Litter Flammability Research Articles

Remotely sensed and ground measurements reveal intraspecific differences in early season needle unfolding and senescence, but lack of variability in litter flammability of Pinus halepensis

Pinus halepensis Mill. is a conifer typical of Mediterranean pinewoods adapted to drought and fire. Although the timing of seasonal events in P. halepensis is strongly associated with these stressors and other environmental factors, needle phenology and litter flammability are insufficiently characterized at the intraspecific level. These traits can be informative of the existence of locally adapted populations across the species’ distribution range, and remote sensing approaches are promising tools to infer phenological changes in forest trees. In this study, we investigated intraspecific differentiation of P. halepensis related to needle phenology at the onset of the fire season (May-June) through vegetation indexes (VIs) obtained from unmanned aerial vehicles. We collected drone images using multispectral and RGB sensors for 56 adult populations of P. halepensis categorized into five ecotypes growing in two common gardens located in Spain under contrasting conditions (dry-continental versus wet-coastal). In the dry trial, we additionally monitored the temporal variation of RGB-derived indexes using two flights spaced over a one-month period. We also performed four consecutive ground measurements of needle pigments and fuel moisture content and obtained litter flammability traits for a subset of populations. Most in situ measurements and flammability traits did not show population differentiation. Regarding RGB-derived VIs, we did not detect obvious temporal patterns which differed among populations. However, we observed greener canopies in June than in May, which are indicative of the pace of current-year needle development. We also deduced an earlier phenology (i.e., earlier current-year needle unfolding) in the dry-continental trial. Ecotypic differentiation was found for some vegetation indexes related to needle unfolding (i.e., TCARI/OSAVI) and old needle senescence (i.e., PRSI). These differences were generally consistent across trials and time, and fundamentally indicated that sub-humid ecotypes typical of the eastern Mediterranean showed earlier needle unfolding and old needle senescence than semiarid ecotypes thriving in western continental Mediterranean areas. The intraspecific divergence observed in early season phenology is potentially related to the existence of contrasting life-history strategies present at the intraspecific level for the species. In particular, some semi-arid ecotypes, which are known to optimize the trade-off between carbon gain and water loss, exhibited an early old needle senescence (high likelihood of crown fire development) coupled with a late needle unfolding (conservative water use). These results indicate a possible trade-off between drought and fire resistance for the species, which suggest the importance of considering the intraspecific characteristics of Aleppo pine in forest management actions.

Evaluation of Litter Flammability from Dominated Artificial Forests in Southwestern China

Southwestern China has a large area of artificial forests and has experienced massive environmental and social losses due to forest fires. Evaluating the flammability of fuels from dominated forests in this region can help assess the fire risk and predict potential fire behaviors in these forests, thus guiding forest fire management. However, such studies have been scarcely reported in this region. In this study, the flammability of litter from nine forest types, which are common in southwestern China, was evaluated by measuring organic matter content, ignition point, and calorific value. All these flammability characteristics of fuels varied significantly across forest types. By using principal component analysis and K-means clustering, litters were classified into three groups: highly susceptible to ignition with low fire intensity (Pinus densata, Pinus densata-Populus simonii, Pinus yunnanensis, Larix gmelini, Pinus armandii), less susceptible to ignition with high fire intensity (Abies fabri-Populus simonii), and median ignitibility and fire intensity (Abies fabri, Abies fabri-Picea asperata, Platycladus orientalis). Our study can help predict the risk and intensity of fires in the studied forests and serve as a source of information for fire management in southwestern China.

Multivariate roles of litter traits on moisture and flammability of temperate northeastern North American tree species

