Posts Tagged ‘germination’

Seed dormancy, bet-hedging, and best-bet

September 2nd, 2022 No comments

Seed dormancy is a key plant characteristic that occurs among many species worldwide. One mechanism that select for seed dormancy is the bet-hedging strategy. In unpredictable environment (i.e., with high interannual variability) there is a benefit in spreading the germination over a number of years to reduce year-to-year variation in fitness but taking advantage of exceptionally good years for establishment. In those environments, seed dormancy is adaptive; each year there is a small fraction of the seed crop that germinates and the other seeds remain dormant in the soil. Because the environmental conditions of most years are poor, successful establishment only occurs in good (wet) years. Thus bet-hedging selects for seed dormancy and it is a mechanism for living in highly unpredictable environments such as arid ecosystems [1]

There is another environmental setting that also selects for seed dormancy: seasonal (predictable) climate with a dry season during which the vegetation is highly flammable and thus wildfires are frequent (e.g., mediterranean, savanna, warm temperate, and dry boreal ecosystems). In those ecosystems, seed dormancy is adaptive and fire provide both a mechanism for dormancy release (proximate cause) and conditions (postfire) optimal for germination and establishment (low competition, high resource availability, low predation, low pathogen load) that increase fitness and allow maintenance of the population (ultimate cause) [1,2]. Dormant seeds survive the passage of fire and the heat or the chemicals from the combustion (collectively called ‘smoke’ [2,3]) are the stimulus for the seed to recognize a fire gap to germinate. That is, postfire recruitment occurs in a single pulse after fire. Here selection does not favor spreading the risk of recruitment failure over many years (as in the bet-hedging strategy) but, instead, maximizes germination in a single year when conditions are optimal, after fire. We call this strategy the best-bet strategy [1] or environmental matching [2]. This strategy selects for seed dormancy to accumulates seeds in the soil seedbank but also selects for serotiny to accumulate seeds in the canopy seedbank [4]; in both cases, species recruit mostly after fire and not during the interfire period.

There is a further driver that selects for seed dormancy but it does not imply the formation of seed banks (in contrast with bet-hedging and best-bet). Many seeds have acquired seed dormancy to facilitate long-distance dispersal. The clearest example is dispersal by vertebrate frugivores (endozoochory). Frugivores consume the fruit pulp and defaecate or regurgitate the seeds far from the mother plant. This means that seeds need to resist passage through the gut and remain intact until arriving at a new microsite for germination. Thus, seeds of fleshy fruited species typically are dormant, and scarification through the gut releases their dormancy. While bet-hedging spreads germination of seeds over time, this strategy spread the seeds across the space and thus it could be viewed as a spatial bet-hedging strategy.

Figure: Schematic representation of the dynamics of seed recruitment for plants lacking seed dormancy (nondormant; top panel), and for plants with dormant seeds following the bet-hedging strategy (middle panel) and the best-bet strategy (bottom panel). The figure shows the moment of flowering (red asterisk; spring), the germination (black bars; autumn), the seed bank in autumn (empty bars), the recruitment 2 months later (green bars) and the fire (flame; summer). As an example, the seasons are considered as in the Northern Hemisphere, and vertical dotted lines are the end of the year. From [1]
Table: Main characteristics of the evolutionary strategies that select for seed dormancy and seed banks (bet-hedging, best-bet), together with the nondormant strategy.


[1] Pausas JG, Lamont BB, Keeley JE., Bond WJ. 2022. Bet-hedging and best-bet strategies shape seed dormancy. New Phytol. [ doi | wiley | pdf]

[2] Pausas JG. & Lamont BB. 2022. Fire-released seed dormancy – a global synthesis. Biol. Rev. 97: 1612-1639. [doi | pdf | supp. mat. | data (figshare)

[3] Keeley JE & Pausas JG. 2018. Evolution of ‘smoke’ induced seed germination in pyroendemic plants. South African J. Bot. 115: 251-255. [doi | pdf]  

[4] Lamont BB, Pausas JG, He T, Witkowski, ETF, Hanley ME. 2020. Fire as a selective agent for both serotiny and nonserotiny over space and time. Critical Rev. Pl. Sci. 39:140-172. [doi | pdf | suppl.]

