueden actuar como fuente de ignición e iniciar un incendio forestal. Tenemos muchos ejemplos de incendios generados por rayos (la mayoría sofocados rápidamente por los bomberos); y en los meses de verano, la AEMET detecta miles de rayos potencialmente capaces de generar igniciones (Figura 1).
Figura 1. Imagen del 31 de Julio de 2015 donde se muestra la localización de 12835 rayos que se registraron durante 6 horas en la Península Ibérica. Los diferentes colores indican diferentes horas, entre las 12 y las 18h. Fuente: Agencia Estatal de Meteorología.
Por lo tanto, las tres condiciones arriba mencionadas se dan de manera natural en nuestros ecosistemas, y por lo tanto podemos afirmar que sí hay incendios naturales. Pero, ¿cuantos?
Las estadísticas de incendios actuales nos dicen que los incendios generados por rayos son una minoría, comparado con la gran cantidad de incendios generados por los humanos. ¿Podría esta minoría de incendios por rayo representar la cantidad de los incendios esperables en condiciones naturales? La respuesta es no. Una gran cantidad de rayos cae en suelo sin vegetación combustible (zonas agrícolas y urbanas) y por lo tanto no producen los incendios que producirían en unas condiciones más naturales. Además, de los rayos que sí generan igniciones en el monte, la mayoría son apagados por los bomberos forestales cuando aun son solo conatos o incendios muy pequeños. Cabe recordar que nuestros bomberos apagan la inmensa mayoría de las igniciones y sólo un porcentaje muy pequeño se escapa y se transforma en los incendios que aparecen en la prensa. Y además, de los incendios que realmente progresan, la mayoría son más pequeños de lo que serían esperable en condiciones más naturales, porque los apagan los bomberos, o porque se detienen en zonas no inflamables (zonas agrícolas, urbanas, cortafuegos, etc.). Como consecuencia, las estadísticas de incendios por rayos, ya sea en número de incendios como en área afectada, no reflejan la importancia que tendrían los incendios en condiciones naturales, sino que los subestima. Algunos de los incendios que actualmente se dan por actividad humana, en realidad están sustituyendo a incendios naturales.
Es decir, en unos paisajes más naturales (con menos presión humana) que los actuales, sería de esperar que hubiese menos incendios que en la actualidad porque habría muchas menos igniciones (la actual elevada población genera muchas igniciones), pero en muchos casos, esos incendios podrías ser más grande. En cualquier caso, el balance probablemente sería de menos área afectada por incendios que actualmente; pero sí habría incendios frecuentes. A todo esto hay que añadirle que actualmente estamos cambiando el clima, de manera que la estación con incendios tienden a ser más larga, y las olas de calor más frecuentes, y todo ello incrementa la actividad de los incendios; pero ahora no entraremos en detalle en ello.
Además, hablar de condiciones o paisajes ‘más naturales’ es complicado por varias razones. ¿Cuanto hacia atrás en el tiempo son esas condiciones ‘más naturales’? Los humanos han poblado la Península ibérica desde hace muchos años, modificando las igniciones, cambiando la estructura de la vegetación, así como la cantidad y tipo de herbívoros. Esto ha llevado a continuos cambios en la cantidad y continuidad del combustible y en el régimen (frecuencia, intensidad, y estacionalidad) de incendios. Y si nos vamos a periodos antes de los humanos, tanto el clima como la cantidad y tamaño de los herbívoros (también consumidores de biomasa, como el fuego) era bastante diferente. Por lo tanto, lo importante no es si el régimen de incendios actual es ‘natural’ o no. Lo importante es si el régimen de incendios actual y futuro es ecológica y socialmente sostenible, considerando el cambio climático. Eliminar los incendios es imposible, antinatural y ecológicamente insostenible. Nuestra sociedad ha de aceptar la existencia de incendios, aprender a convivir con ellos, adaptar las estructuras y los comportamientos, y gestionar las zonas semi-urbanas y los paisajes rurales para que el régimen de incendios sea ecológica y socialmente sostenible. Esto incluye gestionar y planear la zonas semi-urbanas, la plantaciones forestales, y los parques naturales, pensando que lo normal es que un día les llegue un incendio.
Louis Trabaud (born in Montpellier, 2nd Feb. 1937) has recently passed away (Collioure, 16th April 2017). He was a research on plant ecology at Centre d’Ecologie Fonctionnelle et Evolutive (CEFE) of the CNRS, France. He was a pioneer of fire ecology in the Mediterranean Basin and set the basis of this topic for the region; he was especially influential to the fire ecologist of Spain (including me), and was awarded Professor Honoris Causa by the Univeristy of León (Spain) [1]. He was also award in France as Chevalier du Mérite Agricole (Order of Agricultural Merit). As a person, Louis Trabaud was very kind and always happy to help any student.
