Fire as a pre-emptive evolutionary trigger among seed plants

Plants in fire-prone ecosystems have evolved a variety of mechanisms to resist or adapt to fire. Post-fire resprouting is a key adaptation that promotes rapid ecosystem recovery and hence has a major impact on the terrestrial carbon cycle. However, our understanding of how the incidence of resprouting varies in different fire regimes is largely qualitative. The increasing availability of plant trait data and plot-based species cover data provides an opportunity to quantify the relationships between fire-related traits and fire properties. We investigated the quantitative relationship between fire frequency (expressed as the fire return time) and the proportion of resprouters in woody plants using plot data on species cover from Australia and Europe. We also examined the relationship between the proportion of resprouters and gross primary production (GPP) and grass cover, where GPP was assumed to reflect fuel loads and hence fire intensity, while grass cover was considered to be an indicator of the likelihood of ground fire and the speed of fire spread, using generalised linear modelling. The proportion of resprouting species decreased significantly as the fire return time increased. When the fire return time was considered along with other aspects of the fire regime, the proportion of resprouters had significant negative relationships with the fire return time and grass cover and a significant positive relationship with GPP. These findings demonstrate that plants with the ability to resprout occur more often where fire regimes are characterised by high-frequency and high-intensity crown fires. Establishing quantitative relationships between the incidence of resprouting and the fire return time and fire type provides a basis for modelling resprouting as a consequence of the characteristics of the fire regime, which in turn makes it possible to model the consequences of changing fire regimes on ecosystem properties.

The variation in climate conditions and fire-related traits across Pinus (Pinaceae) species

AimGlobal change factors, such as warming, heatwaves, droughts and land‐use changes, are intensifying fire regimes (defined here as increasing frequency or severity of fires) in many ecosystems worldwide. A large body of local‐scale research has shown that such intensified fire regimes can greatly impact on ecosystem structure and function through altering plant communities. Here, we aim to find general patterns of plant responses to intensified fire regimes across climates, habitats and fire regimes at the global scale.LocationWorldwide.Time periodStudies published 1962–2023.Major taxa studiedWoody plants, herbs and bryophytes.MethodsWe carried out a global systematic review and meta‐analysis of the response of plant abundance, diversity and fitness to increased fire frequency or severity. To assess the context dependency of those responses, we tested the effect of the following variables: fire regime component (fire frequency or severity), time since the last fire, fire type (wildfire or prescribed fire), historical fire regime type (surface or crown fire), plant life form (woody plant, herb or bryophyte), habitat type and climate.ResultsIntensified fire regimes reduced overall plant abundance (Hedges' d = −0.24), diversity (d = −0.27), and fitness (d = −0.69). Generally, adverse effects of intensified fire regimes on plants were stronger due to increased severity than frequency, in wildfires compared to prescribed fires, and at shorter times since fire. Adverse effects were also stronger for woody plants than for herbs, and in conifer and mixed forests than in open ecosystems (e.g. grasslands and shrublands).Main conclusionsIntensified fire regimes can substantially alter plant communities in many ecosystems worldwide. Plant responses are influenced by the specific fire regime component that is changing and by the biotic and abiotic conditions.

BackgroundClimate change is driving global fire regimes toward greater extremes, potentially threatening plant species that are adapted to historic fire regimes. Successful conservation of threatened plant species depends upon improving our understanding of how they respond to these changing fire regimes in fire prone regions. The 2019–2020 Australian megafires burnt at very high to extreme severity across an unprecedented extent and overlaid a complex history of prescribed burns and wildfires, providing an ideal foundation to study the consequences of multiple fire regime elements. We examined the recruitment response of Pomaderris bodalla, one of many threatened obligate-seeding shrub species growing in wet sclerophyll (mesic) forest in south-east Australia. We surveyed seedling recruitment at sites across a gradient of fire severity and frequency. Our aims were to (i) confirm in vitro results that suggest a positive relationship with fire severity; (ii) determine the species response to fire frequency and (iii) identify the nature of the effect of fire severity and fire frequency in combination.ResultsWe found that recruitment had a positive response to fire severity, peaking at high severity sites as soil temperatures reached optimal levels for dormancy-break but declining, while still remaining positive, at moderate and extreme severity sites. The pattern of response matched in vitro studies, which had established that physically dormant P. bodalla seeds had minimal dormancy broken at low fire-related temperatures, peak dormancy broken at high fire-related temperatures and heat-induced mortality at extreme temperatures. Fire frequency had an overall negative effect on recruitment, with fewer recruits at more frequently burnt sites and this effect appeared to be additive with fire severity.ConclusionOur findings indicate that increased fire frequency poses an ongoing threat to P. bodalla and similar obligate-seeding shrub species. The hump-shaped relationship with fire severity suggests that future large-scale extreme fires will cause seed mortality-induced reduction in recruitment, with the potential to exacerbate the negative effects of high fire frequency. Informed management of threatened species requires detailed knowledge of species responses to multiple fire regime elements, and novel fire response traits like seed dormancy can provide beneficial insights for robust conservation strategies.