Fire-released seed dormancy

April 8th, 2022 No comments

Many plants concentrate their seedling recruitment after the passage of a fire. This is because postfire conditions are especially optimal for germination and establishment of many species as fires create extensive vegetation gaps that have high resource availability, minimal competition, and low pathogen load. Thus we propose that fireprone ecosystems create ideal conditions for the selection of seed dormancy as fire provides a mechanism for dormancy release and optimal conditions for germination [1]. We compiled data from a wide range of fire-related germination experiments for species in different ecosystems across the globe and identified four dormancy syndromes: heat-released (physical) dormancy, smoke-released (physiological) dormancy, non-fire-released dormancy, and non-dormancy. In fireprone ecosystems, fire, in the form of heat and/or chemical by-products (collectively termed ‘smoke’), are the predominant stimuli for dormancy release and subsequent germination, with climate (cold or warm stratification) and light sometimes playing important secondary roles. Fire (heat or smoke)-released dormancy is best expressed where woody vegetation is dense and fires are intense, i.e. in crown-fire ecosystems (e.g., mediterranean-type ecosystems). In grassy fireprone ecosystems (e.g. savannas), where fires are less intense but more frequent, seed dormancy is less common and dormancy release is often not directly related to fire (non-fire-released dormancy). Fire-released dormancy is rare to absent in arid ecosystems and rainforests. Heat-released dormancy can be traced back to fireprone floras in the ‘fiery’ mid-Cretaceous, followed by smoke-released dormancy, with loss of fire-related dormancy among recent events associated with the advent of open savannas and non-fireprone habitats. Anthropogenic influences are now modifying dormancy-release mechanisms, usually decreasing the role of fire. We conclude that contrasting fire regimes are a key driver of the evolution and maintenance of diverse seed dormancy types in many of the world’s natural ecosystems.

Fig. 1. Percentage germination of 68 populations or species subjected to simulated fire- (y axis) and summer-type (warm stratification) temperature (x-axis) (C., Cistus; F., Fumana; U., Ulex; A., Acacia; M., Mimosa). Points above the dotted line (1:1) have higher germination levels after fire heat than after summer heat. Note that all points at or below the line are for species in savannas [S], while the others are from mediterranean shrublands and other crown-fire ecosystems. That is, in crown-fire ecosystems, fire is the most likely selective agent for dormancy. From [1].
Fig. 2. Dated phylogeny for major clades in the New and Old World Cistaceae together with closely related ancestral clades. Pie charts at the tips show the fraction of species that occur in crown-fire ecosystems (red), surface-fire ecosystems (orange), those with physical dormancy – hard seeds (green), and those with heat-released dormancy (blue). Blank sectors mean that the trait is absent. Letters at the tips refer to growth forms in the clade (T, tree; S, shrub or subshrub; H, herb/annual). Black dots indicate the crown age of diversification of the corresponding clade. From [1].


[1] Pausas J.G. & Lamont B.B. 2022. Fire-released seed dormancy – a global synthesis. Biological Reviews  [doi | pdf | supp. mat. | data (figshare)]

Foc i germinació

August 30th, 2018 3 comments

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.

Related posts: fire drive trait divergence in smoke-induced germination | smoke-stimulated recruitment | seed dormancy as a fire adaptation | smoke-stimulated germination | heat and smoke as germination cues


A fire ecology lesson from the Florida scrub

June 11th, 2018 No comments

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).



[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]


Postfire germination in Chile

July 22nd, 2017 No comments

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).



[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!

More on: fire and Chile | fire and germination |

A new pyroendemic annual plant

January 21st, 2017 No comments

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].


[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]


Fire drives trait divergence: smoke-induced germination

April 3rd, 2014 No comments

There is an increasing evidence that recurrent fires are driving within species phenotypic variability, and that different fire regimes can generate trait divergence among populations [1]. For instance, populations of the annual species Helenium aromaticum (Asteraceae) growing under different fire histories in Chile have different seed traits in such a way that the anthropogenic increase in fire frequency selected for an increasing in seed pubescence [2]. In the Mediterranean Basin there is also evidence of phenotypic divergence among populations under different fire regimes: Ulex parviflorus (Fabaceae) plants living under high fire frequency are more flammable than those growing in sites that have not suffered fires [3-5]; Pinus halepensis and P. pinaster living under high crown-fire frequency have higher serotiny that those living in areas that rarely burn in crown fires [6]

A recent paper add further examples of this fire-driven trait divergence: Vandvik et al. show that smoke-induced germination is observed in populations of Calluna vulgaris (Ericaceae) from traditionally burnt coastal heathlands of Norway but it is lacking in populations from other habitats with infrequent fires [7]. The results are also consistent with the suggestion that smoke-induced germination is a fire adaptation [8-9].


Figure: Probability of germination of Calluna vulgaris in relation to time (days) since sowing for smoke-treated (pink) and control (grey) seeds, in coastal and inland heathlands of Norway. From Vandvik et al. 2014 [7].