His research was especially focused on mediterranean shrublands around Montpellier (garrigue), i.e., shublands dominated by Quercus coccifera, Cistus species, Rosmarinus officinalis, Fumana species, etc… sometimes with an overstory of Pinus halepensis. He performed the first fire experiments in the Mediterranean region to study the regeneration of these shrublands, where he recurrently burned them in different seasons to demonstrate their high resilience. He also performed the first studies in the Mediterranean Basin on heat-stimulated germination and on flammability traits. He produced many papers, some in French (the most earlier ones) and others in English, and also wrote or edited some books. The full Trabaud’s publications list (books and scientific papers) is available here.
Louis Trabaud, together with his friend Roger Prodon, organized the International Workshop on Fire Ecology in Banyuls-sur-Mer (south of France) in the years 1992, 1997, 2001; these workshops were key in building the knowledge on fire ecology for the region; they were the meeting point for all mediterranean fire ecologist; we all met there for the first time and we all have very good memories from those meetings.
Participants of the 2001 Banyuls meeting organized by Louis Trabaud (in the middle, with glasses and a pale sweater) and Roger Prodon (second from the right).
El 12 de Mayo de 2017 impartí una charla titulada Incendios forestales y biodiversidad en el IVIA (Valencia), en la que expliqué las principales adaptaciones de las plantas mediterráneas a los incendios, y cómo estudiamos esas adaptaciones en el marco de la ecología del fuego. La conclusión es que el fuego explica una parte de la biodiversidad de nuestros ecosistemas. La charla tuvo cierto impacto en los medios (enlace). Aquí podéis ver la charla integra así como la discusión posterior:
In the cover of the March issue of the Journal of Ecology (105:2) there is a picture of Palicourea rigida (Rubiaceae), a plant growing in the Brazilian savannas (cerrado). It is an example of a plant that survives in a very flammable environment (grassy savanna) thanks to a set of traits conferring very low flammability, including a very low specific leave area and a thick corky bark. Grasses generates fast fires of low intensity (fast-flammable strategy), and in this environment, having low flammability is adaptive as it increases survival (non-flammable strategy). That is, different (contrasted) flammability strategies allows coexistence. For the definition of the different flammability strategies see [1].
(photo by J.G. Pausas)
[1] Pausas J.G., Keeley J.E., Schwilk D.W. 2017. Flammability as an ecological and evolutionary driver. Journal of Ecology 105: 289-297. [doi | wiley | pdf | blog | brief]
Some of the scientists attending the recent MEDECOS (International Conference on Mediterranean Ecosystems, Sevilla, February 2017 [1]) wrote a declaration on the recent wildfires that affected very large areas of Chile [2]. The declaration is composed of 10 statements (a decalogue) and is available here:
Professor Leopoldo (Léo) M. Coutinho (1934–2016; Fig. 1) from the University of Sao Paulo, Brazil, studied fire adaptations in Brazilian savannas (cerrado) during the 1970s, when very few researchers recognized fire as an evolutionary force. One of his important contribution on the cerrado ecology was on fire-stimulated flowering (Fig. 2), but he also studied serotiny, nutrient cycling, fire germination, water balance, among other topics [1,2]. However, his research is little known, partly because he was not part of the dominant Anglo-Saxon culture but also because he was ahead of his time, when fire and evolution were still distant concepts [1].
Figure. 1. Professor L. M. Coutinho in a Brazilian cerrado (photos by A. C. Coutinho)
Figure 2: Frequency distribution of the flowering intensity index (from 0 to 4) after fire (shaded; 90 days post-fire) and in control conditions (white) in 47 species (belonging to 20 families) of a cerrado ecosystem (prepared from data in Coutinho 1976). The 31 species with the highest post-fire flowering belong to 17 different families. From [1]
References
[1] Pausas J.G. 2017. Homage to L. M. Coutinho: fire adaptations in cerrado plants. Intern. J. Wildland Fire, [doi | pdf]
[2] Pivello, V.R. 2016. Professor Leopoldo Magno Coutinho: a visão de uma discípula. Biodiversidade Brasileira, 6(2): 4-5.
Esta entrada se ha realizado en colaboración con Susana Paula (ICAEV, Universidad Austral de Chile)
En las últimas semanas una gran cantidad de incendios han afectado cerca de 600 mil hectáreas en la zona central de Chile, con unas 1600 casas destruidas, 11 fallecidos y varios miles de afectados [1]. Esto ha generado una alarma social, y se han publicado numerosas opiniones, muchas de ellas sin datos o con poco rigor. Aquí intentamos analizar lo ocurrido, de manera muy breve, partiendo de una base científica y de los datos oficiales proporcionados por el Sistema de Información Digital para el Control de Operaciones (SIDCO) de la CONAF (Gobierno de Chile).