BackgroundTree litter is the primary fuel affecting surface fire behavior in most fire-prone forest and woodland ecosystems in northeastern North America. Fire exclusion and land use changes have dramatically altered fire regimes in many of these ecosystems, shifting toward species that reinforce the continued diminution of fire. Comparative litter flammability studies provide a useful approach to identify the corresponding leaf and litter bed traits that drive the positive feedback termed “mesophication” in these ecosystems.ResultsIn this study, we measured laboratory flammability and moisture characteristics of leaf litter from 17 common tree species in temperate forests of northeastern North America. Principal components analysis of flammability and moisture data resulted in species separating along a pyrophyte-to-mesophyte gradient. Pyrophytic Quercus litter was the most flammable of all species, and Pinus litter dried the most rapidly and flamed for the longest duration. The remaining mesophytic species grouped into either a high moisture retention-poor burning cluster (Acer rubrum, Betula alleghaniensis, Nyssa sylvatica, Carya glabra, C. tomentosa) or an intermediate flammability cluster (Acer saccharum, C. ovata, Tilia americana). Several litter traits were associated with multivariate factors. Leaf length, density, curling, surface area-to-volume, and litter bed bulk density were associated with the factor comprising saturation moisture content, flaming time, and consumption. Specific leaf area was related to moisture time-lag and flame height. Litter bed packing ratio was most related to smoldering duration.ConclusionsInterspecific variability in leaf and litter bed traits explained substantial differences in moisture dynamics and flammability among the species examined. These findings extend our understanding of litter traits’ influence on the combustion environment and affirm the potential effects of pyrophytic and mesophytic tree species on contemporary fire regimes in northeastern North American forests.

Experimental evidence that leaf litter decomposability and flammability are decoupled across gymnosperm species

Abstract Biological decomposition and wildfire are two predominant and alternative processes that can mineralize organic C in forest litter. Currently, the relationships between decomposition and fire are still poorly understood. We provide an empirical test of the hypothesized decoupling of surface litter bed decomposability and flammability, and the underlying traits and trait spectra. We employed a 41‐species set of gymnosperms of very broad evolutionary and geographic spread, because of the wide range of (absent to frequent) fire regimes they are associated with. We found that the interspecific pattern of mass loss proportions in a “common garden” decomposition experiment was not correlated with any of the flammability parameters and an RDA analysis also showed that the decomposability and flammability of leaf litter in litter layers were decoupled across species. This decoupling originates from the former depending mostly on size and shape spectrum traits and the latter on PES traits and those trait spectra being virtually uncorrelated. Synthesis: Our results show that, indeed, leaf litter decomposability and flammability parameters are decoupled across species, and this decoupling can be explained by their different drivers in terms of trait spectra: chemical traits for decomposability and size‐shape traits for litter layer flammability.

Evidence of local adaptation in litter flammability of a widespread fire‐adaptive pine

Abstract Fire is a strong ecological and evolutionary driver of plant species and terrestrial ecosystem dynamics. Numerous studies have shown that plant species vary widely in their flammability and associated traits that coincide with clear strategies of persistence in fire‐prone ecosystems. Yet, intraspecific variation and phenotypic plasticity in plant flammability has received substantially less attention. Some evidence has demonstrated high plasticity in flammability among populations in species adapted to low‐frequency, high severity fire regimes which suggests the capacity of local adaptation; however, intraspecific variability of species in high‐frequency, low severity fire regimes has not been conducted. Given that frequent fire‐adaptive species tend to produce fast‐flammable litter that is associated with survival following fire, we hypothesized that these species would have highly conserved leaf traits with low variation in litter flammability. We used an existing common garden study to test our hypothesis in Pinus rigida (pitch pine) with source populations that span most of its distribution in eastern North America. We examined both phenotypic and genotypic variation as well as phenotypic plasticity (relative distance plasticity index) in P. rigida leaf traits and litter flammability among populations grown in situ (population) and the same populations grown in a common garden or provenance study (provenance). Phenotypic and genotypic variation in fuel traits and litter flammability differed among source locations. Phenotypic plasticity in litter flammability among source locations were associated with fuel trait plasticity and biogeographic factors. Populations located in warmer and drier climates with presumably more frequent historical fire regimes were associated with greater plasticity in physical litter traits and their resulting flammability. Synthesis. Our study demonstrates the utility of common garden experiments to examine intraspecific variation and phenotypic plasticity in litter flammability. Our results indicate that litter flammability in Pinus rigida has the capacity for local adaptation. We hypothesize that intraspecific variation in plant flammability provides ecologically and evolutionary relevant information and that other frequent fire‐adaptive species will similarly demonstrate local adaptation in plant flammability.

