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Background Foliar moisture content influences crown fire ignition and behaviour. Some spatial variation in foliar moisture is attributable to differences in stand conditions but evidence describing this role is lacking or contradicting. Aims To examine the role of stand conditions on tree physiology and foliar moisture content in Oregon white oak (Quercus garryana) and Douglas-fir (Pseudotsuga menziesii). Methods We monitored foliar moisture content in both species, and tree physiology (stomatal conductance and leaf water potential) in oak, across three stand conditions, including intact stands (unencroached), stands invaded by Douglas-fir (encroached), and thinned stands with Douglas-fir removed. Key results Encroached stands had higher foliar moisture content than intact or thinned stands. Higher stand density was associated with higher foliar moisture content in both species and foliage ages. Encroached stands also had higher midday leaf water potential compared with intact or thinned stands. Conclusions These findings provide strong evidence that stand conditions and thinning treatments can influence foliar moisture content in Oregon white oak ecosystems, with likely implications for other ecosystems. Implications Better understanding of the role of stand conditions on foliar moisture content may contribute to improved spatial and temporal prediction of foliar moisture content and modelling of potential crown fire behaviour.

Background Within California’s chaparral ecosystems, fuel reduction treatments are commonly used to reduce the negative impacts of wildfire but the durability of fuel treatment changes to fuels and vegetation when exposed to wildfire is less well understood. This study examined the interactive effects of 15-year-old fuel treatments and an extreme wildfire on burn severity, fuel loading, and vegetation in chaparral and oak vegetation types in Whiskeytown National Recreation Area in northern California, USA. Fuel treatment types included hand thinned, mechanical mastication, mechanical mastication + prescribed burning, and prescribed burning only. Results Vegetation and substrate burn severity was characterized as moderate across the study site and did not differ among treatments. Contrasting with higher pre-fire shrub density in the mastication + burning treatment, 2-year post-fire live shrub density did not differ among treatments. Higher pre-fire fine woody fuel loading in the mastication treatment did not correspond to post-fire fuel loading among treatments, while the hand thinned treatment was the only treatment where fine fuel loading was not significantly reduced post-fire. Total plant species richness increased in all treatment types following wildfire, largely driven by an increase in exotic species. Native cover decreased, and exotic cover increased in oak and chaparral types, but greater exotic species cover in the mastication + burning treatment in chaparral was maintained following wildfire. Conclusions Pre-fire differences in fuel and vegetation responses among treatments largely did not persist or were not detectible 1 to 2 years following wildfire. These findings suggest that the extreme wildfire conditions superseded long-term treatment differences in many fuel and vegetation metrics observed prior to wildfire. Despite subtle treatment differences, the hand thinned treatment resulted in the lowest change in fuel loading relative to all other treatments. Lastly, pre-fire differences in exotic species among fuel treatments were retained following wildfire, suggesting some treatments may have greater potential for exotic species expansion or type conversion to exotic grasslands.

Background Tree 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. Results In 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. Conclusions Interspecific 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.

Background Wildland fires are fundamentally landscape phenomena, making it imperative to evaluate wildland fire strategic goals and fuel treatment effectiveness at large spatial and temporal scales. Outside of simulation models, there is limited information on how stand-level fuel treatments collectively contribute to broader landscape-level fuel management goals. Our objective here is to present a framework designed to measure fuel treatment effectiveness from stands to landscapes to inform fuel treatment planning and improve ecological and social resilience to wildland fire. Results Our framework introduces the concept of a fuel management regime, an iterative and cumulative evaluation from the stand to the landscape of fire hazard, fuel treatments, and wildland fire behavior and effects. We argue that the successfulness of fuel treatments within this regime must be evaluated based on pre-treatment fire hazard and post-wildland fire fuel treatment outcomes over large spatial and temporal scales. Importantly, these outcomes can be evaluated from the stand level to across a landscape through time, based on preidentified management objectives that define condition-based criteria that account for social values and environmental and ecological indicators used to determine the effectiveness of fuel treatments within a fuel management regime. Conclusions Evaluating the cumulative ability of fuel treatments to change landscape patterns of fire behavior and effects is challenging. By quantifying fire hazard, followed by evaluating outcomes of wildfires on environmental and ecological indicators and social values, it becomes possible to assess how individual fuel treatments placed within the context of a fuel management regime are effective based on desired conditions that address management objectives. This conceptual framework offers a much-needed middle-ground planning, monitoring, and reporting approach between overly simplistic annual reporting summaries of the area treated, number of fires, and burned area and detailed fire simulation modeling outcomes by putting individual treatments and fires in the context of current and desired vegetative conditions and social values. Our fuel treatment effectiveness framework examines the state of fuels through the lens of fire hazard and connects fuels to subsequent fire behavior and effects over time and space. The framework provides a way to focus regional and national fuel management planning efforts toward creating fuel management regimes that increase social and ecological resilience from wildfire.