Los ecosistemas de Chile central parece que hayan tenido una actividad historia de incendios naturales (durante el Cuaternario) menor que los otros ecosistemas mediterráneos. Esto es debido a que la elevación los Andes durante el Mioeno bloqueó las tormentas estivales y los rayos asociados, y por lo tanto limitó los incendios forestales naturales [2]. Los incendios devienen importantes en la zona central de Chile con la llegada de los humanos. Por lo tanto, muchas especies nativas de los ecosistemas de Chile no están especialmente adaptadas a un régimen con incendios relativamente frecuentes e intensos, ni han adquirido características que les confiere una especial inflamabilidad. Esto contrasta con las especies que viven en otros ecosistemas mediterránenos del mundo donde se encuentras plantas que se ven favorecidas por los incendios, incluyendo plantas muy inflamables en las cuales su reproducción incrementa con el fuego. En cualquier caso, existen en Chile muchas plantas que rebrotan bien después de incendio. De manera que los incendios actuales en Chile podrían generar efectos negativos en la biodiversidad de los bosques nativos (p.e, mortalidad de no rebrotadoras, invasión de exóticas), aunque habrá que evaluar la regeneración con detalle. Sin embargo, cabe destacar, que gran parte del paisaje ardido no corresponden a sistemas naturales, sino a plantaciones forestales de especies exóticas (Figura 1).
Figura 1. Superficie afectada por incendios durante este verano (hasta la fecha), en las diferentes regiones de Chile (de izquierda a derecha: de norte a sur), separando la superficie de bosque nativo (en verde) y de plantaciones de eucaliptos y pino (en azul). La linea y puntos, representa el promedio afectado por incendios en cada región, durante el periodo 1977-2016. Elaboración propia a partir de datos oficiales de SIDCO-CONAF (Chile).
Para que se den grandes incendios, se requiere igniciones, baja humedad y elevado combustible. En general, en las zonas altamente pobladas, las igniciones antrópicas son muy frecuentes, y se generan frecuentes conatos o incendios pequeños que son fácilmente extinguidos. Sólo se generan grandes incendios de difícil extinción, si el clima y el combustible son apropiados para ello. La gran actividad de incendios de estos días en Chile responde, en gran manera, a esos dos factores. Las condiciones climatológicas de este periodo, han sido muy propicias para los incendios. Según la Dirección Meteorológica de Chile, este enero es el mes con la temperatura máxima, la mínima y la media más altas desde que se tienen datos [3,4]. Por lo tanto, las condiciones meteorológicas para los incendios eran óptimas, más que nunca.
A ello cabe añadir que Chile central tiene un paisaje forestal muy inflamable, formado por grandes plantaciones de pinos y eucaliptos utilizados para la producción de papel y madera (Figura 1, [5-7]). Ninguna de estas especies son nativas de Chile, sino de zonas donde el fuego es una perturbación natural, y donde ser una planta inflamable no es necesariamente un problema, incluso es beneficioso para la reproducción. En Chile, estas plantaciones proporcionan gran cantidad de combustible (elevada biomasa, formaciones densas), de elevada inflamabilidad (los pinos y los eucaliptos tienen resinas y compuestos volátiles que les hacen muy inflamables), y con unas estructura muy homogénea (plantaciones densas, monoespecíficas y coetáneas); todo ello facilita la propagación de los incendios. Además, estas plantaciones, en muchos casos llegan hasta el límite con poblaciones, poniendo en riego a la gente en caso de incendio.
Un análisis de las regiones con mayor superficie quemada (superior al valor promedio histórico, Fig. 1; es decir, las regiones de Valparaiso (V), Metropolitana (RM), O’Higgins (VI) y Maule (VII)), sugiere que, en general, los incendios seleccionan las plantaciones de manera positiva, y los bosques nativos y zonas agrícolas de manera negativa (Figura 2). Es decir, que las plantaciones se quemas más (desproporcionadamente), que el resto del paisaje, cosa que enfatiza la elevada inflamabilidad y combustibilidad de las plantaciones actuales de Chile (Figura 3). Un reciente estudio, realizado de manera independiente y utilizando datos de satélite, llega a similares conclusiones [8].
Figura 2. Análisis de las áreas afectadas por incendios según tipos de uso (Plantaciones, Bosque nativo, matorral+pastos, y zonas agrícolas), en relación a lo disponible en cada una de las 4 regiones que más han ardido (V, RM, VI, VII; ver Figura 1). Los datos positivos, significan que el fuego ha seleccionado de manera positiva ese tipo de uso (se ha quemado más de lo esperado por la superficie que ocupa); los datos negativos indican que el fuego tiende a evitar ese tipo de uso. Por ejemplo, en la Región Metropolitana (RM, en verde) se ha quemado más o menos lo que se espera según las proporciones en paisaje de plantaciones y nativo (valores cercanos a 0). En cambio, el las demás regiones, hay una fuerte tendencia a que las plantaciones se quemen más de lo esperado según su abundancia en el paisaje (valores positivos), mientras que los bosques nativos, el matorral, o las zonas agrícolas se queman de manera similar o menos de lo esperado según su abundancia (valores negativos). La región VII (Maule) es la más extrema en selección positiva de plantaciones y negativa del resto de usos, y es la región donde más superficie ha sido afectada (Fig. 1). Elaboración propia a partir de datos oficiales de SIDCO-CONAF (Chile).
Las grandes plantaciones forestales de Chile pueden haber sido económicamente rentables, y haber contribuido a la economía del país, pero todo indica que son social y ecológicamente poco apropiadas (véase vídeo ilustrativo, abajo). Da la impresión que la política forestal de Chile está pensada en una época con una escala de valores y un clima del pasado. Dada la importancia de la industria forestal en Chile, la política forestal requiere actualizarse urgentemente, considerando el cambio climático, los incendios, y la calidad de vida de la población local.
