A comparative assessment of plant flammability through a functional approach: The case of woody species from Argentine Chaco region

Recent changes to fire regimes in many regions of the world have led to renewed interest in plant flammability experiments to understand and predict the consequences of such changes. These experiments require the development of practical and standardised flammability testing protocols. The research aims were (i) to compare plant flammability assessments carried out using two different approaches, namely functional trait analysis and testing with a shoot‐level device; and (ii) to evaluate the effect of disturbances and seasonal variability on flammability. The study area was located in the Western Chaco region, Argentina, and 11 species were selected based on their representativeness in forests. We studied six functional traits related to flammability, growth habit and foliar persistence, in forests without disturbances over the three last decades as well as in disturbed forests. The seasonal variation of these functional traits was evaluated over two consecutive years. Functional trait flammability index (FI) and shoot‐level measurements followed standard protocols. Sixty per cent of the species measured presented a high to very high FI. The results of both assessment methods were significantly correlated. Both methods identified the same species as having medium flammability, but differed in regards to the most flammable species. Senegalia gilliesii was identified as the most flammable species when using functional trait analysis, whereas shoot‐level assessments found Larrea divaricata and Schinus johnstonii to be the most flammable. There were no disturbance effects on the FI but there was seasonal variation. Our results validate the use of functional traits as a predictive method of flammability testing and represent the first global effort comparing flammability obtained through functional trait analysis with empirical measurements. The significant correlation between both methods allows the selection of the one that is more appropriate for the size of the area to be evaluated and for the availability of technical resources. Abstract in Spanish is available with online material.     

How do slow‐growing,fire‐sensitive conifers survive in flammable eucalypt woodlands?

Question: To what extent do low flammability fuel traits enhance the survival and persistence of fire‐sensitive (slowing‐growing, non‐serotinous, non‐resprouting) dominant trees in highly flammable landscapes, under varying fire‐weather conditions? Location: Mixed forests co‐dominated by flammable Eucalyptus species and fire‐sensitive Callitris glaucophylla in Pilliga State Forest, southeast Australia. Methods: The influence of vegetation composition (relative abundance of Callitris and flammable Eucalyptus) on fire intensity and survival of fire‐sensitive Callitris was assessed across gradients of Callitris abundance in mixed Eucalyptus–Callitris forests that burned under low‐moderate and extreme fire‐weather conditions. Results: In areas that burned under low‐moderate fire‐weather conditions, as Callitris abundance increased, fire intensity declined and Callitris survival increased (46%). By comparison, in extreme fire‐weather conditions, lower fire intensity at higher levels of Callitris abundance, was not sufficient to increase Callitris survival (4%). Callitris survival was also positively related to trunk diameter. Ground fuel type, but not biomass, varied with vegetation composition. Conclusions: These results demonstrate that flammable feedbacks, mediated by low flammability fuel traits of dominant trees, can provide an important mechanism for enhancing the survival and persistence of slow‐growing, non‐serotinous, non‐resprouting, fire‐killed trees in highly flammable landscapes. By modifying vegetation and fuel structure, patches of fire‐sensitive Callitris reduce fire intensity, and thereby reduce Callitris mortality, enhancing population persistence. However, this feedback loop is insufficient to ensure Callitris survival under extreme fire‐weather conditions, when fire intensity is greater. After burning, stands remain vulnerable to future fires, until trees grow large enough to modify fuel levels and reduce stand flammability.     

Genetic component of flammability variation in a Mediterranean shrub

Recurrent fires impose a strong selection pressure in many ecosystems worldwide. In such ecosystems, plant flammability is of paramount importance because it enhances population persistence, particularly in non‐resprouting species. Indeed, there is evidence of phenotypic divergence of flammability under different fire regimes. Our general hypothesis is that flammability‐enhancing traits are adaptive; here, we test whether they have a genetic component. To test this hypothesis, we used the postfire obligate seeder Ulex parviflorus from sites historically exposed to different fire recurrence. We associated molecular variation in potentially adaptive loci detected with a genomic scan (using AFLP markers) with individual phenotypic variability in flammability across fire regimes. We found that at least 42% of the phenotypic variation in flammability was explained by the genetic divergence in a subset of AFLP loci. In spite of generalized gene flow, the genetic variability was structured by differences in fire recurrence. Our results provide the first field evidence supporting that traits enhancing plant flammability have a genetic component and thus can be responding to natural selection driven by fire. These results highlight the importance of flammability as an adaptive trait in fire‐prone ecosystems.     