book reviews ISSN 1948-6596 The burning bush Fire in Mediterranean Ecosystems: Ecology, Evolution and Management. Jon E. Keeley, William J. Bond. Ross A. Bradstock, Juli G. Pausas & Philip W. Rundel, 2011, Cambridge University Press. 522 pp. US$120 (hardback). ISBN: 978-1-139-15321-8. http://www.cambridge.org This book examines the role that fire plays in the five Mediterranean -type climate (MTC) regions of the world: California, central Chile, the Mediterranean Ba- sin, the south-western portions of the West- ern Cape Province of South Africa, and south -western Western Aus- tralia and portions of South Australia. These re- gions are characterized by winter rains and sum- mer drought and all have vast areas subject to high fire risk. Although the book is focused on the MTC regions, this text is of more general interest: the authors have succeeded in producing an in- sightful study of fire as an important determinant of ecosystem assembly and distribution. The au- thors use the MTC regions as a way to explore the interactions among vegetation, fire regime, cli- mate and geology. The sclerophyllous shrublands within the MTC regions (the Mediterranean-type vegetation) have received a great deal of attention from plant ecologists and evolutionary biologists and have served as some of the textbook examples of con- vergent evolution (e.g. Mooney 1977). Such work necessarily emphasized the broad similarities in climate across these five regions and often as- sumed that climate was the major cause of trait evolution and community composition. One strength of this book is the authors' use of the dissimilarities in environmental factors across these regions to tease apart the potential drivers of vegetation distribution. The authors consider the full range of plant communities found in these regions and also discuss the occurrence of Medi- terranean-type vegetation outside of areas strictly defined by a Mediterranean-type climate. This comparative perspective allows the authors to form some conclusions regarding when winter rain or summer drought are likely to be the impor- tant climate drivers of vegetation and allows them to examine the feedback interaction between fire and plant communities that can create sharp vegetation shifts within a climate region. The book is structured into three sections. The first section provides an overview of ecosys- tem convergence and indicates the major similari- ties and differences in both vegetation and fire across these regions. The chapters on fire regime and on fire-related plant traits in this section pro- vide an excellent, stand-alone, brief introduction to the ecology of plants and fire. The second section includes a chapter on each of the five MTC regions. Another strength of this book is in having the detailed chapters on each region explicitly compare similarities and differences. Some minor stylistic inconsistencies across chapters betray the multiple authored na- ture of the book (with authors based in four of the five MTC regions), but the chapters on individual regions are quite good and each covers the major vegetation types, the floristic composition and evolutionary history of the lineages that comprise the vegetation types, and detailed information on the historical and current role of fire. The final section covers trait evolution, community diversity and fire management. These chapters synthesize and build upon the informa- tion thus far presented. The authors make the strong argument for fire playing a key role in plant evolution and provide a good review of the accu- mulating experimental and comparative evidence for fire adaptations. A chapter on origins of Medi- terranean-type vegetation is especially useful be- cause it not only covers the paleoclimatic and pa- leobotanical information available, but also rigor- ously examines the assumptions inherent in many previous investigations of convergent evolution. As they do throughout the book, the authors here attempt to determine which elements of climate frontiers of biogeography 5.3, 2013 — © 2013 the authors; journal compilation © 2013 The International Biogeography Society