Figura 3. Impacto de un incendio cerca de Penco (Región del Bío-Bío), donde alternan plantaciones y bosque nativo. En primer plano, un peumo (Cryptocarya alba, especie del bosque nativo) parcialmente afectado. Foto: Fernando Saenger.
[5] Peña-Fernánde F. & Valenzuela-Palma, L. 2008. Incremento de los incendios forestales en bosques naturales y plantaciones forestales en Chile. En: González-Cabán, Armando, Coord. 2008. Proceedings of the second international symposium on fire economics, planning, and policy: a global view. Gen. Tech. Rep. PSW-GTR-208, Albany, CA [PDF en: español | inglés]
The XIV International Conference on Mediterranean Ecosystems (MEDECOS), has been successfully held on Sevilla, Spain, 31st Jan – 4th Feb, 2017, together with the conference of the Spanish Society of Terrestrial Ecology (AEET). For details, see the web page of the meeting and the post conference comments in the J. Ecol. blog. My contributions to this MEDECOS include two talks on fire and biotic interactions, two on resprouting, and one on fire hazard:
Pausas J.G – Fire and biotic interactions: the benefits of the disruption
García Y., Castellanos M.C., Pausas J.G. – Fires do not jeopardize reproduction of Chamaerops humilis despite disrupting its pollination
Tavsanoglu C. & Pausas J.G. – Resprouting ability encapsulates the most functional variability in the Mediterranean Basin flora
Paula, S. & Pausas J.G – Worldwide geographic and phylogenetic distribution of lignotubers
Cáceres M. de, Casals P., Álvarez D., Pausas J.G., Vayreda J., Beltrán M. – The role of understory fuel characteristics in the fire hazard of Mediterranean forests
The last day I attended to the field trip to Los Alcornocales Natural Park (a mosaic of cork oak forests and heathlands), where I enjoyed a long conversation on alternative vegetation states in non-tropical ecosystems (e.g., PDF) with William Bond (photo below).
Thank you very much to the organizers of MEDECOS, especially to Juan Arroyo (Universidad de Sevilla) and Montse Vilà (Doñana-CSIC) for this nice and smooth conference.
William Bond (left) and myself (right) in a Cork oak forest (photo: F. Ojeda)
Recently, the annual plant Chaenorhinum rubrifolium (Plantaginaceae) has been recorded for the first time in Turkey, and it was found in a recently burned area only (8 months after a fire); no individuals were found outside the burn perimeter [1, 2]. To understand the mechanisms of germination, the authors performed a range of germination tests in which seeds were submitted to different fire-related treatments like heat shocks, smoke treatments, and the application of some chemical compounds present in the smoke (NO3, karrikinolide) or analogue to those in the smoke (mandelonitrile, a cyanohydrin type compound). The results are pretty clear (Figure below): the chemical compound of smoke break their seed dormancy and stimulates the germination [1].
Overall C. rubrifolium is a clear example of a postfire seeder species, but given their strong dependency of fire, at least in Turkey, we can call it a pyroendemic plant, that is, a plant in which seedling germination and successful recruitment is restricted to immediate postfire environments [3]. Pyroendemic annuals are common in mediterranean-climate regions [4], but they have been little studied in the Mediterranean basin [5,6].
It would be interesting to study the germination of this species from other localities (e.g., it is not rare in Spain); previous research comparing plant regeneration traits from shared species between the East and the West of the Mediterranean basin show that intraspecific variability is higher at the local scale than between distant regions [7]. At least in the West, there are some varieties of C. rubrifolium that are unlikely to be pyroendemics as the ones occurring in dune systems.
Figure: Summary of the germination response of Chaenorhinum rubrifolium to fire-related treatments: Control (untreated seeds), Heat (a range of heat shocks were tested), Smoke (mean value from a range of smoke concentrations), and different chemical compounds related to smoke: NO3 (nitrate), MAN (mandelonitrile), and KAR1 (karrikinolide). Seeds were 4 month-old; the germination for Smoke and KAR1 treatments were nearly 100% when using 2 year-old seeds (after-ripening). For details see [1].
References
[1] Tavşanoğlu Ç, Ergan G, Çatav ŞS, Zare G, Küçükakyüz K, Özüdoğru B. 2017. Multiple fire-related cues stimulate germination in Chaenorhinum rubrifolium (Plantaginaceae), a rare annual in the Mediterranean Basin. Seed Sci. Res. [doi]
[2] Zare G., Özüdoğru B., Ergan G., Tavşanoğlu Ç. (submitted) Taxonomic notes on the genus Chaenorhinum (Plantaginaceae) in Turkey.
