j.g. pausas' blog

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 [doi | pdf]

[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]

Fire are often considered disasters when they destroy infrastructure and lives (independently of the ecological effects); this is specially important in mediterranean-type climate regions where there is a high density of houses in the wildland-urban interface. The most common management action to avoid these disasters are based of fuel reduction (fuel breaks, prescribed fires, etc). However, these actions do not seems to be very successful, at least in crowded regions and where fire occur in extreme weather situations. For instant, in southern California nearly 1000 homes per year have been destroyed by wildfires since 2000, despite their fire management plans. In addition, strong fuel reduction actions have negative biodiversity implications (vegetation degradation, alien invasion, etc.). In a recent paper, Syphard et al. [1] suggest that urban planning could be more efficient in reducing disasters and property losses from fire than fuel management. They found that at the regional scale, fuelbased maps did not predict property loss as well as maps developed using a combination of factors that included housing arrangement and location. Consequently land use planning and housing development policies should be important components of fire risk management plans for the wildland-urban interface.

[1] Syphard A.D., Keeley J.E., Massada A.B., Brennan T.J. & Radeloff V.C. (2012). Housing arrangement and location determine the likelihood of housing loss due to wildfire. PLoS ONE, 7, e33954.

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

How old are wildfires? Probably as old as terrestrial ecosystems [1]. The origin of fire is tied to the origin of land plants, which are responsible for two of the three elements essential to the existence of fire: oxygen and fuel. The third element, a heat source, has probably been available throughout the history of the planet (mainly through lightning). There is charcoal evidencs of fires already in the Silurian (440 Ma). However, the existence of fire does not necessarily mean that fire was playing an evolutionary role at that time. So when did fire start to play an evolutionary role generating fire adaptations [2, 3]? By mapping fire adaptation onto a dated phylogeny of Pinaceae, we recently demonstrated [4] that at least, and for this family, fire was an agent of natural selection since about 90-125 Ma! This is far back from what was known until now [4]. At this time, fire-protective thick barks were originated in Pinus species as response to surface fires. With increasing fire intensity, thicker barks and serotiny appeared by 70-90 Ma. These innovations appear at the same time as the Earth’s paleoatmosphere experienced elevated oxygen levels that led to high burn probabilities (mid-Cretaceous). That is, the fiery environments of the Cretaceous strongly influenced trait evolution in Pinus. Whether fire had an evolutionary role prior to this is a challance for future research.

Fotos: In many pines, the thick bark and the discontinuity between the canopy and the understory (self-pruning) allows survival after surface fires (left: Pinus nigra, eastern Spain). Serotinous cones allow a quick seed regeneation after crown fire (right: P. halepensis, eastern Spain). Photos: JG Pausas

References
[1] 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 | jstor | BioOne | pdf]

[2] 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 | trends | pdf] | For managers]

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

[4] He T., Pausas J.G., Belcher C.M., Schwilk D.W., Lamont B.B. (2012). Fire-adapted traits of Pinus arose in the fiery Cretaceous. New Phytologist 194: 751-759 [doi | wiley | pdf]

Resprouting is a mechanism that allows individual plants to persist in disturbance-prone ecosystems. It is often considered a binary trait, defining species as resprouters or non-resprouters [1]. Although this dichotomous classification accounts for a high proportion of the interspecific variability in resprouting, it does not account for the intraspecific variability, as not all individuals of resprouting species successfully resprout [2], even if they are subject to a similar disturbance. In a recent paper, we proposed a conceptual model that disaggregates the process of resprouting into three sequential steps: initial ability to resprout, resprouting vigour and post-resprouting survival [3]. Intraspecific variability in resprouting supported the importance of: a) the pre-disturbance state of the plant (i.e. plant size and stored resources) on the initial ability to resprout and on the resprouting vigour, and b) the initial post-disturbance capacity to acquire resources (i.e., resprouting vigour) on the post-resprouting survival. The proposed three-step model of resprouting provides a mechanistic description of the factors driving intraspecific variability in resprouting.