Large sugar pine mortality was associated with recent growth and defense measures. Many old-growth pine forests across the western United States have encountered widespread and concerning increases in tree mortality attributed to increased competition and reduced vigor associated with prolonged fire exclusion that can make trees more vulnerable to bark beetles. We investigated the importance of growth and resin duct defense on recent mortality of large sugar pine (Pinus lambertiana) in a fire-excluded, mixed-conifer forest of the Sierra Nevada, in California, USA. Growth and defense were measured from tree rings for 33 pairs of live and dead sugar pine. The 10-year trend in basal area increment prior to sampling declined by 1.78 cm2 year− 1 in sugar pine that died and increased by 0.34 cm2 year− 1 in paired live sugar pine. The 10-year trend in resin duct total area showed declines of 0.0154 mm2 year− 1 in trees that died and increases of 0.0068 mm2 year− 1 in trees that survived. The most informative models of large sugar pine mortality included measures of both growth and defense. The top model included the trends of basal area increment and resin duct total area 10 years before mortality, and growth variability 5 years before mortality. In general, trends and variability in growth and defense over shorter time windows (

Significance Forests are experiencing growing risks of drought-induced mortality in a warming world. Yet, ecosystem dynamics following drought mortality remain unknown, representing a major limitation to our understanding of the ecological consequences of climate change. We provide an emerging picture of postdrought ecological trajectories based on field indicators of forest dynamics. Replacement patterns following mortality indicate limited short-term persistence of predrought dominant tree species, highlighting the potential for major ecosystem reorganization in the coming decades. The great variability of the observed dynamics within and among species reinforces the primary influence of drought characteristics and ecosystem legacies, modulated by land use, management, and past disturbances, on ongoing drought-related species turnover and their potential implications for future forest biodiversity and ecosystem services., Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern postdrought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.

AIM: Functional 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 PERIOD: Western continental USA, present day. MAJOR TAXA STUDIED: Twenty‐nine common conifer tree species. METHODS: We 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. RESULTS: Species 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 engelmannii and Pinus 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 CONCLUSIONS: Quantifying 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.

California faces crisis conditions on its forested landscapes. A century of aggressive logging and fire suppression in combination with conditions exacerbated by climate change have created an ongoing ecological, economic, and public health emergency. Between commercial harvests on California’s working forestlands and the increasing number of acres the state treats each year for fire risk reduction and carbon sequestration, California forests generate millions of tons of woody residues annually—residues that are typically left or burned in the field. State policymakers have turned to biomass electricity generation as a key market for woody biomass in the hope that it can support sustainable forest management activities while also providing low-carbon renewable electricity. However, open questions surrounding the climate and air pollution performance of electricity generation from woody biomass have made it difficult to determine how best to manage the risks and opportunities posed by forest residues. The California Biomass Residue Emissions Characterization (C-BREC) model offers a spatially-explicit life cycle assessment framework to rigorously and transparently establish the climate and air pollution impacts of biopower from forest residues in California under current conditions. The C-BREC model characterizes the variable emissions from different biomass supply chains as well as the counterfactual emissions from prescribed burn, wildfire, and decay avoided by residue mobilization. We find that the life cycle ‘carbon footprint’ of biopower from woody residues generated by recent forest treatments in California ranges widely—from comparable with solar photovoltaic on the low end to comparable with natural gas on the high end. This variation stems largely from the heterogeneity in the fire and decay conditions these residues would encounter if left in the field, with utilization of residue that would otherwise have been burned in place offering the best climate and air quality performance. California’s energy and forest management policies should account for this variation to ensure desired climate benefits are achieved.

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.

Interest in prescribed fire science has grown over the past few decades due to the increasing application of prescribed fire by managers to mitigate wildfire hazards, restore biodiversity, and improve ecosystem resilience. Numerous ecological disciplines use prescribed fire experiments to provide land managers with evidence-based information to support prescribed fire management. Documenting variation in the context and conditions during prescribed fire experimental treatments is critical for management inference, but inconsistencies in reporting critical experimental details can complicate interpretation. Such details are needed to provide ecological and empirical context for data, facilitate experimental replication, enable meta-analyses, and maximize utility for other scientists and practitioners. To evaluate reporting quality in the recent literature, we reviewed 219 prescribed fire experiments from 16 countries published in 11 refereed journals over the last 5 years. Our results suggest substantial shortcomings in the reporting of critical data that compromise the utility of this research. Few studies had specific information on burning conditions such as fuel moisture (22%), quantitative fuel loads (36%), fire weather (53%), and fire behavior (30%). Further, our analysis revealed that 63% of the studies provided precise coordinates for their study area, while 30% of studies indicated the prescribed fire date. Only 54% of the studies provided descriptions of the ignition characteristics. Given these common deficiencies, we suggest minimum reporting standards for future prescribed fire experiments. These standards could be applied to journal author guidelines, directed to researchers and reviewers by the editor, and promoted in the education of fire ecologists. Establishing reporting standards will increase the quality, applicability, and reproducibility of prescribed fire science, facilitate future research syntheses, and foster actionable science.