Effect of Fire Frequency on the Flammability of Two Mediterranean Pines: Link with Needle Terpene Content

Flammability is a major factor involved in Mediterranean plant evolution that has led to the diversity of fire-related traits according to fire regimes and fire-adaptive strategies. With on-going climate change, new fire regimes are threatening plant species if they do not adapt or acclimate. Studying flammability and terpene content variation according to the different fire frequencies in the recent fire history represents a great challenge to anticipating the flammability of ecosystems in the near future. The flammability of shoots and litter as well as the needle terpene contents of two pine species with different fire adaptive strategies (Pinus halepensis and Pinus sylvestris) were measured according to two fire modalities (0 vs. 1–2 fire events over the last 60 years). Results showed that, regardless of the species and the fuel type, flammability was higher in populations having undergone at least one past fire event even when factors influencing flammability (e.g., structural traits and hydric content) were considered. The terpene content did not vary in P. sylvestris’ needles according to the fire modality, but that of sesqui- and diterpenes was higher in P. halepensis’ needles sampled in the “Fire” modality. In addition, associations made between flammability and terpene content using random forest analyses indicated that the terpene molecules differed between fire modalities for both species and fuel types. The same results were obtained with significant terpenes driving flammability as were highlighted in the PLS analyses, especially for P. halepensis for which enhanced shoot flammability in the “Fire” modality agreed with the adaptive strategy of this species to fire.

Litter Flammability of 50 Southeastern North American Tree Species: Evidence for Mesophication Gradients Across Multiple Ecosystems

Widespread fire exclusion and land-use activities across many southeastern United States forested ecosystems have resulted in altered species composition and structure. These changes in composition and structure have been implicated in positive fire-vegetation feedbacks termed “mesophication” where fire spread and intensity are diminished. In forests and woodlands, inherent flammability of different species is the mechanistic driver of mesophication. To date, there has been limited work on documenting the high diversity of flammability among species in the region, limiting the ability to differentiate among species to restore fuels that sustain fire regimes. Here, we coalesce disparate flammability data and add missing species across the spectrum from species that facilitate fire (so called “pyrophytes”) to those that dampen fire (so called “mesophytes”). We present data on 50 important tree species from across the southeast, all burned using identical laboratory methods. We divide our results for four dominant ecosystems: Coastal Plain uplands, oak-hickory woodlands, Appalachian forests, and bottomland forests. Across ecosystems, the most flammable species were American chestnut (Castanea dentata), a suite of pines (Pinus palustris, P. elliottii, P. serotina, and P. rigida), several oaks (Q. laevis, Q. falcata, Q. margaretta, and Q. alba), and sourwood (Oxydendrum arboreum). At the mesophytic end, the least flammable species were Tsuga canadensis, Acer rubrum, and several other hardwoods previously implicated in mesophication. Each of the four ecosystems we studied contained species that spanned the pyrophytic to mesophytic gradient. These data fill in some key holes in our understanding of southeastern fire adaptations, but also provide context for restoration decisions and fire management prioritization efforts to restore and sustain fire-prone ecosystems of the region.

Litter trait driven dampening of flammability following deciduous forest community shifts in eastern North America

Global change has contributed to substantial vegetation shifts across many ecosystems through altered fire-vegetation feedbacks, but the direction, strength, and mechanism of these processes vary considerably. Most prior studies have focused on fire-vegetation feedbacks that enhance community flammability with less emphasis on examples of feedbacks resulting from dampened flammability. Widespread fire exclusion in many deciduous forests and woodlands of eastern North America has caused substantive changes in overstory species composition. These changes have led to feedbacks that are hypothesized to decrease community flammability (i.e. mesophication) to the detriment of fire-dependent “pyrophytes”. Mesophytic species, such as maple (Acer spp.), are implicated in these fire regime changes and have extensively invaded many oak (Quercus spp.) landscapes across the region. To date, only limited circumstantial evidence is available in support of dampened flammability as the mechanism contributing to mesophication. Here we use laboratory experiments to determine whether red maple (A. rubrum) and sugar maple (A. saccharum) could reduce forest floor flammability through the input of their leaf litter. Wetting, drying, and burning characteristics of three pyrophytic oaks, white oak (Q. alba), northern red oak (Q. rubra), and chestnut oak (Q. montana), and the two mesophytic maples revealed the strong potential for red maple to reduce community flammability. Red maple absorbed the greatest amount of water, was the slowest to dry, and was the least flammable among these species while white oak was the most resistant to water absorption, the fastest to dry, and the most flammable. Northern red oak, chestnut oak, and sugar maple were intermediate in their moisture dynamics and flammability. The moisture and flammability responses of the species examined were a result of their corresponding leaf traits, with drying rate and curling having the strongest influence. Our findings provide substantial evidence that vegetation shifts resulting from an altered fire regime could result in dampened litter flammability that further limit fire and reinforce on-going changes.