Fuel moisture content enhances nonadditive effects of plant mixtures on flammability and fire behavior

Fire behavior of plant mixtures includes a complex set of processes for which the interactive contributions of its drivers, such as plant identity and moisture, have not yet been unraveled fully. Plant flammability parameters of species mixtures can show substantial deviations of fire properties from those expected based on the component species when burnt alone; that is, there are nonadditive mixture effects. Here, we investigated how fuel moisture content affects nonadditive effects in fire behavior. We hypothesized that both the magnitude and variance of nonadditivity in flammability parameters are greater in moist than in dry fuel beds. We conducted a series of experimental burns in monocultures and 2‐species mixtures with two ericaceous dwarf shrubs and two bryophyte species from temperate fire‐prone heathlands. For a set of fire behavior parameters, we found that magnitude and variability of nonadditive effects are, on average, respectively 5.8 and 1.8 times larger in moist (30% MC) species mixtures compared to dry (10% MC) mixed fuel beds. In general, the moist mixtures caused negative nonadditive effects, but due to the larger variability these mixtures occasionally caused large positive nonadditive effects, while this did not occur in dry mixtures. Thus, at moister conditions, mixtures occasionally pass the moisture threshold for ignition and fire spread, which the monospecific fuel beds are unable to pass. We also show that the magnitude of nonadditivity is highly species dependent. Thus, contrary to common belief, the strong nonadditive effects in mixtures can cause higher fire occurrence at moister conditions. This new integration of surface fuel moisture and species interactions will help us to better understand fire behavior in the complexity of natural ecosystems.     

Fuels and landscape flammability in an Australian alpine environment

Factors governing landscape‐scale flammability are poorly understood, yet critical to managing fire regimes. Studies of the extent and severity of the 2003 Australian alpine fires revealed marked differences in flammability between major alpine plant communities, with the occurrence and severity of fire greater in heathland compared to grassland. To understand this spatial variation in landscape flammability, we documented variation in two physical properties of fuel – load and bulk density – at the life‐form and plant community scale. We measured the load (mass per unit area) and bulk density (mass per unit volume) of fine fuels (<6 mm) at 56 sites across the Bogong High Plains, southeastern Australia. Fine fuel load was positively correlated with shrub cover, and fine fuel bulk density was negatively correlated with shrub cover. Furthermore, fine fuel load and bulk density were accurately predicted using simple measures of canopy height and shrub cover. We also conducted a burning experiment on individual shrubs and snowgrass (Poa spp.) patches to assess comparative differences in flammability between these life‐forms. The burning experiment revealed that shrubs were more flammable than snowgrass as measured by a range of flammability variables. Consequently, our results indicate that treeless alpine landscapes of southeastern Australia are differentially flammable because of inherent life‐form differences in both fine fuel load and bulk density. If shrub cover increases in these alpine landscapes, as projected under climate change, then they are likely to become more flammable and may experience more frequent and/or severe fires.     