Premise Dispersal capacity primarily determines the spatial establishment patterns that drive range expansions and contractions in tree species. Seedling establishment in Baker cypress (Hesperocyparis bakeri [(Jeps.) Bartel]) relies predominantly on fire events, due to its cone serotiny, shade intolerance, and small seeds that require the optimal conditions of fire-exposed, mineral soil seedbeds. Methods We quantified the density and spatial distribution of post-disturbance seedlings following the 2014 Eiler Fire in northern California and compared the observed recruitment to predictions from a mechanistic seed dispersal model. Results Postfire recruitment was dense, averaging 11 seedlings/m2 , and occurred primarily in the first year after fire. We estimated the mean descent velocity of the wingless seeds as ~4 m/s, the highest value reported for any putatively wind-dispersed tree species. The rapid seed descent contributed to markedly spatially constrained recruitment. Most seedlings (~81%) established within 5 m of the parent tree, and 94% established within 10 m. The maximum observed dispersal distance was 48.5 m; dispersal distance scaled linearly with canopy height. Distributions of modeled seed dispersal distance and observed seedling establishment in Baker cypress did not differ, demonstrating that wind disperses seeds a short distance because of the lack of a wing, and secondary dispersal appeared to be minimal at this recently burned site. Conclusions If seed dispersal is solely reliant on wind, migration in response to rapid climate change will be impeded and potentially present difficulties in sustaining populations of this and other obligate seeder species with equally constrained dispersal.

The prolonged absence of fire in Oregon white oak (Quercus garryana) woodlands and savannas of the Pacific Northwest has resulted in substantial conifer encroachment over the past century. Use of low intensity prescribed burns often lacks sufficient intensity to kill larger encroached trees, requiring alternative approaches. In the Bald Hills region of Redwood National Park, managers have implemented girdling treatments to kill Douglas-fir (Pseudotsuga menziesii) over the past two decades with the objective of recovering remnant oaks and restoring historical woodland area. We surveyed 258 girdled Douglas-fir to examine the effectiveness of girdling treatments to create snags and the impacts of girdling on tree regeneration and fuel recruitment over a 17 year time since girdling chronosequence. Girdling was successful in killing 91% of the treated Douglas-fir independent of the method used (axe or chainsaw). Larger trees with a low girdle width-to-tree diameter ratio tended to survive girdling. Trees with a girdle width-to-tree diameter ratio ≥1 were most effective at killing trees. Snags generated through girdling decayed over time but did not significantly reduce in height over the time period examined. Bird activity was observed on 19% of snags across the chronosequence but 88% of 10 y old snags had signs of bird foraging. Fine woody fuel loading and fuelbed depths were potentially elevated 7 years after treatment but did not persist through the time period examined. Regeneration of Douglas-fir and oaks were highly variable and did not seem to be clearly linked to girdling activity. Our results indicate that girdling is a highly effective treatment to kill Douglas-fir and aid restoration of Oregon oak woodlands with limited negative impacts on surface fuel hazards or regeneration.

The iconic American chestnut (Castanea dentata) once spanned a large portion of eastern North America before its functional extinction in the early 20th century due primarily to non‐native fungal pathogens. The pronounced loss of this species likely resulted in an abrupt alteration of many ecological processes, including fire. The potential to resurrect this species through resistance breeding or other methods holds promise, but more information on the fire ecology of American chestnut may provide helpful information to assist restoration. Here we summarize the existing direct and indirect research on the fire history and fire ecology within the former range of American chestnut. We found multiple lines of evidence to suggest fire was frequent throughout much of its historical range. A broadscale analysis of historical fire frequency revealed that 88% of the American chestnut range had a mean fire return interval of 20 yr or less, corresponding to finer‐scale fire history and forest structure studies. In much of the historical range of American chestnut, the stand structure was much more open, fire scar studies of associated species were very frequent (mean fire return interval ranged between 1.9 and 19.8 yr), and, in many cases, charcoal abundance and American chestnut pollen were positively related. This evidence coupled with American chestnut’s suite of traits associated with tolerance of frequent fire, such as highly flammable litter, tall stature, rapid growth, and resprouting ability, reinforce concepts that fire was historically an important component of many woodlands and forests containing American chestnut. While these lines of evidence are strongly suggestive, we provide potential areas of further inquiry to expand and refine our understanding of American chestnut fire ecology.

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 palustrisMill. andQuercus stellataWangenh.—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.