[1] Pausas, J. G. and D. W. Schwilk. 2012. Fire and plant evolution. New Phytologist 193 (2). [doi | pdf | blog]

[2] Gómez-González S, Torres-Díaz C, Bustos-Schindler C, Gianoli E, 2011. Anthropogenic fire drives the evolution of seed traits. PNAS 108: 18743-18747. [doi blog]

[3] Pausas J.G., Alessio G., Moreira B. & Corcobado G. 2012. Fires enhance flammability in Ulex parviflorusNew Phytologist 193: 18-23. [doi | pdf | blog]

[4] Pausas J.G. & Moreira B. 2012. Flammability as a biological concept. New Phytologist 194: 610-613. [doi | wiley | pdf]

[5] Moreira B., Castellanos M.C., Pausas J.G. 2014. Genetic component of flammability variation in a Mediterranean shrub. Molecular Ecology 23: 1213-1223. [doi | pdf | suppl. | data:dryad | blog]

[6] Hernández-Serrano A., Verdú M., González-Martínez S.C., Pausas J.G. 2013. Fire structures pine serotiny at different scales. American Journal of Botany 100 (12): 2349-2356. [doi | amjbot | pdf | supp. | blog]

[7] Vandvik, V., J. P. Töpper, Z. Cook, M. I. Daws, E. Heegaard, I. E. Måren, and L. G. Velle. 2014. Management-driven evolution in a domesticated ecosystem. Biology Letters 10 (2): 20131082. [doi]

[8] Pausas J.G. & Keeley J.E. 2009. A burning story: The role of fire in the history of life. BioScience 59: 593-601 [doi | jstor | BioOne | pdf | scribd | ppt slides | post]

[9] Keeley J.E., Pausas J.G., Rundel P.W., Bond W.J., Bradstock R.A. 2011. Fire as an evolutionary pressure shaping plant traits. Trends in Plant Science 16(8): 406-411. [doi | trends | pdf]


Smoke-stimulated recruitment

September 16th, 2013 No comments

In many plant species from mediterranean ecosystems, germination is promoted by fire [1]; this effect may be driven by the heat [e.g., 2-4] or by the chemicals produced by the fire (e.g., smoke, 4,5]). Most information regarding to smoke-stimulated germination in the Mediterranean Basin comes from a few experiments performed in laboratory conditions. This approach does not consider factors that occur in the field, such as species interactions, density-dependent processes or the fact that seeds spent time in the soil. A recent field experiment performed in eastern Spain show that smoke increase overall seedling recruitment, specially seedlings of annual plant species [6]. However, despite most species had higher seedling establishment in the smoke than in the control subplots, there were very few species in which the effect of smoke was statistically significant, suggesting that the community response to smoke cannot be inferred from individual species; it is the sum of small differences in each species towards the same direction that produces a significant pattern at community scale. This emerging property of the community is often neglected by only considering germination experiments in the laboratory. The results also suggest that the effect of smoke in annual species of the Mediterranean Basin might be more relevant than previously thought.

[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] Paula S. & Pausas J.G. 2008. Burning seeds: Germinative response to heat treatments in relation to resprouting ability. Journal of Ecology 96 (3): 543 – 552. [pdf | doi]

[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 | pdfblog]

[4] 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 | post]

[5] Smoke-stimulated germination,, 2/Dec/2011.

[6] 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]

Seed dormancy as a fire adaptation in Mediterranean ecosystems

December 6th, 2012 1 comment

Plant species with physical seed dormancy are common in mediterranean fire-prone ecosystems. Because fire breaks seed dormancy and enhances the recruitment of many species, this trait might be considered adaptive in fire-prone environments [1]. However, to what extent the temperature thresholds that break physical seed dormancy have been shaped by fire (i.e., for post-fire recruitment) or by summer temperatures in the bare soil (i.e., for recruitment in fire-independent gaps) remains unknown [1]. In a recent paper published in PLoS ONE [2], we tested these two alternatives in six woody species (21 populations) occurring in fire-prone areas across the Mediterranean Basin (Spain and Turkey). Seeds from different populations of each species were subject to heat treatments simulating fire (i.e., a single high temperature peak of 100ºC, 120ºC or 150ºC for 5 minutes) and heat treatments simulating summer (i.e., temperature fluctuations; 30 daily cycles of 3 hours at 31ºC, 4 hours at 43ºC, 3 hours at 33ºC and 14 hours at 18ºC).