Shifting tree species composition of upland oak forests alters leaf litter structure, moisture, and flammability

In historically open-canopied and fire-dependent upland oak (Quercus spp.) forests of the central and eastern United States, fire exclusion is contributing to an increase in competing non-oak tree species that are often shade-tolerant and fire-sensitive. As these non-oak species encroach and oak abundance declines, forests are becoming denser and will likely become cooler, moister, and less flammable through a hypothesized feedback loop termed mesophication. To better understand how this gradual shift in forest composition could affect flammability of leaf litter, the primary fuel in these systems, we measured leaf litter traits and moisture dynamics of two pyrophytic oaks and three non-oaks, and implemented experimental burns on plots (~3-m2) with fuel beds comprised of single- and mixed-species leaf litter. Non-oaks produced thinner, smaller leaves with a greater specific leaf area compared to oaks, and had a higher initial fuel moisture content, traits associated with low flammability. Our experimental burns confirmed that non-oak leaf litter flammability was lower than that of oaks and that flammability decreased linearly with increasing non-oak leaf litter contribution to the fuel bed. These species-driven changes in fuelbed flammability may provide a mechanism whereby encroaching non-oak tree species create self-promoting conditions that are less favorable for regeneration of fire-dependent upland oaks. Thus, without other management interventions, our ability to reintroduce fire into these systems as a management tool to prevent further compositional shifts and improve oak regeneration will likely decline as non-oak species’ contribution to the fine fuel bed increases.

Leaf traits predict global patterns in the structure and flammability of forest litter beds

Abstract Fallen plant material such as leaves, needles and branches form litter beds which strongly influence fire ignition and spread. Traits of the dominant species influence litter flammability directly by determining how individual leaves burn and indirectly through the structure of the litter bed. However, we are yet to determine the relative importance of these different drivers across a range of plant species from different biomes. We undertook a meta‐analysis, combining leaf trait, litter structure and flammability data for 106 species from North America, South America, Europe, Asia and Australia. The dataset encompassed broad‐leaved and coniferous species from seven different experimental studies. Relationships between leaf traits, litter structure and key flammability metrics—sustainability, combustibility and consumability—were analysed using bivariate and piecewise structural equation modelling (SEM). Traits which characterise the three‐dimensional nature of the leaf and how much space a leaf occupies showed much stronger associations to litter structure and flammability than other morphological traits. Leaf curl, surface area to volume ratio (SAV) and SLA predominately influence litter flammability indirectly via litter structure with SLA being the only leaf trait which had a negative direct effect on flame duration. Packing ratio and bulk density were influenced by different combinations of leaf traits and, in turn, they aligned with different flammability metrics. Bulk density predicted flame spread rate and flame duration whereas packing ratio predicted consumption. Synthesis. We identified key leaf and litter traits which influence different components of litter bed flammability. Importantly, we show that the effects of these leaf and litter traits are consistent across a wide range of taxa and biomes. Our study represents a significant step towards developing trait‐based models for predicting surface wildfire behaviour. Such models will more flexibly accommodate future shifts in the composition of plant species triggered by altered fire regimes and climate change.

Moisture absorption and drying alter nonadditive litter flammability in a mixed conifer forest

Abstract Moisture content is a strong determinant of forest fuel flammability. However, little is known about the mechanisms by which species‐specific litter traits influence litter moisture absorption and retention. Characterizing how litter fuels retain moisture through time and how fuels from multiple species interact are key steps to adding taxon‐specific information to fire behaviour prediction. To fill these gaps in knowledge, we determined moisture absorption and retention characteristics of the litter from multiple tree species, investigated what litter traits influenced these processes and tested for nonadditive moisture dynamics and nonadditive flammability in multi‐species mixtures. The litter from eight tree species of a mixed conifer forest was saturated and allowed to dry to determine moisture absorption capacity and drying rates. Burn trials were performed in litter beds of single species and of mixtures to establish flammability response to the dry‐down process and to species interactions. Litter mixtures exhibited nonadditivity in flame spread rate and in fuel consumption. This nonadditivity became more positive with fuel drying. Moisture dynamics varied across species with higher specific leaf area being associated with higher maximum absorption and lower litter bulk density being associated with faster drying rates. We found that these differing moisture dynamics resulted in time since wetting having strongly differing effects on flammability across the major litter types typified by different tree taxa. Synthesis. This study is the first to examine ecologically relevant drying rates and their interaction with nonadditive mixture effects on flammability. We find that taxon specific patterns of moisture absorption and drying change flammability rankings under moist compared with dry fuels because traits that increase flame spread rate increase water absorption. Litter mixtures generally burn with fire behaviour more similar to that of the most flammable constituent species and this effect increases as fuels dry.