Severe droughts have the potential to reduce forest productivity and trigger tree mortality. Most trees face several drought events during their life and therefore resilience to dry conditions may be crucial to long-term survival. We assessed how growth resilience to severe droughts, including its components resistance and recovery, is related to the ability to survive future droughts by using a tree-ring database of surviving and now-dead trees from 118 sites (22 species, >3,500 trees). We found that, across the variety of regions and species sampled, trees that died during water shortages were less resilient to previous non-lethal droughts, relative to coexisting surviving trees of the same species. In angiosperms, drought-related mortality risk is associated with lower resistance (low capacity to reduce impact of the initial drought), while it is related to reduced recovery (low capacity to attain pre-drought growth rates) in gymnosperms. The different resilience strategies in these two taxonomic groups open new avenues to improve our understanding and prediction of drought-induced mortality., Resilience to drought is crucial for tree survival under climate change. Here, DeSoto et al. show that trees that died during drought were less resilient to previous dry events compared to surviving conspecifics, but the resilience strategies differ between angiosperms and gymnosperms.

Resin ducts in the secondary xylem of tree rings are a measure of a tree's defense capacity from insects and pathogens. Because resin ducts are permanently embedded within the xylem, retrospective analysis can be performed to quantify changes in defense over time and determine factors that contribute to this change, such as climate and disturbance. Here, we provide methods on how to measure axial resin ducts in secondary xylem. These methods provide the necessary protocols for consistent quantification of xylem resin ducts and terminology, which will also allow easier cross-comparison among studies in the future. We describe:•Steps to prepare tree cores for resin duct measurements.•Procedure to obtain image and measure individual resin ducts.•Software code to compile duct measurements into a complete chronology with both standardized and unstandardized resin duct metrics for further analyses., Graphical abstract Image, graphical abstract

Loss of native foundation tree species to introduced pests profoundly alters the structure and function of many forest ecosystems. Recent advances to resurrect or prevent the loss of species by developing resistant hybrids hold promise, but uncertainty remains about the potential impacts of introducing a novel genotype on ecological processes, such as fire. A classic example of a non-native, pathogen-caused loss of a foundation species is American chestnut (Castanea dentata), a species now functionally extinct from the eastern US but undergoing experimental trials to resurrect the species with a putatively disease-resistant genotype. We compared the litter flammability among American chestnut, Chinese chestnut (C. mollisima), and a population of first intercross, third-generation backcrossed hybrid (BC3F2) using laboratory burning experiments. Litter flammability of American chestnut was consistently greater than Chinese chestnut, whereas the hybrid had an intermediate flammability or more closely resembled Chinese chestnut flammability by some measures. Greater flammability in American chestnut was associated with a longer leaf structure with greater curling when dry. American chestnut had flammability properties consistent with other pyrophytic species present in contemporary fire-prone ecosystems. The loss of American chestnut may have altered litter flammability of some eastern US forest ecosystems, a result more commonly associated with compositional changes in conjunction with fire exclusion and other disturbances. Resurrecting American chestnut with a hybrid genotype may mitigate this change in areas where less flammable species have replaced American chestnut. Resurrection of lost foundation species through introduction of resistant genotypes may represent a resounding ecological success story, but unanticipated changes to ecological processes, such as fire, should be considered.

Fire exclusion has contributed to substantial basal accumulations of forest floor fuels in many historically fire-prone forests in western North America. These accumulations can contribute to undesired impacts (e.g., elevated tree mortality, more smoke production). Thus, managers interested in limiting some of these impacts require a better understanding of the spatial factors contributing to forest floor fuel variation in long-unburned forests. To address this need, we measured the forest floor mound size and depth of each layer at four distances from the tree (base, half, edge, and beyond) in each cardinal direction from the tree, and a subset of density measures for each layer and position around 25 large sugar pine (Pinus lambertiana) in the Klamath Mountains of California. Based on our study we found: 1) duff depths were greater than litter depths and both decreased with distance from tree; 2) duff densities were typically greater than litter but were highly variable with distance to tree; 3) litter and duff depths were positively correlated with loading; 4) use of a previously developed depth-to-loading equation performed well, but tended to underestimate litter loading and overestimate duff loading; and 5) tree size was the most strongly related to basal accumulations of forest floor fuels. Our results indicate there are consistent tree characteristics that can help predict the accumulation of forest floor fuels. This research can help provide managers with site-specific equations to inform fuel and fire treatments in areas with substantial basal accumulation in forests with large sugar pine.