[3] Keeley JE, Pausas JG. 2017. Evolution of ‘smoke’ induced seed germination in pyroendemic plants. South African J. Bot. [doi | pdf]
[4] Keeley JE, Bond WJ, Bradstock RA, Pausas JG, Rundel PW. 2012. Fire in Mediterranean ecosystems: ecology, evolution and management. Cambridge University Press. [the book]
[5] Moreira B, Pausas JG. 2017. Shedding light through the smoke on the germination of Mediterranean Basin flora. South African J. Bot. [doi | pdf] | post]
[6] Tormo J, Moreira B, Pausas JG. 2014. Field evidence of smoke-stimulated seedling emergence and establishment in Mediterranean Basin flora. J. Veget. Sci. 25: 771-777. [doi | wiley | pdf | post]
[7] Moreira B, Tavşanoglu Ç, Pausas JG. 2012. Local versus regional intraspecific variability in regeneration traits. Oecologia 168: 671-677. [doi | pdf | post]
A recent paper suggested that fire-vegetation feedback processes may be unnecessary to explain tree cover patterns in tropical ecosystems and that climate-fire determinism is an alternative possibility [1]. This conclusion was based on the fact that it is possible to reproduce observed broad scale patterns in tropical regions (e.g., a trimodal frequency distribution of tree cover) using a simple model that does not explicitly incorporate fire-vegetation feedback processes. We argue that this reasoning is misleading because these two mechanisms (feedbacks vs fire-climate control) operate at different spatial and temporal scales [2]. It is not possible to evaluate the role of a process acting at fine scales (e.g., fire-vegetation feedbacks) using a model designed for reproducing regional-scale pattern; i.e., there is a mismatch between the scale of the question and the scale of the approach for addressing the question. While the distribution of forest and savannas are partially determined by climate, the most parsimonious explanation for their environmental overlaps (as alternative states) is the existence of feedback processes [3,4], as has been shown in many ecosystems, not only tropical ones [4]. Climate is unlikely to be an alternative to feedback processes; rather, climate and fire-vegetation feedbacks are complementary processes acting at different spatial and temporal scales [2].
Figure: Fire activity (based on remotely sensed data) for savannas and forests located in the range of environmental conditions where both occurs, for Africa and South America (Afrotropics and Neotropics, respectively). From [2,3].
References
[1] Good, P., Harper, A., Meesters, A., Robertson, E. & Betts, R. (2016) Are strong fire–vegetation feedbacks needed to explain the spatial distribution of tropical tree cover? Global Ecol. and Biogeogr. 25, 16-25.
[2] Pausas J.G. & Dantas V.L. 2017. Scale matters: Fire-vegetation feedbacks are needed to explain tropical tree cover at the local sacle. Global Ecol. and Biogeogr. [doi | wiley | pdf]
[3] Dantas V.L., Hirota M., Oliveira R.S., Pausas J.G. 2016. Disturbance maintains alternative biome states. Ecology Letters 19: 12-19. [doi | wiley | pdf | suppl | blog]
[4] Pausas, J.G. 2015. Alternative fire-driven vegetation states. J. Veget. Sci. 26:4-6. [doi | pdf | suppl.] | blog]
La huella del fuego es un documental sobre incendios forestales en España realizado por el equipo del programa Crónica, de La 2 de TVE, y que se emitió el 28 Noviembre 2016. En él participaron algunas de las personas que recientemente realizaron el decálogo sobre incendios forestales (decálogo | blog). Podéis ver un resumen del documental, o el programa entero en www.rtve.es/alacarta.
Nota: el documental no está relacionado con el libro que tiene el mismo título (de L. Otero 2006), que describe la historia de los bosques de Tierra del Fuego.
We live on a flammable planet [1,2] yet there is little consensus on the origin and evolution of flammability in our flora [3]. Part of the problem lies in the concept of flammability. In a recent paper [4] we suggest that flammability should not be viewed as a single quantitative trait or metric, rather we propose that flammability has three major dimensions that are not necessarily correlated: ignitability, heat release, and fire spread rate. These dimensions define three flammability strategies observed in fire-prone ecosystems: the non-flammable, the fast-flammable and the hot-flammable strategy (with low ignitability, high flame spread rate and high heat release, respectively). The non-flammable strategy refers to plants that do not burn (or rarely) in natural conditions despite living in fire-prone ecosystems: this is because they have biomass with very low ignitability (low flammability at the organ scale) or because their plant structure does not allow the ignition of the biomass (low flammability at the individual scale). The hot- and the fast-flammable strategies refer to flammable plants with contrasted heat release and spread rate. Flammability strategies increase the survival or reproduction under recurrent fires, and thus, plants in fire-prone ecosystems benefit from acquiring one of them; they represent different (alternative) ways to live under recurrent fires. This novel framework on different flammability strategies helps us to understand variability in flammability across scales [4].