The results showed that fire treatments broke dormancy and stimulated germination in all populations of all species. In contrast, summer treatments had no effect over the seed dormancy for most species and only enhanced the germination in Ulex parviflorus, although less than the fire treatments. That is, the results suggest that in Mediterranean species with physical dormancy, the temperature thresholds necessary to trigger seed germination are better explained as a response to fire than as a response to summer temperatures (see Figure below). The high level of dormancy release by the heat produced by fire might enforce most recruitment to be capitalized into a single post-fire pulse when the most favorable conditions occur. This supports the important role of fire in shaping seed traits [3]. Given that seed dormancy is heritable, demonstrating that it provides higher chances of recruitment (i.e., higher potential fitness benefits) in response to fire than in response to summer temperatures suggests the temperature threshold for breaking dormancy might be an adaptation to fire [1, 4].

Figure: Germination (%) in fire conditions (y axis) versus germination (%) in summer conditions (x axis) for 6 species (21 populations across the Mediterranean basin). Intraspecific variability (i.e., among populations) is indicated by small symbols (mean population value) emerging from the large symbol (mean species value). The 1:1 line is also shown (dotted line). Species considered are: Cistus albidus, Cistus creticus, Cistus parviflorus, Cistus salviifolius, Fumana thymifolia, and Ulex parviflorus.

[1] Keeley, J. E., J. G. Pausas, P. W. Rundel, W. J. Bond, and R. A. Bradstock. 2011. Fire as an evolutionary pressure shaping plant traits. Trends in Plant Sci. 16:406-411. [doi | pdf]

[2] Moreira, B. and J. G. Pausas. in press. Tanned or burned: The role of fire in shaping physical seed dormancy. PLoS ONE 7(6): e39810. [doi | pdf]

[3] Moreira B., Tavsanoglu Ç., Pausas J.G. 2012. Local versus regional intraspecific variability in regeneration traits. Oecologia 168: 671-677. [doi | pdf]

[4] Pausas J.G. & Schwilk D.W. 2012. Fire and plant evolution. New Phytol., 193, 301-303. [doi | wiley | pdf]

Smoke-stimulated germination

December 2nd, 2011 No comments

It is know that the germination of some species from Mediterranean fire-prone ecosystems is triggered by combustion chemicals which appear in the smoke and the charred wood (for simplicity, we use the term “smoke-stimulate germination”). This smoke-stimulated germination is now known from many post-fire recruiting species in South Africa, Australia, California and the Mediterranean Basin [e.g., 1-4]. Certain nitrogen oxides (NOx) induce germination in a limited number post-fire species [3], but this does not apply to most the smoke-stimulated species. In 2004 two independent studies isolated the active organic compound from the smoke that stimulates germination [5,6]: butanolide (also named karrikinolide). Because this compound is a derived from the combustion of cellulose it was thought to be universal germination cue in all smoke-stimulated plants. However, the fact that smoke-induced germination appears in very distant regions and in species from very different lineages, suggest that unrelated species could had evolve mechanisms that are triggered by different components from the smoke [7]. Later it was demonstrated that some species with smoke-stimulated germination did not responded to butanolide, supporting the idea that could be multiple mechanisms to stimulate germination by smoke [8]. A recent paper has found a new smoke-stimulation mechanism from which burning plant material produces cyanide that stimulate the germination of some species [9]. Little by little we are learning on the role of fire in plant ecology and evolution [7, 10].

Figure:  Germination percentage (mean+s.e.) in relation to time since sowing (days) for Cistus monspeliensis after different heat treatments (A), and for Lavandula stoechas after different smoke treatments (B). From Moreira et al. (2010) [4]

[1] Brown, N. A. C. 1993. Promotion of germination of fynbos seeds by plant-derived smoke. New Phytologist 123:575-584.

[2] Dixon, K. W., S. Roche, and J. S. Pate. 1995. The promotive effect of smoke derived from burnt native vegetation on seed germination of Western Australian plants. Oecologia 101:185-192.

[3] Keeley, J. E. and C. J. Fotheringham. 2000. Role of fire in regeneration from seeds. Pages 311-330 in M. Fenner, editor. Seeds: The ecology of regeneration in plant communities. CAB International, Wallingford, UK.

[4] Moreira, B., J. Tormo, E. Estrelles, and J. G. Pausas. 2010. Disentangling the role of heat and smoke as germination cues in Mediterranean Basin flora. Annals of Botany 105:627-635. [doi | pdf | post]

[5] Van Staden, J., A. Jäger, M. Light, and B. Burger. 2004. Isolation of the major germination cue from plant-derived smoke. South African Journal of Botany 70:654-659.