Drought is an important stressor in forest ecosystems that can influence tree vigor and survival. In the U.S., forest managers use two primary management techniques to promote resistance and resilience to drought: prescribed fire and mechanical thinning. Generally applied to reduce fuels and fire hazard, treatments may also reduce competition for resources that may improve tree-growth and reduce mortality during drought. A recent severe and prolonged drought in California provided a natural experiment to investigate tree-growth responses to fuel treatments and climatic stress. We assessed tree-growth from 299 ponderosa pine (Pinus ponderosa) and Douglas-fir (Pseudotsuga menziesii) in treated and untreated stands during severe drought from 2012 to 2015 in the mixed-conifer forests of Whiskeytown National Recreation Area (WNRA) in northern California. The treatment implemented at WNRA removed 34% of live basal area through mechanical thinning with a subsequent pile burning of residual fuels. Tree-growth was positively associated with crown ratio and negatively associated with competition and a 1-year lag of climate water deficit, an index of drought. Douglas-fir generally had higher annual growth than ponderosa pine, although factors affecting growth were the same for both species. Drought resistance, expressed as the ratio between mean growth during drought and mean growth pre-drought, was higher in treated stands compared to untreated stands during both years of severe drought (2014 and 2015) for ponderosa pine but only one year (2014) for Douglas-fir. Thinning improved drought resistance, but tree size, competition and species influenced this response. On-going thinning treatments focused on fuels and fire hazard reduction are likely to be effective at promoting growth and greater drought resistance in dry mixed-conifer forests. Given the likelihood of future droughts, land managers may choose to implement similar treatments to reduce potential impacts.

Fuel moisture exerts considerable influence on fire behavior and effects but varies substantially spatially and temporally. A portion of the spatial variation in dead surface fuel moisture of woodland and forest ecosystems is likely attributed to differences in stand conditions (i.e. stand density and composition). Existing evidence suggests that the role of stand conditions on dead surface fuel moisture is inconsistent across studies and more research examining the effects of stand conditions on seasonal microclimate and fuel moisture is needed. This study examined the effects of different stand conditions within Oregon white oak (Quercus garryana) woodlands and forests, including intact oak-dominated stands without Douglas-fir (Pseudotsuga menziesii), oak stands that have been invaded by Douglas-fir (encroached), and thinned stands with Douglas-fir removed within Oregon white oak ecosystems of northern California on seasonal variation of microclimate and dead surface fuel moisture. Stand condition had a strong effect on microclimate and fuel moisture over time with thinned stands consistently having warmer and drier conditions with lower fuel moisture than encroached stands in most surface fuel components. Differences in fuel moisture among stand conditions were most pronounced in the late spring and early fall. Observed fuel moisture values did not correlate well with estimates from remotely automated weather stations or process-based calculations, especially when observed fuel moisture values exceeded 20%. Collectively, these findings provide strong evidence that stand conditions can modify microclimate that contributes to variation in dead surface fuel moisture within Oregon white oak ecosystems of northern California. These differences are often not considered within commonly used fuel moisture estimates, emphasizing the need to develop better models and tools to allow managers to more accurately anticipate fuel moisture conditions in varying stands to model landscape variation in fire behavior and effects and to meet fuel reduction and restoration objectives.

Oregon white oak (Quercus garryana Douglas ex Hook.) is experiencing increasing competition from Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) across much of its range at the same time as climate variability is increasing, including more frequent and extreme droughts. These combined factors suggest that Douglas-fir encroachment is not only leading to range reduction from competition, but also likely exacerbating drought stress for a keystone oak species in the Pacific West of North America. Our research examines this by evaluating radial tree growth and drought response in Oregon white oak (n = 104 trees) and Douglas-fir (n = 104 trees) along a gradient of encroachment in an oak woodland in the North Coast region of California. Linear mixed-effects models were used to evaluate the effects of tree diameter, Douglas-fir competition, oak competition, and climate on drought resistance (2013–2015) and recent tree growth (2002–2016). Oregon white oak growth was more resistant to prolonged drought than Douglas-fir. However, Oregon white oak drought resistance declined with increasing Douglas-fir competition and increased with increasing oak competition. Oregon white oak and Douglas-fir growth related to different seasonal climate factors, but both species were more strongly limited by Douglas-fir competition than climate. Oregon white oak may be better suited to future climate conditions than Douglas-fir, although Douglas-fir encroachment will likely continue to reduce Oregon white oak resistance to future drought. These results present a strong case for the need to release Oregon white oak by controlling Douglas-fir in areas that were historically oak woodlands with diverse understory species, wildlife, and important traditional ecological values and services.