Figure: Conceptual model describing the three plant flammability strategies in fire-prone ecosystems. While many plants fall at intermediate levels of these axes (i.e., the null model for flammability), plants in fire-prone ecosystems benefit from being at the extremes, forming the three flammability strategies considered here. From [4]
[3] Pausas J.G. & Moreira B. 2012. Flammability as a biological concept. New Phytol. 194: 610-613. [doi | wiley | pdf]
[4] Pausas J.G., Keeley J.E., Schwilk D.W. 2017. Flammability as an ecological and evolutionary driver. J. Ecol. 105: 289-297 [doi | wiley | pdf | brief for managers]
The first version of this paper was my talk at the University of Campinas, Unicamp: link
UPDATE: paper featured on the cover of J Ecol 105(2): cover | blog
There is a tendency to think that fires will increase in the near future due to global warming. This is because many fire risk prediction are based on climate only. However fire regime changes not only depend on climate [1]; there are other factors, like land-use changes, CO2, plant invasion, fragmentation, etc. that are also important drivers of change in fire activity [1]. Even plant drought stress (and flammability) not only depends on climate [2,3].
A recent simulation study [4] suggests that global burned area is certainly predicted to increase in the following decades when simulations are based on climate only (blue line in the figure below). However, adding the effect increased CO2 reduces the predicted burned area to no increase (green line below). Furthermore, when adding increased population density and urbanization (black and red lines), the model predicts much more area burnt in the last century (black lines 1900-2000) and a reduction of future burned area (red lines). The predicted reduction of fire during 1900-2000 is consistent with global charcoal records [5] and can be explained by increasing agriculture, land use and fragmentation. Overall, this study suggests that global area burned is unlikely to increase in the following decades.
Note that 1) this is a model, so take it with caution! 2) This model is at the global scale, but changes in different directions are expected in different regions, and this can have biodiversity consequences (even if the global balance is steady); for instance, in the Mediterranean Basin, fire are likely to keep increasing as land abandonment and fuels are increasing [6]. And 3) there is a high uncertainty in some fire drivers. For instance, temperature is likely to keep increasing, however, rainfall and wind changes are very uncertain, and landuse and emissions are subject to uncertain changes in environmental policies in different countries. In any case, this study gives us an idea of the possible sensitivity of different parameters.
Figure: Simulation of global area burned for 1900 to 2100 under different scenarios: a) climate only (blue line); b) climate + CO2 (green); c) climate + CO2 + population & urbanization (black lines; red area for the future predictions). From [4].
[2] De Cáceres M, et al. 2015. Coupling a water balance model with forest inventory data to predict drought stress: the role of forest structural changes vs. climate changes. Agr. For. Meteorol. 213: 77–90. [doi | pdf | suppl. | blog]
[3] Luo, Y. & H. Y. H. Chen. 2015. Climate change-associated tree mortality increases without decreasing water availability. Ecol, Let. 18:1207-1215.
[4] Knorr W, Arneth A, & Jiang L, 2016. Demographic controls of future global fire risk. Nature Clim. Change 6:781-785.
[5] Marlon JR, et al. (2008). Climate and human influences on global biomass burning over the past two millennia. Nature Geosci, 1, 697-702.
[6] Pausas J.G. & Fernández-Muñoz S. 2012. Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Climatic Change 110: 215-226. [doi | pdf | blog]
There are plants in which fire can breaks seed dormancy and stimulate germination. In some species, it is the heat of the fire that breaks seed dormancy and triggers germination (heat-stimulated germination, [1, 2]). In others, germination is stimulated by chemicals produced during the combustion of the organic matter (e.g., chemicals found in the smoke and charred wood) [1, 3]; we call this process, smoke-stimulated germination [5]. That is, in fire-prone ecosystems many plants have evolved seeds with sensitivity to heat and/or to chemicals produced by fire [1, 2, 3].
There are many species from a wide phylogenetic range with smoke-stimulated germination [5]; they appear in different regions worldwide and are stimulated by different combustion-related products, both organic and inorganic [4, 5]. All this suggest that smoke-stimulated germination is a trait that has appeared multiple times during the evolution, and thus is another example of convergent evolution [5].
In the Mediterranean Basin we currently know about 67 species (from 19 families) showing a significant increase in germination in response to smoke [6]. Families with many smoke-stimulated species in this region are Lamiaceae, Ericaceae and Asteraceae. However, there is still a lot of research to be done on smoke-stimulated germination in Mediterranean Basin flora, as many species have not yet been tested; in fact, very few annuals has been tested [6] despite there is evidence from field studies (3) and from other Mediterranean regions suggesting that smoke-stimulated germination is important in annuals.
But remember, plants are not the only organisms that have evolved in response to chemicals present in the smoke, humans too! [7].
Figure: Germination (proportion of seeds) in control conditions (light yellow) and after a smoke treatment (blue) for four Mediterranean species in which germination is strongly dependent on smoke: Coris monspeliensis (Primulaceae), Erica umbellata (Ericaceae), Onopordum caricum (Asteraceae) and Stachys cretica (Lamiaceae) See [6].