Figure: Probability of initiating resprouting (as a function of starch concentration in roots), resprouting vigor (as a function of pre-disturbance plant size), and survival (as a function of the resprouting vigor), for Linum suffruticosum [see pictures] in the Valencia (eastern Spain). From Moreira et al. (2012) [2]

References

[1] Pausas, J.G., Bradstock, R.A., Keith, D.A., Keeley, J.E. & GCTE Fire Network. 2004. Plant functional traits in relation to fire in crown-fire ecosystems. Ecology 85: 1085-1100. [jstor | pdf]

[2] Catry F.X., Rego F., Moreira F., Fernandes F.M., Pausas J.G. 2010. Post-fire tree mortality in mixed forests of central Portugal. For. Ecol. Manage. 206: 1184-1192. [doi | pdf | post]

[3] Moreira B., Tormo J, Pausas J.G. 2012. To resprout or not to resprout: factors driving intraspecific variability in resprouting. Oikos [doi | pdf]

Considering fire as evolutionary pressure driving evolution has traditional been neglected, and only now is becoming a topic of research [1-3]. The role of fire as an evolutionary pressure can be elucidated using both macro- and micro-evolutionary approaches. While the micro-evolutionary approach searches for trait divergences in different current selective environments, the macro-evolutionary approach uses dated phylogenies to trace the evolution of traits over long time scales (My). In a previous post [3] we mentioned an example of the macro-evolutionary approach. In a recent paper, S. Goméz-Gonzalez and collaborators [5] provided, for the first time, a clear example of the micro-evolutionary approach to demonstrate natural selection driven by fire.  They presented compelling evidence that the novel anthropogenic fires affecting the Chilean matorral shaped seed traits on a native annual plants (Helenium aromaticum). By studying populations growing on sites with different recent fire histories, they showed that increasing fire frequency selects for increasing seed pubescence (directional selection): a trait that was proven to be heritable and that increased fitness under experimental heat treatments. This paper was also presented in a special session at the MEDECOS Conference [3].

Figure: Habitat of Helenium aromaticum in central Chile [5]

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]

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: 406-411 [doi | trends |pdf]

[3] Pausas, J. G. and D. W. Schwilk. 2012. Fire and plant evolution. New Phytologist 193: [pdf]

[4] Pausas J.G. 2011. Australia born to burn – phylogenetic evidences. URL: jgpausas.blogs.uv.es, 18/03/2011

[5] 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]

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]

References:
[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]

Mediterranean Ecosystem (MEDECOS) conferences are held every 3–5 yrs, rotating venues through all five Mediterranean-type climate (MTC) regions of the world. The first meeting was held in Valdivia (Chile) in 1971. The last MEDECOS was held in Los Angeles (University of California, September 6-9, 2011, [1]), and about 300 scientist from the different MTC regions got together and presented their research on the different aspects of the ecology in mediterranean ecosystems. In this MEDECOS, fire was an important topic, it was explicit in the title at least in the following 5 special sessions:

- Fire as an evolutionary pressure shaping plant traits (6th Sept, Pausas & Schwilk)
- Fire management at the wildland-urban interface (7th Sept, Keeley)
- Global change and fire (7th Sept, van Mantgem)
- Fire ecology in Mediterranean woodlands ans shrublands (8th Sept, O’Leary)
- Fire management (9th Sept, Fotheringham)

Dylan Schwilk and I organised the first one which highlighted several key aspects on the role of fire in plant evolution: First, there is good evidence for vegetation-fire regime feedbacks at different spatial and temporal scales, in such a way that plant flammability is a major driver of plant evolution and vegetation distribution. Second, the evidence that fire acts as a selective force is apparent on both micro- and macro-evolutionary scales, suggesting that fire shapes plant traits and generates fire adaptations. And third, that fire is a complex selective pressure – plants adapt to (and, in turn, influence) particular fire regimes rather than fire in the abstract. This is an exciting time for fire ecologists, as fire is now recognized as fundamental for many ecological and evolutionary processes; the coming macro- and micro- evolutionary studies will certainly reinforce many of the ideas drawn during the meeting [2]. The details of this session, including slides of the talks and a summary of the session [2], are now available here .