Synergistic influences of introduced herbivores and fire on vegetation change in northern Patagonia,Argentina

Question: We investigated how cattle and European hares, the two most widespread exotic herbivores in Patagonia, affect species composition, life‐form composition and community structure during the first 6 years of vegetation recovery following severe burning of fire‐resistant subalpine forests and fire‐prone tall shrublands. We asked how the effects of introduced herbivores on post‐fire plant community attributes affect flammability of the vegetation. Location: Nahuel Huapi National Park, northwest Patagonia, Argentina Methods: We installed fenced plots to exclude livestock and European hares from severely burned subalpine forests of Nothofagus pumilio and adjacent tall shrublands of N. antarctica. The former is an obligate seed reproducer, whereas the latter and all other woody dominants of the shrubland vigorously resprout after burning. Results: Repeated measures ANOVA of annual measurements over the 2001‐2006 period indicate that cattle and hare exclusion had significant but complex effects on the cover of graminoids, forbs, climber species and woody species in the two burned community types. Significant interactions between the effects of cattle and hares varied by plant life forms between the two communities, which implies that their synergistic effects are community dependent. Conclusions: Following severe fires, the combined effects of cattle and hares inhibit forest recovery and favour transition to shrublands dominated by resprouting woody species. This herbivore‐induced trend in vegetation structure is consistent with the hypothesis that the effects of exotic herbivores at recently burned sites contribute to an increase in the overall flammability of the Patagonian landscape.     

Evolutionary history of fire‐stimulated resprouting,flowering, seed release and germination

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 120 million years (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 >100 My 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, 120–65 Mya, and rapidly but fitfully in the Cenozoic, 65–0 Mya, peaking over the last 20 My. The four trait‐types climaxed at different times, with the peak in resprouter speciation over the last 5 My 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 20 My 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.     

Do physical plant litter traits explain non‐additivity in litter mixtures? A test of the improved microenvironmental conditions theory

The decomposition rates of plant litter mixtures often deviate from the averaged rates of monocultures of their component litter species. The mechanisms behind these non‐additive effects in decomposition of litter mixtures are lively debated. One plausible explanation for non‐additive effects is given by the improved microenvironmental condition (IMC) theory. According to this theory, plant litter species, whose physical characteristics improve the microclimatic conditions for decomposers, will promote the decomposition of their co‐occurring litter species. We tested the IMC theory in relation to leaf litter and soil moisture in two contrasting moisture conditions in a dry subarctic mountain birch forest with vascular plant leaf litters of poor and high quality. The non‐additive effects in mass loss of litter mixtures increased when moisture conditions in litter and soil became more favourable for plant litter decomposition. The sign of this increase (antagonistic or synergistic) in non‐additive effects was more predictable for litter mixtures of poor litter quality. Although the specific mechanisms underlying the IMC theory depended on the litter quality of the litter mixtures, a standardized water holding capacity (WHC) was the litter trait most closely related to the non‐additive effects in mixtures of both poor and high quality litter types. Furthermore, we found that higher dissimilarity in WHC traits between the component litter species in a mixture increased synergistic effects in litter mixtures under limiting moisture conditions. However, under improved moisture conditions, increased antagonistic effects were observed. Thus, we found clear support for the IMC theory and showed that climatic conditions and leaf litter physical traits determine whether the non‐additive effects in litter mixtures are antagonistic or synergistic. Our study emphasizes the need to include litter physical traits into predictive models of mixing effects on plant litter decomposition and in general suggests climate specificity into these models.     

Fires without tanoak: the effects of a non-native disease on future community flammability

Non-native pathogens affect forests throughout North America, resulting in changes in species composition, structure, and ecosystem processes. We studied the effects of the emergent disease sudden oak death (SOD) and the resulting functional extinction of tanoak, a highly susceptible native tree with flammable litter, on future community flammability. We quantified four flammability metrics (flame height; flaming duration; smoldering duration; and fuel consumed) for litter from each of five species that co-occur with tanoak and for species mixtures with and without tanoak (n = 14). We combined these flammability metrics in a Principal Components Analysis to evaluate potential shifts in litter flammability from pre-SOD (with tanoak litter) to post-SOD (where tanoak was replaced equally with remaining species). Litter flammability changed following the loss of tanoak, but the differences were species- and community-specific. Mixed-evergreen fuels where coast Douglas-fir litter replaced tanoak experienced consistent declines in flammability. Few substantive changes occurred in communities where coast redwood or other hardwoods co-occurred. We found consistent synergistic flammability effects when multiple species’ litters were mixed; in cases where two or more species were mixed, most (75%) combinations burned with higher intensity (+8.1%) and fuelbeds flamed for a longer duration (+17.1%). Our work demonstrates that potential surface fire behavior in northwestern California may be dampened where Douglas-fir replaces tanoak, while those communities with other hardwoods or coast redwood may have redundant fuels and tanoak’s absence in future fires may be inconspicuous. These results underscore the potential for cascading effects of non-native pathogens on ecosystem function in fire-prone ecosystems.     