References
[1] Moreira B., Tormo J., Estrelles E., Pausas J.G. 2010. Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora. Ann. Bot. 105: 627-635. [pdf | doi | blog]
[2] Moreira B and Pausas JG. 2012. Tanned or Burned: the role of fire in shaping physical seed dormancy. PLoS ONE 7:e51523. [doi | plos | pdf]
[3] Tormo, J., B. Moreira, and J. G. Pausas. 2014. Field evidence of smoke-stimulated seedling emergence and establishment in Mediterranean Basin flora. J. Veget. Sci. 25: 771-777. [doi | wiley | pdf | blog ]
[5] Keeley J.E. & Pausas J.G. 2018. Evolution of ‘smoke’ induced seed germination in pyroendemic plants. South African J. Bot. 115: 251-255 [doi | pdf] <- New
[6] Moreira B. & Pausas J.G. 2018. Shedding light through the smoke on the germination of Mediterranean Basin flora. South African J. Bot. 115: 244-250 [doi | pdf] <- New
Después de una serie de despropósitos sobre el posible uso de cipreses ignífugos [1-4], por fin parece que se encaucen las cosas: Los cipreses que estaban destinados para hacer de barrera cortafuegos en el monte, parece que finalmente se utilizarán en jardinería [5], y esperemos que para jardines urbanos, lejos del monte. En paisajes con incendios recurrentes, plantar cipreses en zonas semi-urbanas (en la interfaz urbano-forestal), no es recomendable, ya que si llega el fuego, o simplemente pavesas, pueden prender de manera intensa y actuar como antorchas. Por ello, los bomberos temen las casas rodeadas de cipreses, y de hecho, está prohibido plantarlos en jardines de diversas zonas de EEUU. Hay evidencias de que los cipreses pueden ejercer de captadores de pavesas (foto). La idea de utilizarlos como cortafuegos estaba fuera de toda lógica [4].
Foto: Valla de cipreses que prendió durante el incendio de La Granadella (4/Sep/2016, La Marina, Alicante). Nótese que el incendio no llegó directamente a la valla (los pinos y campos de cultivo de los alrededores no se vieron afectados); es probable que el fuego llegase con una pavesa, como pasó con los distintos focos de este mismo incendio [6].
[5] La investigación española sobre cipreses cortafuegos acabará en plantas de jardín, eldiario.es
[6] El SEPRONA concluye que todos los focos del incendio de la Granadella fueron provocados por las pavesas (xabiaaldia.com); Una colilla mal apagada provocó el incendio de Xàbia (eldiario.es); El Seprona cree que una colilla originó el incendio y el viento causó los tres focos (levante-emv.com).
¿Será este el último post sobre el tema? ¿Se habrá ganado una pequeña batalla?
(podéis dejar vuestra opinión en los comentarios)
There are many plants that benefit from fire. Typical examples are those that despite they may be killed by fire, the germination of their seeds is stimulated by the fire (either by the heat or by the smoke; [1,2]), and thus they recruit very well (high offspring abundance) and often increase there population size postfire. Species with fire-stimulated flowering [3,4] also benefit from fire. In a recent paper [5] we propose that there may be another mechanisms by which fire may benefit plants: fire may remove seed predators, and thus create a window of opportunity for reproduction under a lower predation pressure (predator release hypothesis). This is specially applicable to specialist plant-insect interactions. We documented two cases: in Ulex parviflorus, a plant species with fire-stimulated germination [1,2], fire eliminated there specialist seed predator weevil (Exapion fasciolatum, Apioninae, Brentidae) and thus increased the available seed number for germination. Similarly, in Asphodelus ramosus, a fire-stimulated flowering species [3], fire reduced the specialist herbivore and seed predator (Horistus orientalis, Miridae, Hemiptera) and increased their fruit production. Thus, fire, by disrupting the antagonistic interactions, benefit plants; the temporal window of this predator release is likely to depend on fire size. For more information see reference [5].
Figure: Proportion of predated fruits of Ulex parviflorus in unburned sites (grey boxes) and at the edge and center of a recently burned area (white boxes), 2 and 3 years postfire. Data from two large wildfires in Valencia (2012) [5]; Edge and Center of the burned area refer to <1 km and >1.5 km from the fire perimeter, respectively. Photo of the seed predator (Exapion) from BioLib.cz.
References
[1] Moreira B., Tormo J., Estrelles E., Pausas J.G. 2010. Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora. Annals of Botany 105: 627-635. [pdf | doi | blog]
[2] Moreira B and Pausas JG. 2012. Tanned or Burned: the role of fire in shaping physical seed dormancy. PLoS ONE 7:e51523. [doi | plos | pdf]
The overview effect is the feeling and awareness reported by some astronauts when viewing the entire Earth during space-flight. Fire ecologists have our own overview effect! When remote sensed fire information was available for the first time at the global scale, it provided a magnificent and unprecedented view of the importance of fires on the Earth, and fires become a global issue. This remotely sensed information was a very valuable data because, for the first time, it was possible study some fire ecology processes at the global scale (for example [1]). Here is an animation for a 10 years period (2000-2010). It shows that on our planet, fires are widespread and something is always burning; we live in a flammable planet.
MODIS Rapid Response System Global Fire Maps, NASA. Each colored dot indicates a location where MODIS detected at least one fire during a 10-day period.
