j.g. pausas' blog

Fire is unique to Earth; it is a characteristic of our planet. As far as we know, no other planet has fire. Here is a video explaining the geological history of fire, including its relation to the evolution of pines and grasses. It concludes that “without fire, there would probably be no grasslands and the forest of the world would likely have a lot less diversity“. By PBS Eons.

References

• Pausas J.G. & Keeley J.E. 2009. A burning story: The role of fire in the history of life. BioScience 59: 593-601 [doi | OUP | pdf]
• He T, Pausas JG, Belcher CM, Schwilk DW, Lamont BB. 2012. Fire-adapted traits of Pinus arose in the fiery Cretaceous. New Phytol. 194: 751-759. [doi | wiley | pdf | suppl.]

En los últimos años hemos aprendido mucho sobre ecología y evolución de las plantas en relación a los incendios forestales [1]. Una de los puntos clave pare ello fue encontrar evidencias de que siembre, a lo largo de toda la historia evolutiva de las plantas, han habido incendios, incluyendo en las primeras comunidades vegetales que colonizaron el medio terrestre [2]. Y todo esto se sabe gracias a los avances en el estudio de los carbones fósiles. Uno de los estudiosos más importantes en paleo-fuegos es el geólogo Andrew Scott, quien ha dedicado gran parte de su vida a mirar carbones fósiles, y que en 2018 resumió sus investigaciones en un libro titulado Burning Planet: The story of fire through time [3]. Ahora tenemos la suerte de que ese libro se ha traducido al castellano, Planeta en llamas: la historia del fuego a través del tiempo (Galaxia Gutenberg, 2020). Creo que este libro es un buen complemento a mi libro sobre la ecología de los incendios forestales [4], y espero que ayude a la comunidad hispánica a entender mejor el papel ecológico, evolutivo, y ancestral que tiene el fuego en nuestros ecosistemas.

Referencias

[1] Ecology & Evolution in fire-prone ecosystems [enlace]

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

[3] Scott A. 2018. Burning Planet: The story of fire through time. Oxford University Press. [versión española: Planeta en llamas: la historia del fuego a través del tiempo. Galaxia Gutenberg, 2020]

[4] Incendios forestales, una visión desde la ecología, CSIC-Catarata, 2012 [enlace]

In recent decades, fires in Mediterranean Europe have become larger and more frequent. This trend has been driven mainly by socioeconomic changes that have generated rural depopulation and changes in traditional land use. This has increased the amount and continuity of vegetation (fuel), and thus an increase in the fire size and area burnt [1-3]. In a recent paper [4] we compared fire statistics of the Western Rif (Morocco) with those form Valencia (eastern Spain) to show that current fire regimes in Mediterranean Africa resemble past fire regimes in the Mediterranean Europe when rural activities dominated the landscape. The temporal fire regime shift observed in different countries of the Mediterranean Europe (from small, fuel-limited fires to drought-driven fires) can be identified when moving from the southern to the northern rim of the Basin. That is, most spatial and temporal variability in fire regimes of the Mediterranean Basin is driven by shifts in the amounts of fuel and continuity imposed by changes in socioeconomic drivers (e.g., rural depopulation). In fact, we can use rural population density as an early warning for abrupt fire regime shift. Consequently we can predict future fire regimes in North Africa, based on the trends observed in southern Europe, and we can better understand past fire regimes in Europe based on the current situation in North Africa [4].

Figure 1. Western Rif (northern Morocco) and Valencia (eastern Spain).

Figure 2. Fire-size distribution in Valencia, for the period 1880-1970 (white boxes) and for the period 1975-2014 (grey boxes), and in the western Rif (red symbols, 2008-2015). For details see [4]

References

[1] Pausas, J.G. 2004. Changes in fire and climate in the eastern Iberian Peninsula (Mediterranean basin). Climatic Change 63: 337-350. [pdf | doi]

[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]

[3] 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]  

[4] 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]

During the last years I’ve been working in many topics related to fire ecology and plant evolution in ecosystems subject to recurrent fires (mainly mediterranean and savanna ecosystems). Because I believe knowledge should be spread around easily, I make my results available to the public in my web page (see publications list) and in this blog. However, having the cumulative list of paper published each year is not very convenient for people searching for a specific topic. For this reason, I’m rearranging most of my articles by topics as follows:

1. Fire history
2. Fire regime: climate & fuel
3. Fire traits (resprouting, postfire germination, serotiny, bark thickness, flammability, data & methods)
4. Fire & plant strategies (in Mediterranean ecosystems, in pines, in savannas, community assembly)
5. Fire & evolution
6. Some fire-adapted species (Pinus halepensis, Quercus suber, Ulex parviflorus)
7. Fire & vegetation modelling
8. Plant-animal interactions
9. Restoration & conservation

See: fire-ecology-evolution.html

Some papers may be repeated if they clearly fit in more than one topic; some papers, mainly old ones, do not fit well in any of these topics and have not been included (at least at the moment), they still can be found in the section of publications sorted by year. I’m still working on this rearrangement, so some modifications are possible; and any comment is welcome.
I hope this is useful for somebody!