Background Increasing frequency and size of wildfires over the past few decades have prompted concerns that populations of obligate seeding species may be vulnerable to repeat, short-interval fires that occur prior to these species reaching maturity. The susceptibility of populations to this risk is partially dependent on the amount and characteristics of fuel loading over time and their influence on fire behavior and effects. This study characterized fuel dynamics and modeled fire behavior across a time-since-fire chronosequence in stands of the rare, serotinous conifer, Baker cypress (Hesperocyparis bakeri [Jeps.] Bartel), ranging in age between 3 and 147 years post fire. Results Litter and fine woody fuel loading (1- to 100-hour) were highest in the 10-year-old and 147-year-old stands, while coarse fuel loading (1000-hour) peaked in the 10-year-old stand and subsequently decreased with time since fire. Duff loading consistently increased with time since fire. Cone production had not occurred in the first 10 yr of stand development. Foliar moisture content in Baker cypress was inversely correlated with stand age, and older foliage had lower moisture content than younger foliage. Modeled surface fire behavior was highest in the 10-year-old and 107-year-old stands in accordance with higher litter, fine woody fuel, or shrub fuel accumulation. While foliar moisture content was higher in younger stands and influenced the critical fireline intensity, we did not observe changes in fire type. Conclusions Fine-fuel loading in Baker cypress stands followed a U-shaped pattern over time (first decreasing, then stable, then increasing), consistent with findings in other forests with stand-replacing fire regimes. Our results indicated that early-successional stages of Baker cypress forests have sufficient fuels to allow for the spread of wildfire and 10-year-old stands could burn with substantive fire behavior prior to cone production. Whenever possible, we recommend suppressing wildfire in stands less than 20 yr old to avoid substantial decreases or local extirpation of these rare Baker cypress populations. Our results highlight the importance of knowing the cone production patterns, fuel dynamics, and corresponding fire behavior over the development of obligate-seeder species to assess the risk of population loss due to short-interval fires.

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.

California’s high density, fire-excluded forests experienced an extreme drought accompanied by warmer than normal temperatures from 2012 to 2015, resulting in the deaths of millions of trees. We examined tree mortality and growth of mixed-conifer stands that had been experimentally treated between 2011 and 2013 with two different thinning treatments, one with more structural variability (HighV) and one with less structural variability (LowV), applied alone or in combination with prescribed burning. Tree mortality between 2014 and 2018 varied by species ranging from 42% of white fir (Abies concolor) to 18% of sugar pine (Pinus lambertiana), 12% of incense cedar (Calocedrus decurrens) and 10% of yellow pine (P. ponderosa and P. jeffreyi). Lower overall tree mortality rates at this location relative to drier locations in the southern Sierra Nevada suggested that drought effects may have been ameliorated by lower water deficits due to our site’s more northerly location and deep, productive soils in combination with reductions in tree competition following thinning and burning. Averaged across burn treatments, thinning reduced the overall mortality rate between 2014 and 2018 from 34% to 11%. A total of 23% of the basal area was lost in the unthinned control treatments during this time period, while basal area was unchanged in the thinned treatments, with growth offsetting mortality. There was no significant difference in mortality or basal area change between LowV and HighV, suggesting that leaving trees at variable spacing may not compromise growth or resilience of the stand during a drought. Overall tree mortality was greater in the prescribed burn treatments, most pronounced in the smaller tree size classes, and varied by species, with burning having a significant effect on incense cedar and all pines, but not white fir. Trees with greater competition (Hegyi index) were more likely to die, particularly when also burned. Burning, however, consumed surface fuels and lowered fire hazard. With predictions of warmer droughts and greater weather variability, reducing forest density (basal area) and keeping surface fuel loads low will be important for building greater resilience to future drought stress and wildfire.

Prescribed fire is a widely used forest management tool, yet the long-term effectiveness of prescribed fire in reducing fuels and fire hazards in many vegetation types is not well documented. We assessed the magnitude and duration of reductions in surface fuels and modeled fire hazards in coniferous forests across nine U.S. national parks in California and the Colorado Plateau. We used observations from a prescribed fire effects monitoring program that feature standard forest and surface fuels inventories conducted pre-fire, immediately following an initial (first-entry) prescribed fire and at varying intervals up to >20 years post-fire. A subset of these plots was subjected to prescribed fire again (second-entry) with continued monitoring. Prescribed fire effects were highly variable among plots, but we found on average first-entry fires resulted in a significant post-fire reduction in surface fuels, with litter and duff fuels not returning to pre-fire levels over the length of our observations. Fine and coarse woody fuels often took a decade or longer to return to pre-fire levels. For second-entry fires we found continued fuels reductions, without strong evidence of fuel loads returning to levels observed immediately prior to second-entry fire. Following both first- and second-entry fire there were increases in estimated canopy base heights, along with reductions in estimated canopy bulk density and modeled flame lengths. We did not find evidence of return to pre-fire conditions during our observation intervals for these measures of fire hazard. Our results show that prescribed fire can be a valuable tool to reduce fire hazards and, depending on forest conditions and the measurement used, reductions in fire hazard can last for decades. Second-entry prescribed fire appeared to reinforce the reduction in fuels and fire hazard from first-entry fires.

