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Flammability is an important plant trait, relevant to plant function, wildfire behaviour and plant evolution. However, systematic comparison of plant flammability across ecosystems has proved difficult because of varying methodologies and assessment of different fuels comprising different plant parts. We compared the flammability of plant species at the leaf‐level (most commonly used in flammability studies) and shoot‐level (which retains aspects of plant architecture). Furthermore, we examined relationships between leaf functional traits and flammability to identify key leaf traits determining shoot‐level flammability. We collated and analysed existing leaf‐ and shoot‐level flammability data from 43 common indigenous perennial New Zealand plant species, along with existing data on leaf morphological and chemical traits. Shoot‐level flammability was decoupled from leaf‐level flammability. Moreover, leaf‐level rankings of flammability were not correlated with rankings of flammability of plants derived from expert opinion based on field observations, while shoot‐level rankings had a significant positive relationship. Shoot‐level flammability was positively correlated with leaf dry matter content (LDMC), phenolics and lignin, and negatively correlated with leaf thickness. Synthesis. Our study suggests that shoot‐level measurements of flammability are a useful and easily replicable way of characterizing the flammability of plants, particularly canopy flammability. With many parts of the world becoming more fire‐prone, due to anthropogenic activities, such as land‐use change and global warming, this finding will help forest and fire managers to make informed decisions about fuel management, and improve modelling of fire‐vegetation‐climate feedbacks under global climate change. Additionally, we identified some key, widely measured leaf traits, such as leaf dry matter content (LDMC), that may be useful surrogates for plant flammability in global dynamic vegetation models. Shoot‐ and leaf‐level flammability were decoupled, and shoot flammability corresponded to rankings based on expert opinion, suggesting that shoot‐level tests are a useful way to characterize the flammability of canopy fuels. Furthermore, we identified some widely measured leaf traits, such as leaf dry matter content, that were highly correlated to shoot flammability and can be useful surrogates for measuring plant flammability.

Background – aimsIt is widely assumed that plant flammability in the Mediterranean region peaks during the summer fire season. We currently lack data that could evaluate these assumptions and have not assessed the mechanisms, e.g. fuel moisture content (FMC) or terpenes, that might drive these patterns.MethodsTo determine the mechanistic drivers of species flammability, we used leaf burning experiments coupled with foliar chemical analyses focusing on Aleppo pine (Pinus halepensis) and three introduced cypresses commonly found at the wildland–urban interface (WUI) in southeastern France.Key resultsTerpenes, FMC and flammability varied over time and across the species studied, with contrasting patterns for each. Rare correlations between FMC and flammability occurred, in only one season and differing among species, while correlations between flammability and terpene compounds were diverse. The best flammability drivers were terpenes (mainly diterpenes), often changing among and within seasons, and their effect on flammability also differed. Overall, FMC was not a significant explanatory parameter of leaf flammability.Conclusions – implicationsHighlighting the temporal variation between flammability and its drivers revealed that species flammability could also be enhanced by terpenes outside the fire season; this should be accounted for in fire prevention, especially at the WUI.

Increased wildfire frequency and size has led to a surge in flammability research, most of which investigates landscape-level patterns and wildfire dynamics. There has been a recent shift towards organism-scale mechanisms that may drive these patterns, as more studies focus on flammability of plants themselves. Here, we examine methods developed to study tissue-level flammability, comparing a novel hot-plate-based method to existing methods identified in a literature review. Based on a survey of the literature, we find that the hot plate method has advantages over alternatives when looking at the specific niche of small-to-intermediate live fuel samples—a size range not addressed in most studies. In addition, we directly compare the hot plate method to the commonly used epiradiator design by simultaneously conducting flammability tests along a moisture gradient, established with a laboratory benchtop drydown. Our design comparison addresses two basic issues: (1) the relationship between hydration and flammability and (2) relationships between flammability metrics. We conclude that the hot plate method compares well to the epiradiator method, while allowing for testing of bigger samples.