Fire has shaped the evolution of many plant traits in fire-prone environments: fire-resistant tissues with heat-insulated meristems, post-fire resprouting or fire-killed but regenerating from stored seeds, fire-stimulated flowering, release of on-plant-stored seeds, and germination of soil-stored seeds. Flowering, seed release and germination fit into three categories of response to intensifying fire: fire not required, weakly fire-adapted or strongly fire-adapted. Resprouting also has three categories but survival is always reduced by increasing fire intensity. We collated 286 records for 20 angiosperm and two gymnosperm families and 50 trait assignments to dated phylogenies. We placed these into three fire-adapted trait types: those associated with the origin of their clade and the onset of fire-proneness [primary diversification, contributing 20% of speciation events over the last 120millionyears (My)], those originating much later coincident with a change in the fire regime (secondary diversification, 30%), and those conserved in the daughter lineage as already adapted to the fire regime (stabilisation, 50%). All four fire-response types could be traced to >100My ago (Mya) with pyrogenic flowering slightly younger because of its dependence on resprouting. There was no evidence that resprouting was always an older trait than either seed storage or non-sprouting throughout this period, with either/both ancestral or derived in different clades and times. Fire-adapted traits evolved slowly in the Cretaceous, 120-65Mya, and rapidly but fitfully in the Cenozoic, 65-0Mya, peaking over the last 20My. The four trait-types climaxed at different times, with the peak in resprouter speciation over the last 5My attributable to fluctuating growing conditions and increasing savanna grasslands unsuitable for non-sprouters. All experienced a trough in the 40-30-Mya period following a reduction in world temperatures and oxygen levels and expected reduced fire activity. Thick bark and serotiny arose in the Mid-Cretaceous among extant Pinaceae. Heat-stimulated germination of hard seeds is ancestral in the 103-My-old Fabales. Smoke-(karrikin)-stimulated germination of non-hard seeds is even older, and includes the 101-My-old Restionaceae-Anarthriaceae. A smoke/karrikin response is detectable in some fire-free lineages that prove to have a fire-prone ancestry. Among clades that are predominantly fire-prone, absence of fire-related traits is the advanced condition, associated either with increased fire frequency (loss of serotiny and soil storage), or migration to fire-free habitats (loss of thick bark, pyrogenic flowering, serotiny or soil storage). Protea (Africa) and Hakea (Australia) illustrate the importance of stabilisation processes between resprouting/non-sprouting in accounting for speciation events over the last 20My and highlight the frequent interchange possible between these two traits. Apart from Pinus, most ancestral trait reconstruction relative to fire has been conducted on predominantly Southern Hemisphere clades and this needs to be redressed. Despite these limitations, it is clear that fire has had a profound effect on fire-related trait evolution worldwide, and set the platform for subsequent evolution of many non-fire-related traits. Genetics of the triggering mechanisms remain poorly understood, except the karrikin system for smoke-stimulated germination. We exhort biologists to include fire-proneness and fire-related traits in their thinking on possible factors controlling the evolution of plants.

Summary Key uncertainties in anticipating future fire regimes are their sensitivity to climate change, and the degree to which climate will impact fire regimes directly, through increasing the probability of fire, versus indirectly, through changes in vegetation and landscape flammability. We studied the sensitivity of subalpine forest fire regimes (i.e. fire frequency, fire severity) to previously documented climate variability over the past 6000 years, utilizing pollen and macroscopic charcoal from high‐resolution lake‐sediment records in Rocky Mountain National Park, Colorado. We combined data from the four lakes to provide composite records of vegetation and fire history within a 200 km2 study area. Rates of forest burning were relatively complacent to millennial‐scale summer cooling and decreased effective moisture. Mean return intervals between fire episodes, defined over 500‐year periods, generally varied between 150 and 250 years, consistent with tree‐ring‐based estimates spanning recent centuries. Variability around these long‐term means, however, was significantly correlated with variability in summer moisture (i.e. more burning with drier summers), inferred from existing lake‐level and supporting palaeoenvironmental records. The most pronounced change in fire regimes was in response to decreased subalpine forest density ca. 2400 cal. year BP, itself a response to regional cooling. This indirect impact of climate was followed by a decrease in charcoal production per fire, a proxy for crown‐fire severity, while the long‐term rate of burning remained unchanged. Over the last 1500 years, increased summer evaporation and drought frequency were associated with increased fire severity, highlighting a direct link between fire and climate. Synthesis. Subalpine forest fire history reveals complacency and sensitivity of fire regimes to changing vegetation and hydroclimate over the past 6000 years. Complacency is highlighted by non‐varying fire frequency over millennia. Sensitivity is evident through changes in biomass burned per fire (and inferred fire severity), in response to climate‐induced changes in forest density and, more recently, increased summer drought. Overall, the palaeo record suggests that (i) fire severity may be more responsive to climate change than fire frequency in Rocky Mountain subalpine forests and (ii) the indirect impacts of climate on vegetation and fuels are important mechanisms determining fire‐regime response to climate change.