Fire is integral to the functioning of terrestrial ecosystems of the southeastern USA and is a strong selective force on plant species. Among woody plants, oak species (Quercus spp. L) have diverse life history traits that appear to reflect their evolution in this fire-prone region. Oaks also occur across wide gradients of fire frequency and intensity, from annually burned savannas to fire-protected forests. As such, oak functional traits are presumed to reflect adaptations to acquire limited resources (i.e., “physiological traits”) or survive environmental stress (i.e., “protective traits”). Oak functional traits may also influence fire regimes (i.e., via “flammability traits”) by altering fire behavior through effects on fuels and their combustion. We synthesized evidence from ecophysiological measurements, laboratory burning and drying experiments, and field experiments to determine the suites of functional traits that reflect fire adaptive strategies in eight Southeastern oaks for which abundant data were available. We found strong correlations among Principal Components Analysis axes for flammability (litter burning and drying), protective (bark and wound responses), and physiological (growth) traits. The eight oaks clustered into three strategies: 1) pyrophytic species that produce highly flammable leaf litter, accrue thick bark rapidly, close wounds rapidly, and grow slowly; 2) mesophytic species that produce low flammability litter, have thin bark, and are fast growing; and 3) fire-avoider species with a mixture of traits from the two extremes. This synthesis clarifies the relative pyrophily of Southeastern oaks and suggests how suites of fire-related traits influence fire regimes and species habitat preferences.

Summary Fire-prone woodlands and savannas world-wide face management challenges resulting from fire exclusion and subsequent encroachment of fire-sensitive trees. In the Pacific Northwest (USA), Quercus garryana oak woodlands and savannas are threatened by encroachment from the native conifer Pseudotsuga menziesii in the absence of fire. In the Bald Hills of Redwood National Park (California, USA), prescribed fire and conifer removal have been used to restore encroached woodlands. We examined the effects of encroachment and restoration on understorey vegetation, comparing four treatments: prescribed fire, prescribed fire and conifer removal, conifer removal, and encroached (control). Treatments including prescribed fire had the greatest native species richness. These two treatments also had the greatest non-native species richness, at both the site level and the treatment level. Woodlands treated with conifer removal and no prescribed fire were intermediate in species richness and diversity compared to burned treatments and encroached woodlands. Encroached woodlands had diminished richness and diversity compared to all restoration treatments. Non-metric multidimensional scaling (NMS) ordination demonstrated that conifer basal area, conifer litter and fine wood were associated with low species richness and diversity and that elevation and thatch were associated with higher species richness and diversity. Indicator species analysis identified that most native species and non-native species were associated with burned woodlands that were never encroached. Synthesis and applications. Our results suggest that both prescribed fire and conifer removal have benefits for understorey plant communities, increasing species richness, diversity and cover in oak woodlands and shifting understorey communities from forest-associated species to more woodland-associated species. Restoration of remnant Quercus garryana oak woodlands is complicated by the persistence and abundance of non-native herbaceous plants.

In many fire-prone woodlands and forests, fire exclusion has resulted in substantial litter and duff accumulations capable of long-duration smoldering once fire is reintroduced. While previous research has shown that the soil heating from smoldering fires resulted in short-term reductions in coarse root non-structural carbohydrates and latewood growth, information on the long-term effects of smoldering fire is lacking. Our study compared the effects of three smoldering fire treatments (root only, stem only, root and stem) and two control treatments (no fire and surface fire only) on longleaf pine (Pinus palustris) growth and defense ten years after treatments. We cored 17–29 similar sized trees per treatment and measured growth as basal area increment and defense as resin duct properties (e.g., resin duct size, % resin duct area). We used generalized linear mixed models to determine the influence of smoldering treatments and climate on basal area increment and resin duct properties. Variation in basal area increment and resin duct size during the previous ten years were positively correlated with the subsequent ten years, r2 = 0.71 and r2 = 0.65, respectively. Additionally, temporal variation in growth and defense were related to climatic factors. Growth had the strongest correlation with current August temperatures (r = −0.50), and defense had the strongest correlation with current December temperatures (r = −0.64). Basal area increment was best predicted by summer and fall Palmer Drought Severity Index, while resin duct size was best predicted by the interaction of treatment and precipitation during the previous November. During the three driest years following treatment smoldering duration had a negative relationship with basal area increment (p