Landscape differences in environmental conditions select for divergences among plant species in strategically important leaf traits such as leaf mass per area (LMA) and leaf area (LA). Interspecific variation in some of these same leaf traits has been associated to varying degrees with differences among species in leaf flammability, including the attributes ignitibility, sustainability, and combustibility. Yet, how environmentally selected variation in leaf traits drives variation in leaf flammability at landscape scales remains largely unknown. Here, we compared leaf traits and flammability attributes between species of sheltered forest vegetation (low light, moist habitat) and plant species of exposed woodland vegetation (high light, dry habitat) in a fire‐prone landscape of south‐eastern Australia. We found that leaves of sheltered forest species were significantly more flammable via both higher ignitibility and combustibility compared with exposed woodland species. These significant differences in leaf ignitibility and combustibility were underpinned by sheltered forest species having leaves with significantly larger LA and lower LMA compared with exposed woodland species. Further, multiple regression analyses revealed that both LA and LMA were significantly and uniquely related to faster time to ignition (TTI; ignitibility) and higher mean mass loss rate (combustibility). Most notably, although significantly higher fuel moisture content (FMC) of leaves of sheltered forest species significantly lengthened TTI, the lower LMA of these species played a more critical role in reducing TTI, with low LMA explaining more unique variation (partial r2 = 0.78) in high leaf ignitibility than low FMC (partial r2 = 0.49). Our findings provide the first evidence that landscape‐scale variation in leaf flammability is tightly coordinated with the primary strategic response of the leaf traits LMA and LA to an environmental gradient. Furthermore, projections for increasing wildfire frequency and intensity in the region will likely allow wildfires to overcome the once protective nature provided by topography to sheltered forest vegetation, which means that higher leaf flammability in sheltered forest species has the potential to exacerbate the effects of changing weather conditions to place sheltered forest habitat, their plants, and their animals, at even higher risk of catastrophic wildfire.

Aims Changes 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. Location North‐western Patagonia, Argentina. Methods We 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). Results Moisture 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. Conclusions Pine 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. The introduction of non‐native pine plantations and subsequent pine invasion is increasingly affecting areas of natural forest in the Southern Hemisphere. We found that pine needles (as a novel element in the litter of the modified habitats) have a negative impact on species composition, quality and flammability of litter – changes that represent increased degradation and fire risk for the area.

The importance of studying the role of volatiles in flammability is well recognized, but the relationship between specific compounds and components of flammability is underestimated. In this study, volatiles emitted from Picea omorika (Pančić) Purk. needles were identified and quantified, and the relationship between volatile emission and moisture content, flammability characteristics and bioclimatic coefficients was investigated. Fresh needles from four different forest communities were analyzed for specific volatile organic compounds (using a gas chromatography-surface acoustic wave analyzer, GC-SAW), moisture content (based on fresh and oven-dried mass), and flammability components (using an epiradiator). Five monoterpenes, one sesquiterpene, and one alcohol were identified and their amounts, moisture content, and flammability components differed between the populations. The amounts of myrcene and β-pinene correlated significantly with moisture content and time to ignition. Myrcene content also correlated with flame duration, site altitude, mean annual temperature, variation in mean annual temperature, and the Ellenberg quotient. The emission of myrcene correlates with ecophysiological, flammability, topographic, and climatic variables. This suggests that myrcene-rich plant species may be of particular interest for research into flammability, especially from the point of view of rapid ignition.

The moisture content of vegetation canopies controls various ecosystem processes such as plant productivity, transpiration, mortality, and flammability. Leaf moisture content (here defined as the ratio of leaf water mass to leaf dry biomass, or live-fuel moisture content, LFMC) is a vegetation property that is frequently used to estimate flammability and the danger of fire occurrence and spread, and is widely measured at field sites around the globe. LFMC can be retrieved from satellite observations in the visible and infrared domain of the electromagnetic spectrum, which is however hampered by frequent cloud cover or low sun elevation angles. As an alternative, vegetation water content can be estimated from satellite observations in the microwave domain. For example, studies at local and regional scales have demonstrated the link between LFMC and vegetation optical depth (VOD) from passive microwave satellite observations. VOD describes the attenuation of microwaves in the vegetation layer. However, neither were the relations between VOD and LFMC investigated at large or global scales nor has VOD been used to estimate LFMC. Here we aim to estimate LFMC from VOD at large scales, i.e. at coarse spatial resolution, globally, and at daily time steps over past decadal timescales. Therefore, our objectives are: (1) to investigate the relation between VOD from different frequencies and LFMC derived from optical sensors and a global database of LFMC site measurements; (2) to test different model structures to estimate LFMC from VOD; and (3) to apply the best-performing model to estimate LFMC at global scales. Our results show that VOD is medium to highly correlated with LFMC in areas with medium to high coverage of short vegetation (grasslands, croplands, shrublands). Forested areas show on average weak correlations, but the variability in correlations is high. A logistic regression model that uses VOD and additionally leaf area index as predictor to account for canopy biomass reaches the highest performance in estimating LFMC. Applying this model to global VOD and LAI observations allows estimating LFMC globally over decadal time series at daily temporal sampling. The derived estimates of LFMC can be used to assess large-scale patterns and temporal changes in vegetation water status, drought conditions, and fire dynamics.