Resistance of Drakensberg grasslands to compositional change depends on the influence of fire-return interval and grassland structure on richness and spatial turnover

The focus of the study is to investigate regional and local past, present and future changes in fire regimes of tropical and subtropical Queensland and shifts in vegetation composition and structure. Fire has been shown to be a significant driver of ecosystem evolution, composition and distribution through its impact on biota. Within Australia fire has long played a role in shaping the landscape, with increased fire frequency, associated with heightened aridity, over the last five million years promoting the expansion of fire adapted sclerophyll vegetation across the continent. Evidence of anthropogenic fires date back to approximately 50 ka (thousand years ago) with the advent of Aboriginal occupation and fire-stick practices, however with the arrival of Europeans there was a decline in fire frequencies, related to fire exclusion that observes an increase in fire intensity and severity.A review of the introduction of tropical African perennial grasses to improve grazing in tropical and semi-arid regions of northern Australia was also undertaken. This introduction has resulted in some exotic grass species such as Gamba grass (Andropogon gayanus), Mission grass (Cenchrus polystachios syn. Pennisetum polystachion) and Guinea grass (Megathyrsus maximus syn. Panicum maximum Jacq. var. trichoglume) becoming invasive pests. Invasion by these exotic grasses has serious implications for ecosystem function, altering fire regime dynamics through increasing the distribution and abundance of fine fuels. With increased fine fuels there is a serious danger that there will be an increase in fire frequency and intensity resulting in higher severity burns and higher vegetation mortality, with possible local species extinctions and habitat modification or change.Macro charcoal and pollen records were used from Fraser Island, subtropical eastern Australia to identify fire and vegetation histories, which show substantial temporal and spatial changes in past fire frequencies and vegetation composition for the last 24,000 years. Pollen records show pyrophobic rainforest taxa dominated and then declined while pyrogenic sclerophyll arboreal taxa increased correlating with an increase in fire frequencies, and a dryer climate. This was followed by a dramatic increase in Restionaceae values at the beginning of the Holocene (~10,000 years ago) that dropped off as a marked peak in mangroves, primarily the Rhizophoraceae and Melaleuca occurred, possibly linked with sea level rise approximately 6000 to 5000 years ago, which was also associated with lower fire frequencies. Restionaceae then recovered from around 2 ka to the European settlement period, when a dramatic change in fire frequency occurred linked to fire suppression and was followed by vegetation thickening (i.e. increase in arboreal taxa) in the mid to late 20th century.Vegetation thickening was investigated on Fraser Island through land change analysis of aerial photographs and survey data between 1958 and 2016 of a wetland system at Moon Point. This was undertaken using the Land Change Modeller (IDRISI TerrSet), with results showing that forest and woodland communities have invaded the fringes of a restiad dominated wetland. Pollen results from adjacent sediment cores support the occurrence of vegetation thickening that appears to be linked to marked changes in fire regimes on the island associated with European management since the 19th century. A projection of further landscape change was made to 2066 and this suggested a 30% loss in wetland extent by this time under present fire frequencies (i.e. with a mean of approximately one fire every plus/minus 12 years).Identifying past and present fire regimes and vegetation composition are important for fire modelling as this provides possible scenario based probabilities for changes in fire frequency and intensity. Modelling is useful in that it provides managers with a tool to ascertain possible scenario based outcomes depending on the input values. Here the FireBGCv2 fire simulation model has been applied to an Australian context to provide a research simulation platform for exploring fire, vegetation, and climate dynamics that can be directly applied to fire management applications.Fire has played an integral role in shaping the Australian landscape, with fire regimes driven by both climatic and anthropogenic factors during the late Quaternary period. Evidence of shifts in vegetation and fire regimes for the subtropics of Australia can be seen from pollen and charcoal analysis, with dramatic changes occurring over the past 24,000 years on Fraser Island. With the arrival and settlement of Europeans in the mid19th century, fire regimes were once again changed that resulted in further vegetation shifts due to a fire exclusion policy. Further shifts in fire regimes can be seen in tropical northern Australia through the introduction on invasive grasses increasing fuel loads and fire frequency, resulting in a transformed landscape, perpetuating a fire grass cycle. However in the subtropics alterations in fire management have seen a reduction in fire frequency with a thickening of vegetation along the ecotone of E. minus wetlands and sclerophyllous forests at Moon Point on Fraser Island. The complexities of fire regimes for managers is obvious, therefore there is a need for a dynamic mechanistic fire simulation model that managers can use as a tool to project present and future fire events.