Species composition,functional and phylogenetic distances correlate with success of invasive Chromolaena odorata in an experimental test

Biotic resistance may influence invasion success; however, the relative roles of species richness, functional or phylogenetic distance in predicting invasion success are not fully understood. We used biomass fraction of Chromolaena odorata, an invasive species in tropical and subtropical areas, as a measure of ‘invasion success’ in a series of artificial communities varying in species richness. Communities were constructed using species from Mexico (native range) or China (non‐native range). We found strong evidence of biotic resistance: species richness and community biomass were negatively related with invasion success; invader biomass was greater in plant communities from China than from Mexico. Harvesting time had a greater effect on invasion success in plant communities from China than on those from Mexico. Functional and phylogenetic distances both correlated with invasion success and more functionally distant communities were more easily invaded. The effects of plant‐soil fungi and plant allelochemical interactions on invasion success were species‐specific.     

Larger phylogenetic distances in litter mixtures: lower microbial biomass and higher C/N ratios but equal mass loss

Phylogenetic distances of coexisting species differ greatly within plant communities, but their consequences for decomposers and decomposition remain unknown. We hypothesized that large phylogenetic distance of leaf litter mixtures increases differences of their litter traits, which may, in turn, result in increased resource complementarity or decreased resource concentration for decomposers and hence increased or decreased chemical transformation and reduction of litter. We conducted a litter mixture experiment including 12 common temperate tree species (evolutionarily separated by up to 106 Myr), and sampled after seven months, at which average mass loss was more than 50%. We found no effect of increased phylogenetic distance on litter mass loss or on abundance and diversity of invertebrate decomposers. However, phylogenetic distance decreased microbial biomass and increased carbon/nitrogen (C/N) ratios of litter mixtures. Consistently, four litter traits showed (marginally) significant phylogenetic signal and in three of these traits increasing trait difference decreased microbial biomass and increased C/N. We suggest that phylogenetic proximity of litter favours microbial decomposers and chemical transformation of litter owing to a resource concentration effect. This leads to a new hypothesis: closely related plant species occurring in the same niche should promote and profit from increased nutrient availability.     

Plant domestication disrupts biodiversity effects across major crop types

Plant diversity fosters productivity in natural ecosystems. Biodiversity effects might increase agricultural yields at no cost in additional inputs. However, the effects of diversity on crop assemblages are inconsistent, probably because crops and wild plants differ in a range of traits relevant to plant–plant interactions. We tested whether domestication has changed the potential of crop mixtures to over‐yield by comparing the performance and traits of major crop species and those of their wild progenitors under varying levels of diversity. We found stronger biodiversity effects in mixtures of wild progenitors, due to larger selection effects. Variation in selection effects was partly explained by within‐mixture differences in leaf size. Our results indicate that domestication might disrupt the ability of crops to benefit from diverse neighbourhoods via reduced trait variance. These results highlight potential limitations of current crop mixtures to over‐yield and the potential of breeding to re‐establish variance and increase mixture performance.     