Feasibility of Igniting Prescribed Fires in Bottomland Hardwood Forests

AbstractThere is emerging interest in using prescribed fire to manage bottomlands for wildlife habitat, invasive species control, and overall forest function. We evaluated the feasibility of conducting prescribed fires in bottomland hardwood forests in west-central Alabama as part of a broader strategy to control the invasive shrub Chinese privet (Ligustrum sinense). We used 22 small-scale plots (0.04 hectares) in areas with residual slash from privet cutting operations and initiated prescribed fires on each to assess the overall feasibility and the relation of in-stand weather (i.e., microclimate), stand composition, and litter measurements to fire behavior. Overall, prescribed fire ignition was difficult, and only half the trials successfully burned >10 percent of the plot. We found that stand composition was most correlated with percent plot burned, and plots with higher proportions of tree species with flammable leaf traits (e.g., Quercus spp.) tended to burn best. Although further investigation is warranted, managers interested in using prescribed fire for bottomland hardwoods likely face short time windows and limited forest conditions in which fires can be reliably set.Study ImplicationsThere is increasing interest in using prescribed fires in bottomland hardwood forests. This exploratory study evaluated whether prescribed fires could be reliably set in bottomlands. Prescribed fires were difficult to establish and tended to be very patchy with fire spread related to tree canopy composition (because of differences in leaf litter flammability) and litter loads. Results suggest that it would be difficult to apply fire on a large scale in bottomland hardwood forests and that small-scale fires could only be set under certain conditions.

Increasing cuticular wax concentrations in a drier climate promote litter flammability

Several plant chemical traits (cellulose, tannins and terpenes) have been related to plant flammability. Contrastingly, no study has focused on the relationship between plant flammability and physico-chemical leaf litter traits with a focus on cuticular wax concentration. This study focuses on alkane cuticular waxes because of their relatively low flash point and storage in the cuticle of all vascular plant species. The sclerophyllous species Quercus coccifera is the model species since it is the main shrub species in the Mediterranean basin and all previously investigated sclerophyllous species feature a high cuticular wax content. Litter was collected in a Mediterranean garrigue where Q. coccifera grows under natural drought and recurrent aggravated drought (consisting of 5.5 years of rain restriction). These different drought conditions were expected to imply different alkane wax concentrations since one of the major roles of cuticular waxes is evapotranspiration limitation during drought. Litter flammability was assessed through ignition delay, flame residence time and flame height (assessed using an epiradiator) and gross heat of combustion (using an adiabatic bomb calorimeter). Results showed that the higher cuticular alkane concentrations reached under aggravated drought were associated with an increased leaf litter flammability as expected. These results confirm that all potentially flammable organic metabolites (terpenes as previously reported in other studies, and cuticular alkane waxes) are drivers of vegetation flammability. It is suggested that Q. coccifera flammability (considered as low to moderate), could increase under a drier scenario in the Mediterranean area. We hypothesize that fire severity would accordingly be intensified in shrubs dominated by this sclerophyllous species without necessarily increasing vulnerability of Q. coccifera to fire since this is a resprouter species after fire and is one of the main pioneer species during post-fire vegetation succession.

Pine Species That Support Crown Fire Regimes Have Lower Leaf-Level Terpene Contents Than Those Native to Surface Fire Regimes

Fire is increasingly being recognised as an important evolutionary driver in fire-prone environments. Biochemical traits such as terpene (volatile isoprenoid) concentration are assumed to influence plant flammability but have often been overlooked as fire adaptations. We have measured the leaf-level flammability and terpene content of a selection of Pinus species native to environments with differing fire regimes (crown fire, surface fire and no fire). We demonstrate that this biochemical trait is associated with leaf-level flammability which likely links to fire-proneness and we suggest that this contributes to post-fire seedling survival. We find that surface-fire species have the highest terpene abundance and are intrinsically the most flammable, compared to crown-fire species. We suggest that the biochemical traits of surface fire species may have been under selective pressure to modify the fire environment at the leaf and litter scale to moderate fire spread and intensity. We indicate that litter flammability is driven not only by packing ratios and bulk density, but also by terpene content.