Predicting how belowground carbon storage reflects changes in aboveground vegetation biomass is an unresolved challenge in most ecosystems. This is especially true for fire-prone savannas, where frequent fires shape the fraction of carbon allocated to root traits for post-fire vegetation recovery. Here I review evidence on how root traits may respond to frequent fires and propose to leverage root traits to infer belowground carbon dynamics in fire-prone savannas. Evidently, we still lack an understanding of trade-offs in root acquisitive vs. conservative traits in response to frequent fires, nor have we determined which root traits are functionally important to mediate belowground carbon dynamics in a frequently burned environment. Focusing research efforts along these topics should improve our understanding of savanna carbon cycling under future changes in fire regimes.

Both C3 and C4 photosynthetic pathways and smoke‐released seed dormancy occur among grasses. C4 species evolved from C3 species as seasonality and fire frequency increased and might therefore imply that their smoke sensitivity increased. I searched the worldwide literature for reports on germination responses among grasses, whose photosynthetic pathway was known, to treatment by smoke. Data were obtained for 217 species and 126 genera. While subfamilies tended to be C3 (Pooideae), C4 (Chloridoideae) or a mixture (Panicoideae), a beneficial smoke response was independent of their photosynthetic pathway. The only exceptions were Danthonioideae (C3, non‐smoke responsive) and Triodia (C4, smoke responsive). One third of both C3 and C4 genera were smoke responsive. Even within genera, 90% of species showed contrasting smoke responses, confirming that smoke sensitivity is rarely taxonomically constrained. Data on photosynthetic pathway, climate, fire regime and vegetation were compiled for 15 regions that formed four distinct groups: 1) In warm climates with aseasonal rainfall, C4 grasses are moderately better represented, with crown fires and limited smoke responses. 2) In cool regions, most species are C3, with surface‐crown fires and lack smoke responses. 3) In warm regions with summer rain (savannas), most species are C4, with surface fires and lack smoke responses. 4) In Mediterranean‐climate regions with summer drought, most species are C3, with crown fires and smoke‐released dormancy. Thus, even though C3 and C4 grasses are equally capable of expressing smoke sensitivity, their response depends on the region’s climate and fire regime that also dictate which photosynthetic pathway dominates.

<p>Boreal forests are among the ecosystems most significantly impacted by wildfires as a consequence of climate warming. A large proportion of the global boreal forest area is located in Siberia, however, its vast extent and restricted access limit datasets recording changes in wildfire activity, especially from a longer-term perspective. Such long-term records of wildfire activity are vital to understanding how fire regimes vary with changes in climate, vegetation composition and human-vegetation interaction, as well as the impacts of wildfires on boreal forests.</p><p>Here, we explore how patterns in fire regime (biomass burned, fire frequency, fire type) have changed over the Holocene. We focus on the relationship between fire regime, forest density and the fire-related traits of the main tree species, and peatland hydrology. We used charcoal-morphologies based reconstructions of fire regimes, along with pollen-based assessments of vegetation composition and testate amoebae-based hydro-climate reconstructions in Pinus-Betula dominated peatlands from central-western Siberia, Tomsk Oblast, Russia.</p><p>The occurrence of more severe fires (i.e., higher biomass burning per fire episode and abundant woody morphotypes) were recorded between 7500 and 5000 cal yr BP. Higher temperatures during that time, likely enhanced peatland dryness and fuel flammability creating conditions conducive to peat and forest fires. Drier peatland conditions also affected forest composition and density by favouring the expansion of a mix of light taiga and fire resisters (e.g., Pinus sylvestris, P. sibirica, Larix) with denser taiga and fire avoiders (Picea obovata and Abies sibirica) on the peatland. A shift to the lowest biomass burning and fire types affecting mostly litter and understorey vegetation, was registered between 4000 and 1500 cal yr BP. Temporally, it coincides with an increase in peatland surface moisture and a change in forest composition characterised by a decline in fire resisters, while fire avoiders remained abundant. An almost synchronous intensification in fires frequency and severity from ca. 2000 cal yr BP to the present at all sites, was concurrent with the rise to dominance of fire-invader species (Betula), as well as a more abundant biomass in the understory layer (shrubs, herbs, ferns, moss), while fire resisters and avoiders declined substantially. We found that Picea obovata to be highly vulnerable tree taxa to frequent, severe fires.</p><p>This long-term perspective demonstratesthat peatland hydrology is connected to, and feedbacks on peatland and forest composition and fuel dryness and ultimately fire regime. It also shows that more frequent fires of higher severity can lead to compositional or structural changes of forests, if trees cannot reach reproductive ages prior to the next burning events. Future predicted increases in temperatures are likely to enhance peatland drying, with cascading effects on forest and peat plant composition, subsequently exacerbating wildfire activity. This study thus contributes to an understanding of disturbance regimes in boreal forests and considers their potential to adapt to new climate conditions and fire regimes.</p><p> </p>