[1 ] MEDECOS 2011:  web | program | abstracts | final resolution [2] Pausas, J. G. and D. W. Schwilk. 2012. Fire and plant evolution. New Phytologist 193 (2). [doi | pdf]

Recurrent fires are a strong evolutionary pressure shaping plants [1,2]. It has been hypothesized that in fire prone-ecosystems, natural selection has favoured the development of traits that enhance flammability [3]. Consistent with this idea, in a recent study [4] we found that Ulex parviflorus (Fabaceae) populations that inhabit in recurrently burn areas (HiFi populations) are more flammable than populations of this species growing in old-fields where the recruitment was independent of fire (NoFi populations). That is, HiFi plants ignite quicker, burn slower, release more heat and have higher bulk density than NoFi plants. Thus, it appears that repeated fires select for individuals with higher flammability, and thus driving trait divergence among populations living in different fire regimes. These results provide some field support for the ‘kill thy neighbour’ hypothesis [3], but they also highlighted the need for heritability studies to unambiguously demonstrate natural selection driven by fire. This study together with other studies recently commented in this blog [5, 6] are placing flammability as a fundamental trait in plant evolution.

Figure: Flammability experiments using an epiradiator [4].

References

[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 Science 16:406-411. [doi | pdf]

[2] Pausas J.G. & Keeley J.E. 2009. A burning story: The role of fire in the history of life. BioScience [doi | jstore | pdf]

[3] Bond, W. J. and J. J. Midgley. 1995. Kill thy neighbour: an individualistic argument for the evolution of flammability. Oikos 73:79-85.

[4] Pausas J.G., Alessio G., Moreira B., Corcobado G. (2011). Fires enhance flammability in Ulex parviflorus. New Phytologist 193:18-23 [doi | pdf]

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

[5] Pausas JG. 2011. Australia born-to-burn: a phylogenetic approach. jgpausas.blogs.uv.es, 18/March/2011 [link]

[6] Pausas JG. 2011. Fire and evolution: Cretaceous fires and the spread of angiosperms. jgpausas.blogs.uv.es, 9/Sept/2011 [link]

Resprouting is a very important process in plants living in disturbance-prone ecosystems, and the December issue of the journal Plant Ecology is going to be dedicated to this topic (Ecology of plant resprouting in fire-prone ecosystems). During the recent years, and starting from the PERSIST project, we have been comparing functional traits between resprouters and non-resprouters in Mediterranean fire-prone ecosystems, and the last comparison (physiological traits [5]), is included in this special issue. Resprouters and non-resprouters are two plant syndromes in Mediterranean ecosystems that also differ in their evolutionary history [1]. Resprouters tend to exhibit a deeper root-system than non-resprouters that inverse less resources on roots. So one could think that resprouters are better adapted to drought. However, both resprouters and non resprouters coexist, and non-resprouters counteract their lower root allocation by different traits that confer higher drought resistance [2]. Non-resprouters have higher drought resistance at leave level because they have higher water use efficiency (WUE) and higher leaf mass per area (LMA; i.e., higher sclerophylly, lower SLA) [3]. The seedling root structure of non-resprouters also allows them to more efficiently explore the upper soil layer [4]. A recent paper also shows that, when water is non-limiting, non-resprouters showed a better performance of leaf gas exchange traits (higher assimilation, stomatal conductance and transpiration) than resprouters [5]; that is non-resprouters have higher efficiency in resource capture, and thus a better capacity to take advantage of water when it is freely available. In addition, resprouters and non-resprouters also differ in their post-fire germination, as non-resprouters tend to have a greater capacity to both (i) persist after fire by means of recruiting (greater heat-tolerance) and (ii) increase their population after fire (greater heat-stimulated germination), than resprouters [4]. All these results suggest that resprouters and non-resprouters are two contrasted syndromes or functional types in the Mediterranean Basin [6].

Figure: Arbutus unedo resprouting after a fire.