Plant functional traits along environmental gradients in seasonally dry and fire‐prone ecosystem

Questions: How does the abundance and richness of plant assemblages with different functional (regeneration and nutrient acquisition) traits vary with fire regime, moisture availability and substrate fertility? What is the role of different functional traits in maintaining plant diversity under changing environmental conditions in seasonally dry and fire‐prone environments? Location: Southwest Western Australia. Methods: Plant species abundance and soil nutrients were determined at 16 forest sites with variable fire histories across an aridity gradient. All plant species were classified based on their functional traits as (1) perennial or annual, (2) ectomycorrhizal, arbuscular mycorrhizal, ericoid mycorrhizal, orchid mycorrhizal, proteoid or other non‐mycorrhizal, (3) resprouters or seeder, and (4) nitrogen fixer or non‐fixer. We used a multivariate (fourth‐corner) technique to simultaneously test the significance and direction of the relationship between each of these traits and fire frequency, fire interval length, aridity, and soil N, P and C fractions. Results: The functional response of the vegetation to fire regime was minor and restricted to annual species, which comprised only ～4% of taxa. Proteoid and ectomycorrhizal species dominated over species with arbuscular and orchid mycorrhizal roots, N‐fixers dominated over non‐fixers, and seeders dominated over resprouters when N fertility was low but organic labile P was high. Further, proteoid and ectomycorrhizal species richness increased with aridity, while arbuscular mycorrhizal species richness decreased. Conclusions: While the functional composition of southwest Australian vegetation is largely insensitive to changes in fire regime, nutrient acquisition and, to a lesser extent, regeneration traits provide mechanisms for the vegetation community to adjust to changes in resource availability. Thus, diversity responses to environmental change in seasonally dry and fire‐prone ecosystems are likely to be primarily mediated by the composition of nutrient acquisition traits in the vegetation community.     

Phylogenetic structure of Brazilian savannas under different fire regimes

Questions: Fire is a strong filter in fire‐prone communities and is expected to assemble closely related species when functional traits are conserved in plant lineages. Do frequent fires assemble savannas with closely related species (phylogenetic clustering)? If so, what are the clades pruned by fire in the phylogenetic trees? Are species of semi‐deciduous seasonal forests, where fires are not frequent, less related than expected by chance (phylogenetic over‐dispersion)? Are life forms conserved in the phylogeny of the species? Location: Central and SE Brazilian savannas (Emas National Park, 18°18′S, 52°54′W; Brasília, 15°56′–15°57′S, 47°53′–47°56′W and Corumbataí‐Itirapina, 22°13′–22°15′S, 47°37′–47°39′W); and close semi‐deciduous seasonal forests (in Pirenópolis, 15°45′S, 49°04′W; Brasília, 15°33′S, 47°51′W; and São Carlos, 21°55′S, 47°48′W). Methods: We recorded woody species in savannas under different fire regimes and in semi‐deciduous seasonal forests. We obtained data from the literature and from field sampling. We compared mean phylogenetic distance of species of savanna and of nearby semi‐deciduous seasonal forest sites. We obtained significance by randomizing the species among the tips of phylogenetic trees. We also assessed whether life forms were evolutionary conserved across phylogeny of the studied plants (phylogenetic signal) with tests based on the variance of phylogenetic independent contrasts. Results: Some sites of savanna under high fire frequency were characterized by phylogenetic over‐dispersion of woody species whereas, in contrast, some sites of semi‐deciduous seasonal forest were characterized by phylogenetic clustering. We found phylogenetic signals in the traits across the phylogeny of the 801 species investigated. Conclusion: Fire may have different roles in assembling plant species in Brazilian savannas than in other fire‐prone communities. We postulate that the absence of phylogenetic clustering in the cerrado is mainly due to the persistence of long‐lived resprouting species from different plant lineages.     

Phylogenetic community structure and phylogenetic turnover across space and edaphic gradients in western Amazonian tree communities

Ecological and evolutionary processes influence community assembly at both local and regional scales. Adding a phylogenetic dimension to studies of species turnover allows tests of the extent to which environmental gradients, geographic distance and the historical biogeography of lineages have influenced speciation and dispersal of species throughout a region. We compare measures of beta diversity, phylogenetic community structure and phylobetadiversity (phylogenetic distance among communities) in 34 plots of Amazonian trees across white‐sand and clay terra firme forests in a 60 000 square kilometer area in Loreto, Peru. Dominant taxa in white‐sand forests were phylogenetically clustered, consistent with environmental filtering of conserved traits. Phylobetadiversity measures found significant phylogenetic clustering between terra firme communities separated by geographic distances of <200–300 km, consistent within recent local speciation at the watershed scale in the Miocene‐aged clay‐soil forests near the foothills of the Andes. Although both distance and habitat type yielded statistically significant effects on both species and phylogenetic turnover, the patterns we observed were more consistent with an effect of habitat specialization than dispersal limitation. Our results suggest a role for both broad‐scale biogeographic and evolutionary processes, as well as habitat specialization, influencing community structure in Amazonian forests.     