Forest fires resulting from long periods of drought cause extensive forest ecosystemdestruction and can impact on the carbon balance and air quality and feed back to theclimate system, regionally and globally. Past fire frequency is reconstructed for TuvanScots pine stands using dendrochronology and statistics. Central Tuvan Scotspine (Pinus sylvestris) stands are subject to annual fire regimes; however highintensity fires are rare but they are responsible for most of the damage. Low,medium, and high severity fires have shaped the multi-story Scots pine communities,locally and regionally. Fire type and frequency are directly related to weather andclimate and are also dependent on anthropogenic influences. The primary dryperiod, which promotes fire ignition and spread, in Tuva occurs in April andMay. In some years, the precipitation deficit combined with high air temperaturesinduces long periods of drought. Unlike the typical surface fire regime, forest firesthat burn during these extreme droughts often become crown fires that resultin substantial forest damage and carbon release. The mean fire interval (MFI)is found to be 10.4 years in Balgazyn stands, and the landscape-scale MFI is22.4 years. High severity, stand-replacing crown fires have a longer MFI. Thewarmer and dryer weather that is predicted by global climate models is evident inTuva, and we believe that these changes in weather and climate have resulted inincreased fire intensity and severity, rather than fire frequency in the Tuvan region.

Understanding the responses of rare species to altered fire disturbance regimes is an ongoing challenge for ecologists. We asked: are there associations between fire regimes and plant rarity across different vegetation communities? We combined 62 years of fire history records with vegetation surveys of 86 sites across three different dry sclerophyll vegetation communities in Booderee National Park, south-east Australia to: (1) compare associations between species richness and rare species richness with fire regimes, (2) test whether fire regimes influence the proportion of rare species present in an assemblage, and (3) examine whether rare species are associated with particular fire response traits and life history. We also sought to determine if different rarity categorisations influence the associations between fire regimes and plant rarity. We categorised plant rarity using three standard definitions; species' abundance, species' distribution, and Rabinowitz's measure of rarity, which considers a species' abundance, distribution and habitat specificity. We found that total species richness was negatively associated with short fire intervals but positively associated with time since fire and fire frequency in woodland communities. Total species richness was also positively associated with short fire intervals in forest communities. However, rare species richness was not associated with fire when categorised via abundance or distribution. Using Rabinowitz's measure of rarity, the proportion of rare species present was negatively associated with fire frequency in forest communities but positively associated with fire frequency in woodland communities. We found that rare species classified by all three measures of rarity exhibited no difference in fire response traits and serotiny compared to species not classified as rare. Rare species based on abundance differed to species not classified as rare across each life history category, while species rare by distribution differed in preferences for seed storage location. Our findings suggest that species categorised as rare by Rabinowitz's definition of rarity are the most sensitive to the effects of fire regimes. Nevertheless, the paucity of responses observed between rare species with fire regimes in a fire-prone ecosystem suggests that other biotic drivers may play a greater role in influencing the rarity of a species in this system.

Intraspecific trait variability shapes leaf trait response to altered fire regimes.