References:

[1] Pausas J.G. & Verdú M. 2005. Plant persistence traits in fire-prone ecosystems of the Mediterranean Basin: A phylogenetic approach. Oikos 109: 196-202. [pdf |doi]

[2] Pausas J.G. 2010. Fire, drought, resprouting: leaf and root traits. URL: jgpausas.blogs.uv.es, 22/Oct/2010.

[3] Paula S. & Pausas J.G. 2006. Leaf traits and resprouting ability in the Mediterranean basin. Functional Ecology 20: 941-947. [pdf | [doi]

[4] Paula S. & Pausas J.G. 2011. Root traits explain different foraging strategies between resprouting life histories. Oecologia 165:321-331. [doi | pdf | blog]

[5] Hernández E.I., Pausas J.G. & Vilagrosa A. 2011. Leaf physiological traits in relation to resprouter ability in the Mediterranean Basin. Plant Ecology 212:1959-1966 [doi| pdf]

[4] 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]

[6] Pausas, J.G., Bradstock, R.A., Keith, D.A., Keeley, J.E. & GCTE Fire Network. 2004. Plant functional traits in relation to fire in crown-fire ecosystems. Ecology 85: 1085-1100. [pdf | jstor] [Ecological Archives E085-029]

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].

References

[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| doi | blog]

[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 | pdf | post]

[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]

There are still people believing that wildfires are a catastrophic disturbance to ecosystems, and that are the product of humans. However there is an increasing evidence from paleoecological records and from phylogenetic analyzes suggesting that fire is a very old process in the history of life, dating back to the origin of land plants [1, 2, 3]. As a consequence many plants have evolved in the presence of recurrent wildfires and acquired adaptive traits to persist and reproduce in those conditions. Examples of these traits are the resprouting ability, germination by head or smoke, and serotiny; all of these confer fitness advantage in fire-prone ecosystems. However, plants are not adapted to fire per se but to fire regimes. Species that exhibit traits adaptive under a particular fire regime can be threatened when that regime changes, like the recent human-induced fire regime changes (e.g., increasing or decreasing fire frequency or severity in relation to the historic fire regime).

In a recent paper, Keeley et al. [4] proposed five scenarios of change in a trait state (Figure 1). An adaptive trait might not change through time regardless of the selective environment (scenarios 1 and 2 in Figure 1). Such traits cannot be described as adaptations to the current selective (fire-prone) environment as there is no evidence that natural selection shaped this trait. Other adaptive traits that were shaped by natural selection under a previous evolutionary pressure, but not under the current (fire-prone) environment (scenario 3 in Figure 1) would be adaptations to previous evolutionary pressures and exaptations to the current (fire) environment [4, 5]. Fire adaptations are those adaptive traits in which natural selection is acting under the current fire-prone environment to shape the trait, and it is independent of how long this pressure has been present (scenarios 4 and 5 in Figure 1). For instance, there are clear examples of lineages that resprout after fire, but their origin and evolution is hardly liked to fire. However the most plausible scenario of lineages that resprouting from lignotubers is the number 4 in Fig. 1 (old origin of resprouting reshaped by current recurrent fires). Similarly serotiny and thick barks are traits that has been reshaped by natural selection under the framework of recurrent fires and thus they also fit under the concept of adaptation to fire (scenario 4 or 5 in Fig. 1).

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] Pausas J.G. 2011. Australia born to burn – phylogenetic evidences. URL: jgpausas.blogs.uv.es, 18/03/2011.

[3] Pausas J.G. 2010. Fire and evolution: Cretaceous fires and the spread of angiosperms. URL: jgpausas.blogs.uv.es, 9/Sep/2010.

[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: 406-411 [doi | trends | pdf]

[5] Endler J.A. (1986) Natural selection in the wild. Princeton University Press.

Figure. 1. Five possible evolutionary scenarios of change in a trait state along the evolutionary time (simplified). For each scenario, different line types are periods under different dominant evolutionary pressures (e.g., the continuous line represents a period in which fire acted as an evolutionary pressure; dashed line the period with a different previous selective environment). The scenarios are:
1) and 2) No change along the time axis and no sign of natural selection (no adaptation to fire);
3) trait shaped during the first evolutionary pressure, but no change (with persistence of the state of the trait) during the second evolutionary pressure; natural selection acted during the first period only (no adaptation to fire, but exaptation);
4) and 5) Trait shaped during the whole period; natural selection acts during the whole period even if the dominant evolutionary pressure changed (adaptation to fire).
From Keeley et al. (2011, [4]).