Soil conditioning effects of Phragmites australis on native wetland plant seedling survival

Interactions between introduced plants and soils they colonize are central to invasive species success in many systems. Belowground biotic and abiotic changes can influence the success of introduced species as well as their native competitors. All plants alter soil properties after colonization but, in the case of many invasive plant species, it is unclear whether the strength and direction of these soil conditioning effects are due to plant traits, plant origin, or local population characteristics and site conditions in the invaded range. Phragmites australis in North America exists as a mix of populations of different evolutionary origin. Populations of endemic native Phragmites australis americanus are declining, while introduced European populations are important wetland invaders. We assessed soil conditioning effects of native and non‐native P. australis populations on early and late seedling survival of native and introduced wetland plants. We further used a soil biocide treatment to assess the role of soil fungi on seedling survival. Survival of seedlings in soils colonized by P. australis was either unaffected or negatively affected; no species showed improved survival in P. australis‐conditioned soils. Population of P. australis was a significant factor explaining the response of seedlings, but origin (native or non‐native) was not a significant factor. Synthesis: Our results highlight the importance of phylogenetic control when assessing impacts of invasive species to avoid conflating general plant traits with mechanisms of invasive success. Both native (noninvasive) and non‐native (invasive) P. australis populations reduced seedling survival of competing plant species. Because soil legacy effects of native and non‐native P. australis are similar, this study suggests that the close phylogenetic relationship between the two populations, and not the invasive status of introduced P. australis, is more relevant to their soil‐mediated impact on other plant species.     

Fire severity mediates climate‐driven shifts in understorey community composition of black spruce stands of interior Alaska

Question: How do pre‐fire conditions (community composition and environmental characteristics) and climate‐driven disturbance characteristics (fire severity) affect post‐fire community composition in black spruce stands? Location: Northern boreal forest, interior Alaska. Methods: We compared plant community composition and environmental stand characteristics in 14 black spruce stands before and after multiple, naturally occurring wildfires. We used a combination of vegetation table sorting, univariate (ANOVA, paired t‐tests), and multivariate (detrended correspondence analysis) statistics to determine the impact of fire severity and site moisture on community composition, dominant species and growth forms. Results: Severe wildfires caused a 50% reduction in number of plant species in our study sites. The largest species loss, and therefore the greatest change in species composition, occurred in severely burned sites. This was due mostly to loss of non‐vascular species (mosses and lichens) and evergreen shrubs. New species recruited most abundantly to severely burned sites, contributing to high species turnover on these sites. As well as the strong effect of fire severity, pre‐fire and post‐fire mineral soil pH had an effect on post‐fire vegetation patterns, suggesting a legacy effect of site acidity. In contrast, pre‐fire site moisture, which was a strong determinant of pre‐fire community composition, showed no relationship with post‐fire community composition. Site moisture was altered by fire, due to changes in permafrost, and therefore post‐fire site moisture overrode pre‐fire site moisture as a strong correlate. Conclusions: In the rapidly warming climate of interior Alaska, changes in fire severity had more effect on post‐fire community composition than did environmental factors (moisture and pH) that govern landscape patterns of unburned vegetation. This suggests that climate change effects on future community composition of black spruce forests may be mediated more strongly by fire severity than by current landscape patterns. Hence, models that represent the effects of climate change on boreal forests could improve their accuracy by including dynamic responses to fire disturbance.     

