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Increasing fire severity and warmer, drier postfire conditions are making forests in the western United States (West) vulnerable to ecological transformation. Yet, the relative importance of and interactions between these drivers of forest change remain unresolved, particularly over upcoming decades. Here, we assess how the interactive impacts of changing climate and wildfire activity influenced conifer regeneration after 334 wildfires, using a dataset of postfire conifer regeneration from 10,230 field plots. Our findings highlight declining regeneration capacity across the West over the past four decades for the eight dominant conifer species studied. Postfire regeneration is sensitive to high-severity fire, which limits seed availability, and postfire climate, which influences seedling establishment. In the near-term, projected differences in recruitment probability between low- and high-severity fire scenarios were larger than projected climate change impacts for most species, suggesting that reductions in fire severity, and resultant impacts on seed availability, could partially offset expected climate-driven declines in postfire regeneration. Across 40 to 42% of the study area, we project postfire conifer regeneration to be likely following low-severity but not high-severity fire under future climate scenarios (2031 to 2050). However, increasingly warm, dry climate conditions are projected to eventually outweigh the influence of fire severity and seed availability. The percent of the study area considered unlikely to experience conifer regeneration, regardless of fire severity, increased from 5% in 1981 to 2000 to 26 to 31% by mid-century, highlighting a limited time window over which management actions that reduce fire severity may effectively support postfire conifer regeneration.

Fire is an integral component of ecosystems globally and a tool that humans have harnessed for millennia. Altered fire regimes are a fundamental cause and consequence of global change, impacting people and the biophysical systems on which they depend. As part of the newly emerging Anthropocene, marked by human-caused climate change and radical changes to ecosystems, fire danger is increasing, and fires are having increasingly devastating impacts on human health, infrastructure, and ecosystem services. Increasing fire danger is a vexing problem that requires deep transdisciplinary, trans-sector, and inclusive partnerships to address. Here, we outline barriers and opportunities in the next generation of fire science and provide guidance for investment in future research. We synthesize insights needed to better address the long-standing challenges of innovation across disciplines to (i) promote coordinated research efforts; (ii) embrace different ways of knowing and knowledge generation; (iii) promote exploration of fundamental science; (iv) capitalize on the "firehose" of data for societal benefit; and (v) integrate human and natural systems into models across multiple scales. Fire science is thus at a critical transitional moment. We need to shift from observation and modeled representations of varying components of climate, people, vegetation, and fire to more integrative and predictive approaches that support pathways toward mitigating and adapting to our increasingly flammable world, including the utilization of fire for human safety and benefit. Only through overcoming institutional silos and accessing knowledge across diverse communities can we effectively undertake research that improves outcomes in our more fiery future.

We review science-based adaptation strategies for western North American (wNA) forests that include restoring active fire regimes and fostering resilient structure and composition of forested landscapes. As part of the review, we address common questions associated with climate adaptation and realignment treatments that run counter to a broad consensus in the literature. These include the following: (1) Are the effects of fire exclusion overstated? If so, are treatments unwarranted and even counterproductive? (2) Is forest thinning alone sufficient to mitigate wildfire hazard? (3) Can forest thinning and prescribed burning solve the problem? (4) Should active forest management, including forest thinning, be concentrated in the wildland urban interface (WUI)? (5) Can wildfires on their own do the work of fuel treatments? (6) Is the primary objective of fuel reduction treatments to assist in future firefighting response and containment? (7) Do fuel treatments work under extreme fire weather? (8) Is the scale of the problem too great? Can we ever catch up? (9) Will planting more trees mitigate climate change in wNA forests? And (10) is post-fire management needed or even ecologically justified? Based on our review of the scientific evidence, a range of proactive management actions are justified and necessary to keep pace with changing climatic and wildfire regimes and declining forest heterogeneity after severe wildfires. Science-based adaptation options include the use of managed wildfire, prescribed burning, and coupled mechanical thinning and prescribed burning as is consistent with land management allocations and forest conditions. Although some current models of fire management in wNA are averse to short-term risks and uncertainties, the long-term environmental, social, and cultural consequences of wildfire management primarily grounded in fire suppression are well documented, highlighting an urgency to invest in intentional forest management and restoration of active fire regimes.

The destructive wildfires that occurred recently in the western US starkly foreshadow the possible future of forest ecosystems and human communities in the region. With increases in the area burned by severe wildfire in seasonally dry forests expected to result from climate change, judicious, science-based fire and restoration strategies will be essential for improving the resilience of forest ecosystems. We argue that fire use treatments (including prescribed fires and managed wildfires) as well as restoration thinning strategies, rather than conflicting with existing environmental objectives, will provide numerous co-benefits, including enhanced biodiversity, increased water availability, greater long-term and more sustainable carbon storage, improved forest resilience and adaptation to climate change, and reduced air pollution. Timber production, however, may have to be better aligned with fire management goals to achieve these co-benefits. Taking immediate actions today to promote positive ecological outcomes in seasonally dry forests should be a primary focus of management, particularly in the western US.

Climate influences vegetation directly and through climate-mediated disturbance processes, such as wildfire. Temperature and area burned are positively associated, conditional on availability of vegetation to burn. Fire is a self-limiting process that is influenced by productivity. Yet, many fire projections assume sufficient vegetation to support fire, with substantial implications for carbon (C) dynamics and emissions. We simulated forest dynamics under projected climate and wildfire for the Sierra Nevada, accounting for climate effects on fuel flammability (static) and climate and prior fire effects on fuel availability and flammability (dynamic). We show that compared to climate effects on flammability alone, accounting for the interaction of prior fires and climate on fuel availability and flammability moderates the projected increase in area burned by 14.3%. This reduces predicted increases in area-weighted median cumulative emissions by 38.3 Tg carbon dioxide (CO2) and 0.6 Tg particulate matter (PM1), or 12.9% and 11.5%, respectively. Our results demonstrate that after correcting for potential over-estimates of the effects of climate-driven increases in area burned, California is likely to continue facing significant wildfire and air quality challenges with on-going climate change.