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).

Traditionally wildfires were considered a disturbance linked to the recent history of the Quaternary, and specially linked to the humans. However, evidence are accumulating about the ancient role of wildfires in terrestrial ecosystems [1]. In Australia, the flammable continent, the current believe is that fires started to be important during the onset of seasonal aridity in the Miocene (25 Ma). However, two recent and independent papers demonstrate, using phylogenetic techniques, that fire-dependent traits appeared about 60 Ma ago (early Paleocene), implying that fire was already an effective agent of selection by then. Crisp et al [2] studied the Myrtaceae family and showed that post-fire epicormic resprouting (typical of many eucalypts) is an ancient trait linked to the flammable sclerophyll biomes originated about 60-62 Ma. He et al. [3] studied the Banksia genus (Proteaceae) and showed that serotiny (fire dependent dispersal; figure below) and dead floret retention around the cones (enhanced flammability around serotinous cones) co-originate with the first appearance of Banksia 60.8 Ma ago. The coincidence of the two independent papers, using two different taxa (Myrtaceae and Banksia) is amazing, and clearly suggests that fire was a selective force in Australia during the Paleocene.  These papers are part of the accumulating research on the prominent and ancient role of fire in shaping plant species and biodiversity [1, 4 ].

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] Crisp MD, Burrows GE, Cook LG, Thornhill AH, Bowman DMJS. 2011. Flammable biomes dominated by eucalypts originated at the Cretaceous-Palaeogene boundary. Nature Communications 2: 193. [doi]

[3] He T, Lamont BB, Downes KS. 2011. Banksia born to burn. New Phytol. [doi]

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

Figure: Banksia cone opened by the fire to release seeds (serotiny).

The thickness of the bark is a trait of paramount importance in trees living in ecosystems with frequent surface (understory) fires (e.g., some coniferous forests, savanna woodlands, etc.). This is because the bark is a good insulator protecting vital tissues from the heat of the fire. Having a bark few millimeter thicker provide an advantage in such fire-prone ecosystems. Thus there has been a selection for thick barks in surface fire ecosystems [1]. A prominent example of a tree with a very thick and insulating bark is the Cork oak (Quercus suber) that growth in the western part of the Mediterranean Basin [2]. In such species the thicker is the bark, the better is the response after fire [3, 4]. This bark is so thick and insulating that it is used not only as bottle tops, but also as insulating material in many industrial applications. However the Mediterranean Basin has been densely populated from long ago and it is very difficult (if possible) to find Cork oak woodlands in “natural” conditions, and thus it is not easy to know how thick the bark of Cork oak could attain in natural conditions. Most trees are frequently debarked for obtaining cork (frequencies ranging from every 9 to every 12 years, depending of the site conditions).

Few days ago I visited an ethnographic museum in Aggius (Sardinia) and found a piece of Cork oak bark of about 22 cm thick (see picture below), which is pretty thick. I only know of one record of a thicker bark: 27 cm in a 140 years-old Cork oak that was never debarked [5]. Do you know of any tree (of the same or another species) in the world with a thicker bark? Is Cork oak the world record on bark thickness?

Figure: Piece of bark from a Cork oak (Quercus suber), in the ethnographic museum of Aggius (Sardinia).

References:

[1] Pausas J.G. 2009. Convergent evolution. jgpausas.blogs.uv.es, 8/Nov/2009. [link]

[2] 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.  [link]

[3] Pausas, J.G. 1997. Resprouting of Quercus suber in NE Spain after fire. J. Veg. Sci. 8: 703-706. [doi | pdf]

[4] Catry F.X., Rego F., Moreira F., Fernandes F.M., Pausas J.G. 2010. Post-fire tree mortality in mixed forests of central Portugal. Forest Ecology & Management 206: 1184-1192. [doi | pdf]

[5] Natividade J.V. 1950. Subericultura. Direçao Geral dos Serviços Florestais e Aquícolas Lisbon, Portugal.