Estimation of the surface area-to-volume ratios of litter components of the Brazilian rainforest and their impact on litter fire rate of spread and flammability

The surface-area-to-volume ratio (SAV) is a significant parameter in vegetation description, although different researchers have presented diverging estimates for forest litter. However, considering litter as a unique entity and disregarding the SAV of its components provides a misleading fire rate of spread. Undoubtedly, the leaves are more relevant than the twigs for the rate of spread of surface fires, as the former has a higher SAV. Therefore, based on the proven hyperdominance of tree species in the Brazilian rainforest, this work presents an accurate method to estimate the SAV. Furthermore, it demonstrates the critical role played by the SAV in the litter flammability and fire rate of spread assessments and shows that the leaf moisture content does not influence its SAV. Besides, comparing SAV data using the methodology presented herein with other authors’ assessments, the results differ by up to 49%. Finally, using the Brazilian rainforest litter in the methodologies of two distinct authors showed a difference of at least 36% in the results. The primary outcome of this study is the estimation of the SAV of leaves and thin, medium, and thick twigs, representative of the Brazilian rainforest, at 12,680 m−1, 1360 m−1, 620 m−1, and 310 m−1, respectively. The estimated SAV values are essential input data to numerical simulation models, and for comparison purposes with other authors’ work, the litter SAV is 8460 m−1.

Radiant heating rapidly increases litter flammability through impacts on fuel moisture

BackgroundLitter is the predominant fuel that drives surface fire behavior in most fire-prone forest and woodland ecosystems. The flammability of litter is driven by fuel characteristics, environmental factors, and the interactive effects of the two. Solar radiation can influence litter flammability through its effect on fuel moisture and temperature. The direct influence of radiative heating on flammability, however, is poorly understood and the interactive effects of forest structure, composition, fuels, and micrometeorology may have fine-scale ecological consequences in fire-dependent ecosystems.ResultsIn this study, we measured laboratory flammability of two dominant southeastern USA litter fuels—Pinus palustris Mill. and Quercus stellata Wangenh.—and evaluated the interactive effects of fuel moisture and infrared radiation. Heating of litter fuels increased their flammability primarily through enhanced litter drying. Heated litter quickly became more flammable than unheated litter when fuels were undergoing drying after saturation, a result of more rapid moisture loss. Litter mass did not change the effect of heating on drying rates, but heavier fuels burned with greater flammability.ConclusionsInteractions among surface fuels, overstory structure, and solar heating may be important in understanding fine-scale heterogeneity in both fire behavior and effects, with strong implications toward increasing the effectiveness of prescribed burning.

Biogeography of fire regimes in western U.S. conifer forests: A trait‐based approach

AbstractAimFunctional traits are a crucial link between species distributions and the ecosystem processes that structure those species’ niches. Concurrent increases in the availability of functional trait data and our ability to model species distributions present an opportunity to develop functional trait biogeography (i.e., the mapping of functional traits across space). Functional trait biogeography can improve process‐based predictions about the resistance of certain species assemblages to changing environmental conditions across landscape scales. We illustrate this concept by developing the first trait‐based, quantitative ranking of fire resistance (adult tree survival) in North American conifer species and mapping that fire resistance across space.Location and time periodWestern continental USA, present day.Major taxa studiedTwenty‐nine common conifer tree species.MethodsWe compiled six traits for each species: three relating to tree morphology and three relating to litter flammability. We combined these traits into a single fire resistance score and used community‐weighted averaging to estimate the fire resistance scores of different forest communities, using interpolated species distribution and relative abundance data.ResultsSpecies associated historically with frequent fire have high fire resistance scores (e.g.,Pinus ponderosa), reflected by thick bark, tall crowns and flammable litter. Species associated with subalpine or arid conditions have low fire resistance scores (e.g.,Picea engelmanniiandPinus edulis), reflected by thin bark, short stature, poor self‐pruning and low litter flammability. A map of forest community fire resistance across the western USA reveals agreement with independent assessments of historical fire regimes, while also identifying areas where community‐wide species traits might be mismatched with historical fire regimes.Main conclusionsQuantifying the functional traits that confer resistance to tree‐killing fire provides a direct link between ecosystem disturbance and community resistance. Understanding this link is crucial to evaluation of the long‐term resilience of different forest types under dynamic fire regimes. Our work represents the first known spatial representation of fire resistance traits at a regional scale and, as such, provides a link between functional traits and biogeography relevant to a critical ecosystem process.