[6] Flematti, G. R., E. L. Ghisalberti, K. W. Dixon, and R. D. Trengove. 2004. A compound from smoke that promotes seed germination. Science 305:977.

[7] Pausas, J. G. and J. E. Keeley. 2009. A burning story: The role of fire in the history of life. Bioscience 59:593-601. [doi | pdf | post]

[8] Downes, K. S., B. B. Lamont, M. E. Light, and J. van Staden. 2010. The fire ephemeral Tersonia cyathiflora (Gyrostemonaceae) germinates in response to smoke but not the butenolide 3-methyl-2H-furol[2,3-c]pyran-2-one. Annals of Botany 106:381-384.

[9] Flematti, G. R., D. J. Merritt, M. J. Piggott, R. D. Trengove, S. M. Smith, K. W. Dixon, and E. L. Ghisalberti. 2011. Burning vegetation produces cyanohydrins that liberate cyanide and stimulate seed germination. Nature Comm. 2:360.

[10] Keeley, J. E., J. G. Pausas, P. W. Rundel, W. J. Bond, and R. A. Bradstock. 2011. Fire as an evolutionary pressure shaping plant traits. Trends in Plant Science 16:406-411. [doi | pdf | post]

Intraspecific plant variability and the spatial scale

September 24th, 2011 No comments

Variability is a fundamental characteristic of life and the raw material for natural selection, driving speciation and diversification processes. Traditional biogeographical theory would predict that plants in populations that are close each other (e.g., few km) should be more similar among them, than plants in distant populations (e.g., 100s or 1000s km). This is because biogeographical processes such as migration, glacial/interglacial climatic fluctuations and isolation should cause distant plant populations to diverge, and thus enhance intraspecific variability at large scales, while gene flow through close populations should reduce divergences. In contrast, in a recent paper we suggest that in fire prone-ecosystems, where fire may generate local heterogeneity, local variability in traits related to regeneration are quite large, overriding the variability at the larger scale [1]. Studying post-fire regeneration traits in Cistus salviifolius and Lavandula stoechas, in eastern Iberia (IB, Spain) and in south-western Anatolia (AN, Turkey), we found that the trait variability within each region is larger than between regions (separated by about 2600 km, with the sea in the middle). The traits studied were seed size, seed dormancy and germination stimulation by head and by smoke. The two studied species exhibited germination stimulated by the fire-related cues; and independently of the region, the different populations of each species had a similar pattern of response. That is, Cistus salviifolius was stimulated by heat and Lavandula stoechas was mainly stimulated by smoke, although heat also exhibited a positive effect on the latter species (see also [2] for more details on heat- and smoke- stimulated germination). All these results supports the prominent role of fire as an ecological and evolutionary process across the Mediterranean Basin, producing trait variability and shaping biodiversity [3, 4].


[1] Moreira B., Tavsanoglu Ç., Pausas J.G. 2012. Local vs regional intraspecific variability in regeneration traits. Oecologia 168: 671-677 [doi | pdf]

[2] 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| doiblog]

[3] Pausas J.G. & Keeley J.E. 2009. A burning story: The role of fire in the history of life. BioScience 59: 593-601 [doi | pdfpost]

[4] Keeley J.E., Pausas J.G., Rundel P.W., Bond W.J., Bradstock R.A. 2011. Fire as an evolutionary pressure shaping plant traits. Trends in Plant Science 16(8): 406-411. [doi | pdf | For managers]

Heat and smoke as germination cues in the Mediterranean flora

February 25th, 2010 No comments

Until now, the role of fire as a germination cue for Mediterranean Basin plants was unclear. The idea was that heat stimulates germination mainly in Cistaceae and Fabaceae and that smoke had a limited role as a post-fire germination cue, in comparison to other Mediterranean Type Ecosystems (MTE), suggesting that fire-stimulated germination is less relevant in the Mediterranean Basin than in other Mediterranean regions. However, in a recent paper, Moreira et al. (2010) demonstrate that both heat and smoke stimulates the germination (both amount and rate) of a range of woody species from the Mediterranean Basin flora. In addition, some species also showed enhanced seedling growth after the smoke treatment (Figure below). All these results suggest that fire-cued germination in woody plants of Mediterranean Basin may be as important as in other Mediterranean regions, and that fire had a strong role in shaping the Mediterranean species.

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]


Differences in size between seedlings from untreated (control, left) and treated seeds (smoke, right), for Lavandula latifolia (8 days after seedling emergence). The white squares are of 2.5cm width.