Publications: by year | by topic | books

Serotiny is the delayed seed release for more than a year by retaining the seeds in a woody structure [1]. This implies an accumulation of a canopy seed bank. Serotiny confer fitness benefits in environments with frequent crown-fires, as the heat opens the cones and seeds are dispersed in the post-fire bed which is rich in resource and the competition and predation are low. It is typical of many Proteaceae and some conifers, like some pine species [1, 2; figure below].

Two recent papers analyse the serotiny of two mediterranean pines Pinus halepensis and Pinus pinaster [3, 4]. P. halepensis show higher proportion of serotinous cones than P. pinaster, but the latter retain the cones for longer [3]. The two species show high variability of serotiny within and between populations, but they show a clear pattern of higher serotiny in populations subject to high frequency of crown-fires than those living in areas where crown-fires are rare or absent. This is true either considering serotiny as the proportion of serotinous cones or as the age of the cones stored. Compared with other pines worldwide, the strength of the fire-serotiny relationship in P. pinaster is intermediate, and in P. halepensis is among the highest known [3]. For P. halepensis (the species with higher % serotiny), populations in high fire recurrence regimes have higher fine-scale spatial aggregation of serotiny than those inhabiting low fire recurrence systems. This phenotypic spatial structure generated by fire could be a consequence of the spatial genetic structure of the population. The second study used genomic tools to search for a genetic association for serotiny [4]. The analysis of 384 SNPs of 199 individuals of P. pinaster (in 3 populations included in the previous study [3])  shows that 17 loci were associated with serotiny and explain all together ca. 29% of the serotiny variation found in the field. All these results adds further evidence to the emerging view that fire shapes intraspecific variability of traits and generates phenotypic divergence between populations [5, 6, 7].

Figure: Serotinous cones of Pinus pinaster (Foto: K.B. Budde)

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] He T, Pausas JG, Belcher CM, Schwilk DW, Lamont BB. 2012. Fire-adapted traits of Pinus arose in the fiery Cretaceous. New Phytologist 194: 751-759. [doi | wiley | pdf (suppl.)]

[3] 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: 2349-2356 [doi | amjbot | pdf | supp.]

[4] Budde, K. B., Heuertz, M., Hernández-Serrano, A., Pausas, J.G., Vendramin, G.G., Verdú, M. & González-Martínez, S.C. 2014. In situ genetic association for serotiny, a fire-related trait, in Mediterranean maritime pine (Pinus pinaster Aiton). New Phytologist  201: 230-241 [doi | pdf]

[5] 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]

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

[7] Pausas J.G., Alessio G., Moreira B. & Corcobado G. 2012. Fires enhance flammability in Ulex parviflorus. New Phytologist 193: 18-23. [doi | wiley | pdf]

Traditionally, Australian aboriginal people set fires in their landscape to facilitate hunting. A recent study has compared the landscape and fire history from two regions, one where aboriginal people live in a traditional way and the other where fires are “natural” and caused by lightning [1]. The results show that aborigines generate many small fires that are climate-independent, while lightning generates few large climate-driven fires. Anthropogenic fires are smaller even when climatic conditions cause huge fire in the lightning region. The authors suggest that this climate-buffering effects of aboriginal fires has likely been important for many species that benefit both from fine-grained mosaics of alternating resources and from enhanced protection from large catastrophic fires and the predators that hunt within them. This may explain the coincident decline of many small- to medium-sized mammals in the arid regions of Australia with the cessation of aboriginal hunting and burning. That is, the extinction of the aboriginal life style shifted fire regimes from small fires to large climate-driven fires, in a similar manner to the extinction of rural life styles in the Mediterranean Europe [2], and this shift promoted the extinciton of Australian mammals.