1. Exotic plant invasions can alter fire regimes in plant communities. Invaders often possess traits that differ from native plants in the community, resulting in increases or declines in community-level flammability, changing fire regimes and potentially causing long-term modifications to plant community composition. Although considering traits of multiple invaders and native species together is useful to better understand how invasions change community-level flammability, few studies have done this. 2. We measured morphological and flammability traits of 51 native and exotic plant species common in tussock grasslands in New Zealand's south-eastern South Island to examine relationships between morphology and whole-plant and shoot-level flammability. Plant community data from 103 permanent transects in this region measured over a 25-year period (c. 1982-2007) were used to determine how flammability changed with increasing levels of plant invasion. 3. Invasion by exotic plants has led to reduced community-level flammability due to shifts from native tussock grasses with high flammability and high fuel loads to mat-forming exotic forbs with low flammability and little fuel. These changes will likely lead to considerable alterations to the fire regime, resulting in lower intensity fires that burn more patchily and for shorter amounts of time, potentially causing further changes in floristic composition. We found considerable differences in flammability across the wide range of species and growth forms that we studied, emphasising the importance of quantifying species-level flammability and the need to avoid treating grasslands as homogenous in terms of their flammability. Total biomass, leaf length and leaf area were the traits most positively correlated with flammability in these tussock grasslands. 4. Synthesis. We show how plant invasions over decadal time-scales have reduced the community-level flammability of tussock grasslands and, for the first time, demonstrate how this can be driven by exotic forbs. The total biomass of constituent species is a useful surrogate for community flammability across a wide range of species and growth forms in both temperate grasslands and savanna ecosystems and should be used in dynamic global vegetation models to assess how flammability varies under various global change scenarios.

Leaf flammability is a multidimensional plant functional trait with emerging importance for wildfire risk management. Understanding relationships among leaf flammability attributes not only provides information about the properties of leaves as fuels in the wildland–urban interface (WUI), it can also offer an effective way to identify low-leaf-flammability species. We examined relationships between leaf ignitibility, sustainability and combustibility among 60 plant species of the WUI of eastern Australia. We found that leaf ignitibility and sustainability worked in opposition to each other as dimensions of flammability. Species with leaves that were slow to ignite were those with leaves that sustained burning for the longest, whereas species with leaves that were fast to ignite had leaves that burned for the shortest periods of time. Low leaf combustibility was related to short leaf burning sustainability but not to ignitibility. We created an overall leaf flammability index (OLFI) to rank species on emergent properties of ignitibility, sustainability and combustibility attributes in combination. We found that low-leaf-flammability species with low OLFI values had small leaf area, high leaf mass per area and high leaf water content. Our findings have implications for species selection for green firebreaks in the WUI.

A widespread increase in tree mortality has been observed around the globe, and this trend is likely to continue because of ongoing climate-induced increases in drought frequency and intensity. This raises the need to identify regions and ecosystems that are likely to experience the most frequent and significant damage. We present SurEau-Ecos, a trait-based, plant hydraulic model designed to predict tree desiccation and mortality at scales from stand to region. SurEau-Ecos draws on the general principles of the SurEau model but introduces a simplified representation of plant architecture and alternative numerical schemes. Both additions were made to facilitate model parameterization and large-scale applications. In SurEau-Ecos, the water fluxes from the soil to the atmosphere are represented through two plant organs (a leaf and a stem, which includes the volume of the trunk, roots and branches) as the product of an interface conductance and the difference between water potentials. Each organ is described by its symplasmic and apoplasmic compartments. The dynamics of a plant's water status beyond the point of stomatal closure are explicitly represented via residual transpiration flow, plant cavitation and solicitation of plants' water reservoirs. In addition to the “explicit” numerical scheme of SurEau, we implemented a “semi-implicit” and “implicit” scheme. Both schemes led to a substantial gain in computing time compared to the explicit scheme (>10000 times), and the implicit scheme was the most accurate. We also observed similar plant water dynamics between SurEau-Ecos and SurEau but slight disparities in infra-daily variations of plant water potentials, which we attributed to the differences in the representation of plant architecture between models. A global model's sensitivity analysis revealed that factors controlling plant desiccation rates differ depending on whether leaf water potential is below or above the point of stomatal closure. Total available water for the plant, leaf area index and the leaf water potential at 50 % stomatal closure mostly drove the time needed to reach stomatal closure. Once stomata are closed, resistance to cavitation, residual cuticular transpiration and plant water stocks mostly determined the time to hydraulic failure. Finally, we illustrated the potential of SurEau-Ecos to simulate regional drought-induced mortality over France. SurEau-Ecos is a promising tool to perform regional-scale predictions of drought-induced hydraulic failure, determine the most vulnerable areas and ecosystems to drying conditions, and assess the dynamics of forest flammability.