In Mediterranean Basin ecosystems, fires are frequent, and post-fire regeneration is tipically based on native species, that is, there is no invasion of alien species after fire. However, this is not the case in the other Mediterranean climatic regions, where fire frequencies higher than their natural (historic) fire regime favors the invasion of alien plants. This is specially the case in the Mediterranean ecosystems of Chile, where recurrent fires play a little role on the evolutionary history. In Chile, fires appeared with the indigenous settlements, and increased exponentially since the time of the Spanish invasion (1536). This increase in fires, together with heavy grazing, has reduced the native matorral and increased the invasive species. In a recent paper, Gómez-Gonzalez et al. [1] show that fire open the window for the establishment of annual plants, and most of them are alien (from the Mediterranean Basin). The successful establishment of alien annuals was due to their ability to maintain rich seedbanks in burned areas and to the greater propagule arrival compared to native species (annuals or perennials). Thus the results demonstrate that fire is a relevant factor for the maintenance of alien-dominated grasslands in the Chilean matorral and highlight the importance of considering the interactive effect of seed rain and seedbank survival to understand plant invasions patterns in fire-prone ecosystems.

[1] Gómez-González S, Torres-Díaz C., Valencia G, Torres-Morales P, Cavieres L.A., and Pausas J.G (in press). Anthropogenic fires increase alien and native annual species in the Chilean coastal matorral. Diversity and Distributions 17: 58-67 [doi | pdf]

Figure: Chilean matorral recently burnt showing the invasion of Fumaria capreolata (flowering), and annual alien species original from Europe (Foto by S. Gómez-González).

Drought and fire are prevalent disturbances in Mediterranean ecosystems. Plant species able to regrow after severe disturbances (i.e. resprouter life history) have higher allocation to roots and higher water potential during the dry season than coexisting non-resprouting species. However, non-resprouters have higher survival rate after summer drought. We expect that, to counteract their shallow-rooting systems and to maximize seedling survival, non-resprouters have traits that confer higher water-use efficiency and higher efficiency in soil resource acquisition than resprouters.

Some time ago we tested this prediction in relation to leaf traits [1] and found that non-resprouters have higher leaf mass per area (LMA; i.e. lower specific leaf area, SLA), leaf dry matter content (LDMC), area-based leaf nitrogen content (LNCa) and integrated water-use efficiency (δ13C) than resprouters, suggesting that they have higher potential for structural resistance to drought and higher water-use efficiency than resprouters.

In a recent paper we have now tested the prediction for root traits in seedlings [2]. We found that non-resprouters have higher specific root length (SRL) and longer, thinner and more branched lateral roots, especially in the upper soil layers. The external links (i.e. the most absorptive root region) are also more abundant, longer, thinner and with higher SVR for non-resprouters. Thus seedling root structure of non-resprouters species allows them to explore more efficiently the upper soil layer, whereas seedling roots of resprouters will permit both carbon storage and deep soil penetration.

Whereas resprouters tend to maximize the surface and the efficiency of the organs for carbon uptake to ensure carbohydrate storage for resprouting (eg, higher SLA), non-resprouters maximize the root surface (eg, higher SRL), since their survival and growth may be limited by soil resources.

[1] Paula S. & Pausas J.G. 2006. Leaf traits and resprouting ability in the Mediterranean basin. Functional Ecology 20: 941-947. [doi | pdf]

[2] Paula S. & Pausas J.G. 2011. Root traits explain different foraging strategies between resprouting life histories. Oecologia 165:321-331 [doi | pdf]

Relationship between average root diameter and specific root length (SRL; log-scale) for resprouting (R+, closed symbols) and non-resprouting (R-, open symbols) species. Intraspecific variability is indicated by segments emerging from each symbol. From Paula & Pausas (Oecologia [2]).