In this blog we have discussed that some plants have evolved seeds with sensitivity to chemicals produced by fire in such a way that these chemicals stimulate the germination of the plants after a fire; we call this process smoke-stimulated germination [1-3]. Well, plants are not the only organisms that have evolved in response to chemicals present in the smoke, humans too! A recent paper show that modern humans are the only primates (including early hominids as Nearthentals and Denisovans) that carry a mutation increasing tolerance to smoke chemicals produced by fires [4]. This mutation could have given an evolutionary advantage to modern humans in relation to other hominids as allowed them to use fire for many important activities (e.g., cooking, hunting, defense, heating, agriculture). This high exposure to smoke would have also increased the susceptibility to pulmonary infections, and even the evolution of some of them (tuberculosis [5]). The tolerance to smoke also allowed modern humans to have some tolerance to pollution and to smoke cigarettes! That is, the ability to smoke could be a side effect (an exaptation, if you’d like) of been adapted to use fire, and in fact, it currently acts as a secondary sexual character!
Smoking as a secondary sexual character (Woody Allen in Manhattan, 1979).
References
[1] Moreira B., Tormo J., Estrelles E., Pausas J.G. 2010. Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora. Annals of Botany 105: 627-635. [doi | pdf | post]
[2] Tormo, J., B. Moreira, and J. G. Pausas. 2014. Field evidence of smoke-stimulated seedling emergence and establishment in Mediterranean Basin flora. Journal of Vegetation Science 25: 771-777. [doi | wiley | pdf | post]
[5] Chisholm, R.H., Trauer, J.M., Curnoe, D. & Tanaka, M.M. (2016). Controlled fire use in early humans might have triggered the evolutionary emergence of tuberculosis. Proc. Natl. Acad. Sci. USA, 113, 9051-9056.
Recently I came across this figure published in 1962 by Volkmar Vareschi [1] which nicely synthesize variations in temperatures in the flame and in the soil, as well as flame height and flame spread (time and distance) in a simple hand-drawing. It is not easy to see a figure on fire behaviour as simple and illustrative as this one. It refers to a burn of a Trachypogon savanna in Los Llanos, Venezuela. Vareschi (1906-1991) was born in Austria and moved to Venezuela in 1950; he is considered a pioneer in tropical plant ecology; one of his papers was about savanna fires [1].
Figure 2 from [1]
References
[1] Vareschi, V. (1962) La quema como factor ecológico en los Llanos. Boletin de la Sociedad Venezolana de Ciencias Naturales 23, 9-31.
El 27 de Julio de 2015 un incendio forestal afectó unas 1200 ha en Òdena (Anoia, Catalunya central), una zona dominada principalmente por pino carrasco (Pinus halepensis). Pocos días después ya se empezaba a ver un inicio de la regeneración del ecosistema [1, 2]. En una visita reciente (Abril 2016, 9 meses posincendio), vemos que en gran parte de la zona se han cortado y extraído los árboles quemados (y algunos no quemados). Antiguamente, cuando aun no se daba casi ningún valor a los ecosistemas naturales, y sí a la madera, se sacaban los árboles quemados para obtener algún beneficio económico; y algunas veces por motivos “estéticos”. Hoy en día, no parece una acción muy apropiada [3], a no ser que haya una razón de peso, cosa que desconozco en el caso de este incendio.
Los árboles quemados benefician a la regeneración porque retienen un poco el suelo, disminuyen el impacto de las gotas de lluvia en el suelo, mantienen cierta humedad, captan agua de la niebla, sirven de posadero para aves que traen semillas (que contribuyen a la regeneración), y son hábitat para fauna diversa [4]. Cortar los árboles requiere entrar con maquinaría en la zona quemada (con suelos muy sensibles), generar caminos y arrastrar troncos. Esto conlleva la eliminación de todos los beneficios mencionados, ademas de la disminución de parte del suelo y mantillo, la mortalidad de las primeras germinaciones posincendio (por ejemplo del pino), la formación de surcos que pueden ser puntos de inicio de erosión (cárcavas), y disminución de la regeneración natural en general. En general, entrar en una zona recién quemada, y degradar el ecosistema disminuyendo la regeneración y aumentando la erosión, está poco justificado [3]; en algunos casos, estas intervenciones pueden ser más perjudiciales que el propio incendio.
Fotos: a) Pinar con rebrotes de madroño 4 meses después del incendio, antes de cortar los árboles; se aprecia un cierto ambiente forestal. b) surcos del arrastre de troncos durante la extracción de la madera quemada. c) Ambiente 9 meses después del incendio, una vez se han cortado los árboles. d) Germinación de pino 4 meses después del incendio; germinaciones susceptibles a ser eliminadas si se entra con maquinaria o se arrastran troncos. e) pinos vivos (no quemados) cortados y apilados (9 meses posincendio). f) Enebro rojo (Juniperus oxycedrus) que rebrota tras quema y corta (9 meses posincendio). Incendio de Odena, Abril 2016 (fotos: JG Pausas).
[4] Pausas, J.G., Ribeiro, E. & Vallejo, R. 2004. Post-fire regeneration variability of Pinus halepensis in the eastern Iberian Peninsula. Forest Ecology and Management 203: 251-259. [doi | pdf]