Changes in land cover resulting from the introduction of non‐native pine modifies litter traits of temperate forests in Patagonia

AbstractAimsChanges in land cover resulting from the introduction of non‐native pine plantations and subsequent pine invasion is increasingly affecting forest areas in the Southern Hemisphere, and having negative impacts on diverse ecosystem components. We studied the abundance, biophysical attributes, and flammability of litter in temperate forests modified by the introduction of the non‐native Pinus radiata, and later by its post‐fire invasion from adjacent sites. We also evaluated the short‐term effects on litter of mechanical removal of P. radiata from post‐fire invasion sites.LocationNorth‐western Patagonia, Argentina.MethodsWe measured abundance, depth, moisture content and determined species composition of litter samples collected at native forest, plantation, and post‐fire invasion and removal sites. We also performed flammability tests on leaf litter samples of the dominant species from each habitat type. In all cases, litter traits from the modified habitats were compared with those obtained from native forest (reference system).ResultsMoisture content and litter species composition were notably affected by the alteration of habitat. A decrease in species richness and changes in the identity and abundance of the dominant species were the most remarkable shifts recorded in the modified habitats. While pine needles were mainly associated with parameters indicative of high leaf litter flammability, the opposite was found for leaves of the dominant native trees. The removal of invasive pines from early post‐fire invasion did not reveal any biophysical changes in litter that would indicate a tendency towards the conditions found in native forest litter.ConclusionsPine needles as a novel element in litter found in pine‐dominated habitats have a prominent impact on litter traits associated with alterations in key ecosystem properties in comparison with the reference system. For a short‐term recovery of burned, invaded areas, additional ecological practices other than the removal of invasive trees should be conducted to successfully drive successional changes towards conditions similar to those found in native vegetation.

Non-additive effects of leaf and twig mixtures from different tree species on experimental litter-bed flammability

Tree species can affect litter flammability through leaf size and shape. Larger, simpler-shaped leaf litters form better-ventilated, more flammable litter-beds. However, leaves are generally mixed with twigs in the forest litter layer and together they likely contribute most to surface fire behavior. Here we ask: “Do leaf-twig mixtures have non-additive effects on litter-bed flammability?” Using laboratory fires, we tested the direction and magnitude of non-additivity of inter- and intra-specific leaf-twig mixtures on litter-bed flammability for four tree species contrasted in leaf size and shape and widespread in fire-prone temperate-boreal forests. Across species, small needles reduced mixture fuel-bed ignitibility through filling the space between twigs and inhibiting ventilation. Within the small broad-leaved species, the thin, frequently branched and open spaced twigs were too loosely packed to be flammable, while in mixtures the small broad leaves connected these twigs to produce flammable fuel-beds. Once ignited, across species flame spread rate in mixtures was driven by leaves, while fire sustainability was predicted by fuel mass. Fuel-bed flammability was driven more by leaves at larger leaf-to-twig ratio. For the first time, we demonstrated the existence and mechanisms of non-additive effects of leaf-twig mixtures on experimental litter-bed flammability.

How terpene content affects fuel flammability of wildland–urban interface vegetation

Among plant characteristics promoting flammability, terpenes have received little attention, especially regarding the vegetation surrounding housing. Here, mono-, sesqui- and diterpenes were screened in live and dead leaves of ornamental species found in wildland–urban interfaces (WUIs) of south-eastern France. Terpene content and composition were compared among species and between fuel types. Their influence on flammability was assessed through several variables and compared with that of leaf thickness and moisture content. Six of the 17 species examined contained terpenes. Terpene diversity and content differed among species but not between fuel types. Mono-, sesqui- and diterpenes (especially the highly concentrated compounds) were involved to varying degrees in both leaf and litter flammability. Their effects could be opposite according to the flammability variable and the fuel type considered. Leaf sesquiterpene content and litter total terpene content had the strongest influence on maximum combustion temperature; the former also mainly drove leaf flaming duration. The other flammability variables were more strongly associated with either moisture content or leaf thickness. Our findings highlight the idea that fire management in the WUI must also acknowledge the potential for ornamental species containing terpenes, such as Pinus halepensis, to affect fire behaviour.