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]

In September 2003, a mixed forest of central Portugal (Tapada Nacional de Mafra) burned in a large crown fire. We surveyed the survival of more than 700 trees during 4 years postfire. The results are detailed in a recent paper by Catry et al. (2010, [1]) ; the table below provides a summary on the proportion of the trees that  (a) died, (b) survived but were top-killed (stem and crown mortality) and resprouted from the base, and (c) the stem survived, after 4 years postfire (for tree mortality, the value observed after the first year is given in brackets).

Most pines (P. pinaster and P. pinea) died and few, specially of P. pinea, were little affected by fire; there were a significant positive relationship between crown damage and tree mortality.
Most broadleaved trees survived the fire, and whether the stem survived or died (and resprouted from the base) were related to bark thickness and char height (i.e. fire severity). Castanea sativa showed the highest tree mortality, mostly due to post-resprouting mortality after the first year. Fraxinus angustifolia, Olea europaea and Pistacea lentiscus showed no mortality at all; most Olea and Pistacea individuals resprout from the base, while for Fraxinus the crown of most trees were unaffected. The low effect of fire in Fraxinus angustifolia is probably due to the topographic positions where this species occurs; in addition theses trees are quite tall and with relatively thick bark. Quercus coccifera and Q. faginea showed low mortality and most trees resprouted from the base. Quercus suber showed almost no mortality and almost all trees showed epicormic resprouting, due to their extremely thick and insulating bark [2, 3].

References

[1] Catry F.X., Rego F., Moreira F., Fernandes F.M., Pausas J.G. 2010. Post-fire tree mortality in mixed forests of central Portugal. For. Ecol. Manage. 206: 1184-1192. [doi] [pdf]

[2] Pausas, J.G. 1997. Resprouting of Quercus suber in NE Spain after fire. J. Veg. Sci. 8: 703-706. [pdf]

[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] [Ch 1, the tree]

[Update] see: “To resprout ot not to resprout”, Jan 25th, 2012.

En pleno verano, y como cada año, arden unas cuantas hectáreas de nuestros paisajes. En los ecosistemas mediterráneos esto lleva ocurriendo desde hace muchos años, incluso antes de la llegada de los humanos. El fuego y los incendios forestales no es ni mucho menos un invento de los pirómanos, sino que están en la naturaleza desde casi siempre, y por eso hay muchas plantas adaptadas a vivir en zonas con fuegos recurrentes (igual que hay plantas que pueden sobrevivir el pastoreo recurrente). Lo que ha hecho los humanos es modificar el régimen de incendios, aumentado o disminuyendo su frecuencia e intensidad.

A menudo los incendios se ven como una cosa “mala” para la naturaleza; en un artículo de divulgación en la revista “Investigación y Ciencia” (Agosto 2010) damos otra visión del fuego, y hacemos un repaso del papel de este en la evolución de las especies,  la otra cara del fuego. En nuestros ecosistemas, el fuego es una parte integral de los procesos ecológicos, y a lo largo de la historia ha ido moldeando las especies, las comunidades y los paisajes. Sin duda hay ciertos regímenes de incendios que son naturales y característicos de ciertos ecosistemas, y parte de la diversidad de nuestros ecosistemas se explica por la existencia reiterada y predecible de incendios. Sin embargo, también es cierto que hay zonas que están sufriendo regímenes de incendios fuera del rango natural y con graves consecuencias ecológicas. El objetivo de la gestión forestal no debería ser eliminar los incendios, ya que es prácticamente imposible, además de poco natural. Por el contrario, deberían asumirse ciertos regímenes sostenibles de incendios, y aprender a convivir con ellos.

Pausas, J.G. 2010. Fuego y evolución en el Mediterráneo. Investigación y Ciencia, 407 (Agosto): 56-63. [PDF: IyC | jgpausas]

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

Pausas, J.G., Vallejo R. 2008. Bases ecológicas para convivir con los incendios forestales en la Región Mediterránea – decálogo Ecosistemas, 17(2): 128-129 (5/2008). [link] [pdf]

Pausas J.G., Llovet J., Rodrigo A., Vallejo R 2008. Are wildfires a disaster in the Mediterranean basin? – A review. Intern. Journal of Wildland Fires, 17: 713-723. [pdf] [IJWF CSIRO] [doi]