Fire Dreaming, by Malcolm Maloney Jagamarra [from www.aboriginalartcoop.com.au]

References

[1] Bird R.B., Codding B.F., Kauhanen P.G. & Bird D.W. (2012). Aboriginal hunting buffers climate-driven fire-size variability in Australia’s spinifex grasslands. PNAS, 109, 10287-10292. [pnas]

[2] Pausas J.G. & Fernández-Muñoz S. (2012). Fire regime changes in the Western Mediterranean Basin: from fuel-limited to drought-driven fire regime. Clim. Change, 110, 215-226.  [doi | springer | pdf]

We recently analyzed the fire-climate relationship in the Iberian Peninsula (western Mediterranean Basin) [1], and found that climate shapes fire activity on a temporal scale by modifying fuel flammability (i.e., more fire during dry years; left figure below) and on a spatial scale by affecting fuel structure (i.e., more fire in productive Iberian regions). On the temporal scale, fire and climate are not linearly related, but there is a critical aridity level (i.e., the aridity threshold) above which fuels become highly flammable and area burnt increases sharply (left figure below). This aridity threshold is not universal, but rather intrinsic to each ecosystem (i.e., to its landscape structure). The drier the region, the higher the dryness level needed for switching from non-flammable to flammable conditions (right figure below), suggesting that the aridity threshold is mediated by fuel. In productive regions, an ignition may lead to a fire under relatively high moisture conditions (compared to drier regions) due to the high fuel load and connectivity. On the contrary, in dry regions, wildfires are more fuel-limited, so more extreme climatic conditions (higher aridity than in more mesic regions) are needed for fires to successfully spread. The fact that the aridity threshold is intrinsic to the ecosystem emphasizes the importance of landscape structure in determining fire-climate relationship.

Fuel structure does not depend exclusively on environmental conditions (e.g., aridity/productivity); shifts in fire activity have also been related to changes in land-use [2,3] and fire-suppression policies. Gradual historical shifts in land-use may produce abrupt changes in fuel structure across landscapes and thus, in fire activity [3]. Therefore, the fire-climate relationship changes not only with climatic conditions, but also  in response to different land uses and management practices (and often in an abrupt way).

Figure: [left:] Relation between area burnt and monthly aridity ( (PET-AET)/PET) in one of the 13 Iberian regions considered (temporal scale); vertical line indicates the location of the aridity threshold. [Right:] Relationship between the the aridity threshold and the aridity of the region, for 13 Iberian regions (Pausas & Paula, 2012 [1]).

References
[1] Pausas J.G. & Paula S. 2012. Fuel shapes the fire-climate relationship: evidence from Mediterranean ecosystems. Global Ecology and Biogeography 21: 1074-1082 [doi | pdf | supp]

[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 | post]

[3] Pausas, J.G. 2004. Changes in fire and climate in the eastern Iberian Peninsula (Mediterranean basin). Climatic Change 63: 337-350. [doi | springer | pdf]

Finally the new fire ecology book by Keeley et al. (2012) has been published:

For more information, table of contents, etc, see here.

Cambridge UP (uk | usa | au), Amazon (uk | usa | jp), eBooks

We have compiled the longest fire history for a region in the Mediterranean Basin, from contemporary fire statistics plus old newspapers and old forest administration dossiers in Valencia (Spain) [1]. With this information we statistically demonstrated that fire regime has changed during the 1970’s, in such a way that fires increased in annual frequency (doubled), fire size, and area burned (by an order of magnitude). The main driver of this shift was the increase in fuel amount and continuity due to rural depopulation (vegetation and fuel build-up after farm abandonment) suggesting that fires were fuel-limited during the pre-1970s period. Climatic conditions were poorly related to pre-1970s fires and strongly related to post-1970s fires, suggesting that fire are currently less fuel limited and more drought-driven than before the 1970s. Thus, the fire regime shift implies also a shift in the main driver for fire activity. In conclusion, the collapse of the rural lifestyle shifted fire regimes from being fuel-limited to be drought-driven, and this may have consequences in an world where droughts are increasing in frequency [2]. The same process occurred in all the European Mediterranean region, and the large fires in Greece during the 2007 heat wave [link] are a good example of a system drought-driven. Interestingly, simultaneously to our study, a similar work has just been published demonstrating that the collapse of the Soviet Union lead to a reduction in grazing pressure and a subsequent increase in area burned [3].

[1] 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]

[2] Pausas, J.G. 2004. Changes in fire and climate in the eastern Iberian Peninsula (Mediterranean basin). Climatic Change 63: 337-350. [doi | pdf]

[3] Dubinin M., Luschekina A. & Radeloff V.C. 2011. Climate, livestock, and vegetation: What drives fire increase in the arid ecosystems of southern Russia? Ecosystems, doi:10.1007/s10021-011-9427-9

Fig. 1. The abandonment of agriculture and livestock, and the change in the domestic energetic sources (from wood to others) modified the landscape from a mosaic (top image) to an homogeneous fuel bed of flammable vegetation (bottom image). These changes drove the increased fire activity since the 70’s (see Fig. 2 below).