Prescribed Fire and Herbicide Effects on Soil Processes During Barrens Restoration

Prescribed fire has become a common tool of natural area managers for removal of non‐indigenous invasive species and maintenance of barrens plant communities. Certain non‐native species, such as tall fescue (Festuca arundinacea), tolerate fire and may require additional removal treatments. We studied changes in soil N and C dynamics after prescribed fire and herbicide application in remnant barrens in west central Kentucky. The effects of a single spring burn post‐emergence herbicide, combined fire and herbicide treatments, and an unburned no‐herbicide control were compared on five replicate blocks. In fire‐plus‐herbicide plots, fescue averaged 8% at the end of the growing season compared with 46% fescue cover in control plots. The extent of bare soil increased from near 0 in control to 11% in burned plots and 25% in fire‐plus‐herbicide plots. Over the course of the growing season, fire had little effect on soil N pools or processes. Fire caused a decline in soil CO₂ flux in parallel to decreased soil moisture. When applied alone, herbicide increased plant‐available soil N slightly but had no effect on soil respiration, moisture, or temperature. Fire‐plus‐herbicide significantly increased plant‐available soil N and net N transformation rates; soil respiration declined by 33%. Removal of non‐native plants modified the chemical, physical, and biological soil conditions that control availability of plant nutrients and influence plant species performance and community composition.     

Contrasting effects of fire frequency on plant traits of three dominant perennial herbs from Chaco Serrano

Fire frequencies are currently increasing in many regions across the world as a result of anthropic activities, affecting ecological processes and plant population dynamics. Fire can generate important changes in soil properties, altering nutrient dynamics and thereby plant growth. Here we analyse fire frequency effects on soil quality and plant traits of three native perennial herbaceous plants (Cologania broussonetii, Desmodium uncinatum and Rhynchosia edulis; Fabaceae) with the capacity for biological N₂ fixation that resprouts and are abundant after fire in Chaco Serrano forests. Based on 22‐year fire history, we assessed plant traits in sites with low and high fire frequencies along with unburned scenarios. We found significantly lower water content, nitrates and electrical conductivity in frequently burned soils. As a result, the three species showed consistently lower leaf area and specific leaf area in both fire frequencies, implying lower growth rates in comparison to unburned sites. However, total leaf biomass was not affected by fire, leaf phosphorus concentration was higher in R. edulis in high fire frequency and leaf N concentration was twice as large in plants growing in sites of high fire frequency in C. broussonetii and R. edulis. Such an increase in N and phosphorus concentrations is likely a result of both their conservative use of resources and their biological N₂ fixation capacity. To our knowledge, this is the first record of such contrasting fire effects observed consistently in three co‐occurring species: while plant growth decreased with fire frequency, leaf nutritional traits remain unchanged or increased in frequently burned sites. Quality‐depleted and drier soils that result from increased fire frequencies may not only affect trait variation at the intraspecific level but can also drive to a homogenization of the plant community, selecting species with particular combinations of morphological and functional traits.     

Phylogenetic distance among beneficiary species in a cushion plant species explains interaction outcome

Determining which drivers lead to a specific species assemblage is a central issue in community ecology. Although many processes are involved, plant–plant interactions are among the most important. The phylogenetic limiting similarity hypothesis states that closely related species tend to compete stronger than distantly related species, although evidence is inconclusive. We used ecological and phylogenetic data on alpine plant communities along an environmental severity gradient to assess the importance of phylogenetic relatedness in affecting the interaction between cushion plants and the whole community, and how these interactions may affect community assemblage and diversity. We first measured species richness and individual biomass of species growing within and outside the nurse cushion species, Arenaria tetraquetra. We then assembled the phylogenetic tree of species present in both communities and calculated the phylogenetic distance between the cushion species and its beneficiary species, as well as the phylogenetic community structure. We also estimated changes in species richness at the local level due to the presence of cushions. The effects of cushions on closely related species changed from negative to positive as environmental conditions became more severe, while the interaction with distantly related species did not change along the environmental gradient. Overall, we found an environmental context‐dependence in patterns of phylogenetic similarity, as the interaction outcome between nurses and their close and distantly‐related species showed an opposite pattern with environmental severity.