Fire exclusion in eastern North American Quercus–Carya woodlands has resulted in overstory compositional changes, linked to altered fuel composition, structure, and ultimately, altered fire regimes. These compositional changes have been implicated in a dampening effect on fire behavior in formerly fire‐prone ecosystems, the positive feedback termed “mesophication.” Several proposed mechanisms are likely involved in this process; however, few have been examined. We evaluated the potential mechanism of dampened forest floor flammability through changes in litter composition in a southeastern U.S. oak–hickory forest undergoing mesophication. Laboratory drying and burning experiments revealed that increasing the relative contribution of litter from fire‐sensitive mesophytic tree species (Liquidambar styraciflua, Ulmus alata, and Cornus florida) increased moisture retention and diminished litter flammability. Litterbeds composed of ≥66% mesophyte litter gained the most moisture following fuelbed saturation and were consistently wetter than pyrophyte litter following 12 and 24 h of drying. Flammability metrics decreased with increasing contribution of mesophyte litter with reductions most pronounced under more moist conditions. Under dry conditions, where litter moisture did not differ across compositional treatments, mesophyte litter was less flammable than fuelbeds that contained any pyrophyte litter. The combined effects of moisture retention and less flammable litter reveal an important synergistic impact that may result with increased dominance of fire‐sensitive mesophytic species. Restoration efforts in long‐unburned eastern hardwood forests may require altering stand composition to limit mesophytic litter input or developing burn prescriptions that intensify surface fire behavior in less flammable fuels.

Fire behavior and effects in forests and woodlands are influenced by surface fuels and senesced leaf litter in particular. We have known that species exhibit differential flammability for some time, but isolated efforts have often attributed differences to disparate mechanisms. Recent research has expanded the diversity of species evaluated, clarified patterns at the fuelbed level, and provided evidence that the physical and chemical traits of litter or fuelbeds drive flammability. To date, little effort has focused on uniting methods, clarifying the awkward terminology, or, perhaps most importantly, comparing laboratory findings to field observations of fire behavior. Here, we review recent literature and synthesize findings on what we know about the flammability of litter and propose future research directions.

Future climate predictions suggest that recent patterns of increased pinon pine ( Pinus edulis ) mortality related to drought in the southwestern U.S. will continue. However, the importance of facilitation for young pinon pine growing at different edaphic sites is not well understood. We measured the density and stem elongation of juvenile pinon pine that established under nurse plants or in open areas in northern Arizona, USA that ranged in soil substrate age from 0.93 to 750 ky. We found: 1) 4–9 times greater juvenile pinon pine density under nurse plants compared to open areas; 2) 1–2 times greater densities at the two older substrate sites compared to the younger substrate site; 3) nurse plant effects on juvenile pinon density changed with substrate age; 4) 14–19% greater growth in nursed conditions than open areas across all substrate ages; 5) 32–35% greater growth at the two older substrate age sites; and 6) growth beneath dead pinon nurse plants were approximately 20% greater than under live pinon nurse plants, though differences were not statistically significant (P = 0.07). As a result, we propose that projections of pinon pine distributions need to consider influences of edaphic conditions and the abundance and types of nurse plants on regeneration.

Many forest ecosystems with a large pine component in the western United States have experienced environmental stress associated with climate change and increased competition with forest densification in the absence of fire. Information on how changes in climate and competition affect carbon allocation to tree growth and defense is needed to anticipate changes to tree vigor and, ultimately, stand structure. This study retrospectively examined the influence of annual climate and competition measures on the growth and defense of 113 large sugar pines (Pinus lambertiana) in a mixed-conifer forest of the central Sierra Nevada of California. We found that growth in large sugar pine was positively associated with higher January temperatures and lower intraspecific competition. Resin duct size was negatively associated with climatic water deficit and total competition, while resin duct area contrastingly showed a positive relationship with total competition. From 1979 to 2012, the rates of growth increased, while resin duct size decreased. Our results suggest that tree vigor measures can respond differently to climate and competition factors that may lead to separate growth and defense trends over time. Stress associated with warmer temperatures and higher competition may distinctly influence individual tree and stand-level vigor with potential implications for future forest dynamics.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; International audience; Widespread tree mortality associated with drought has been observed on all forested continents and global change is expected to exacerbate vegetation vulnerability. Forest mortality has implications for future biosphere–atmosphere interactions of carbon, water and energy balance, and is poorly represented in dynamic vegetation models. Reducing uncertainty requires improved mortality projections founded on robust physiological processes. However, the proposed mechanisms of drought-induced mortality, including hydraulic failure and carbon starvation, are unresolved. A growing number of empirical studies have investigated these mechanisms, but data have not been consistently analysed across species and biomes using a standardized physiological framework. Here, we show that xylem hydraulic failure was ubiquitous across multiple tree taxa at drought-induced mortality. All species assessed had 60% or higher loss of xylem hydraulic conductivity, consistent with proposed theoretical and modelled survival thresholds. We found diverse responses in non-structural carbohydrate reserves at mortality, indicating that evidence supporting carbon starvation was not universal. Reduced non-structural carbohydrates were more common for gymnosperms than angiosperms, associated with xylem hydraulic vulnerability, and may have a role in reducing hydraulic function. Our finding that hydraulic failure at drought-induced mortality was persistent across species indicates that substantial improvement in vegetation modelling can be achieved using thresholds in hydraulic function