Fig. 2. Annual area burnt (ha × 1000, vertical lines, left axis) for 1873 to 2006 and the rural population density (inhabitants/ha, dotted line, right axis), in Valencia (Spain). From Pausas & Fernández-Muñoz (2011).

Recently we have highlighted the importance of wildfires in the evolution of plants in many ecosystems worldwide [1 | previous post]. In this line, a recent paper by Bond & Scott suggest that the spread of angiosperms in the Cretaceous (145-65 Ma) was promoted by the development of novel fire regimes linked to the evolution of novel, highly productive (and flammable) plants. They suggest that Creatceous angiosperms were similar to current ruderal (weedy) species, i.e., short, with high maximum photosynthetic rates, rapid reproduction and small seeds. This fast-growing angiosperms would not only compete with regenerating gymnosperms, but would also rapidly accumulate fuel. More fuel would promote more frequent fires, which would help to maintain open habitats in which rapid growth traits of angiosperms would be most favoured, promoting rapid fuel accumulation. The authors emphasize the similitude of this “angiosperm–fire cycle” with  the grass fire-cycle that helped to spread C4 grasses in the Miocene (c. 8 Ma) [3] and with the grass fire-cycle replacing forests by invasive grasses in the modern world [4]. This would also imply that forest was slow to develop until the Eocene, when fire activity dropped to very low levels. This hypothesis could also help to explain the ancient origin of some fire traits like resprouting and the abundance and phylogenetically widespread examples of species with smoke-stimulated germination [1, 5]. In conclusion I think this is a nice and stimulating contribution to the evolution of angiosperms.

References

[1] Pausas J.G. & Keeley J.E. 2009. A Burning Story: The role of fire in the history of life. BioScience 59: 593-601. [doi | pdf | post | slides]

[2] Bond, W. J. and Scott, A. C. 2010. Fire and the spread of flowering plants in the Cretaceous. New Phytol. 188: 1137–1150 [doi]

[3] Keeley, J. E. and Rundel, P. W. 2005. Fire and the Miocene expansion of C4 grasslands. Ecol. Lett. 8: 1-8.

[4] D’Antonio, C. M. and Vitousek, P. M. 1992. Biological invasions by exotic grasses, the grass/fire cycle and global change. Annu. Rev. Ecol. Syst. 23: 63-87.

[5] 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]

Fire regimes and vegetation patterns are the product of both climate and humans (Marlon et al. 2008, Pausas & Keeley 2009). For instance, contemporary ignitions are very often linked to human behaviour (arson, negligence, etc.) and fire sizes may be related to landscape fragmentation and suppression efforts; however, the climatic signal on fire regimes is still evident (e.g,. dry summers, Pausas 2004; during heat waves, etc.). A recent paper documents this complex interaction in shaping Holocene fire regimes in the Mediterranean Basin: Climate, microclimate (topography) and human pressure are factors that need to be considered for understanding changes in both vegetation and fire activity during the Holocene of south Europe (Gil-Romera et al. 2010).

• Marlon et al. 2008. Climate and human influences on global biomass burning over the past two millennia. Nature Geoscience, 1, 697-702.
• Pausas J.G. 2004. Changes in fire and climate in the eastern Iberian Peninsula (Mediterranean basin). Climatic Change, 63, 337-350. [doi | pdf]
• Pausas J.G. & Keeley J.E. 2009. A burning story: The role of fire in the history of life. BioScience, 59, 593-601. [doi | pdf]
• Gil-Romera et al. 2010. Fire regime in Southern Iberia: the long-term role of fire as landscape modeller in a western Mediterranean region. Quat. Sci. Rev. 29: 1082-1092. [doi | pdf]

Age (cal Kyr BP)

Figure: Synthetic pollen percentage diagrams for Baza mountains (1900 m asl, Southeastern Spain), with the Normalised Charcoal (NCHAR, right) and millennial averaged Fire Activity Anomalies (FAA, left) shown on top. (XER: xerophytes, MES: Mesophytes, Querdec: Deciduous oaks, Querever: Evergreen oaks, Pining: Pinus nigra + P. sylvestris). The increase of fire activity is assotiated to the decrease of Pinus nigra (non-serotinous thin-barked pines) and the increase of Poaceae and xerophytes. For mode details see Gil-Romera et al. (2010).