Climate change and fire exclusion have increased the flammability of western United States forests, leading to forest cover loss when wildfires occur under severe weather conditions. Increasingly large high-severity burn patches limit natural regeneration because of dispersal distance, increasing the chance of conversion to non-forest. Post-fire planting can overcome dispersal limitations, yet warmer and drier post-fire conditions can still reduce survival. We examined how two shrub species with different structures affect below-shrub microclimate and survival rates of planted tree seedlings (Pinus ponderosa Lawson & C. Lawson, Pinus edulis Engelm., Pinus strobiformis Engelm., Pseudotsuga menziesii (Mirb.) Franco) following a high-severity fire in northern New Mexico. We expected that Gambel oak (Querus gambelii Nutt.), with its denser canopy, would buffer below-shrub climate causing higher survival rates of planted seedlings more than the lower canopy density New Mexico locust (Robinia neomexicana A. Gray). Seedlings planted under Gambel oak had survival rates 10% to 35% greater than those planted under New Mexico locust. Higher light availability beneath New Mexico locust corresponded to higher temperatures, lower humidity, and higher vapor pressure deficit, impacting the mortality of planted tree seedlings. These results indicate that by waiting for post-fire shrub establishment, selective use of shrubs can buffer microclimate and increase post-fire planting success in the southwestern United States. Key words: climate change, forest transition, post-fire management, Gambel oak, New Mexico locust, early successional vegetation. Le changement climatique et la suppression des incendies ont accru l'inflammabilite des forets de l'ouest des Etats-Unis, entrainant la perte de couvert forestier lorsque les feux de foret surviennent en presence de conditions meteorologiques severes. Les feux de forte intensite qui brulent des zones de plus en plus vastes limitent la regeneration naturelle parce que leur dimension excede la distance de dispersion des graines, ce qui augmente les risques de conversion en zone non forestiere. La plantation apres feu peut surmonter les contraintes associees a la distance de dispersion, mais des conditions apres feu plus chaudes et plus seches peuvent toujours reduire la survie. Nous avons etudie de quelle facon deux especes d'arbustes qui ont differentes structures influencent le microclimat et le taux de survie des semis d'arbre (Pinus ponderosa Lawson & C. Lawson, Pinus edulis Engelm., Pinus strobiformis Engelm., Pseudotsuga menziesii (Mirb.) Franco) plantes sous le couvert de ces arbustes a la suite d'un feu de forte intensite dans le nord du Nouveau Mexique. Nous nous attendions a ce que le chene de Gambel (Quercus gambelii Nutt.), etant donne son couvert forestier plus dense, tempere le climat sous les arbustes, entrainant des taux de survie plus eleves des semis plantes que le couvert forestier moins dense du robinier du Nouveau Mexique (Robinia neomexicana A. Gray). Les semis plantes sous le chene de Gambel avaient un taux de survie 10 a 35% superieur a ceux qui avaient ete plantes sous le robinier du Nouveau Mexique. La plus grande disponibilite de la lumiere sous le robinier du Nouveau Mexique correspondait a des temperatures plus elevees, une plus faible humidite et un deficit de saturation plus eleve, ce qui avait un impact sur la mortalite des semis d'arbre plantes. Ces resultats indiquent qu'en attendant l'etablissement apres feu d'arbustes l'utilisation selective d'arbustes peut proteger le microclimat et augmenter le succes de la plantation apres feu dans le sud-ouest des Etats-Unis. [Traduit par la Redaction] Mots-cles : changement climatique, transition forestiere, amenagement apres feu, chene de Gambel, robinier du Nouveau Mexique, vegetation de debut de succession., 1. Introduction The ongoing warming associated with climate change is increasing the flammability of western United States (US) dry forests, which is compounding the problems associated with increased biomass from [...]

Live shrubs in forest understories pose a challenge for mitigating wildfire risk with prescribed fire. Factors driving shrub consumption in prescribed fires are variable and difficult to explain. This study investigated spatial patterns and drivers of Sierra Nevada mixed-conifer forest shrub consumption in prescribed fires through analysis of high-resolution imagery taken before and after prescribed fire. We applied a spatially explicit, generalized additive model to assess tree cover and coarse woody material as potential drivers of shrub consumption. Shrub cover in two experimental stands prior to burning was 38% and 59% and was 36% and 45% one-year post-burn. In both stands shrub patch density increased, while area-weighted mean patch size and largest patch index decreased. Increased local percent cover of coarse woody material was associated with increased shrub consumption. These findings provide information for prescribed fire managers to help better anticipate shrub consumption and patchiness outcomes under similar conditions. Key words: high resolution imagery, Teakettle Experimental Forest, mixed- conifer, fire hazards, fuel treatments, coarse woody debris. Les arbustes vivants dans les sous-bois constituent un defi pour l'attenuation des risques d'incendie lors d'un brulage dirige. Les facteurs responsables de la combustion des arbustes lors des brulages diriges sont varies et difficiles a cerner. Cette etude se penche sur les patrons spatiaux et les facteurs determinants de la combustion des arbustes dans la foret mixte de coniferes de la Sierra Nevada lors de brulages diriges au moyen de l'analyse d'images a haute resolution prises avant et apres un brulage dirige. Nous avons applique un modele additif generalise, spatialement explicite, pour evaluer le couvert arborescent et le materiel ligneux grossier en tant que facteurs determinants potentiels de la combustion des arbustes. Le couvert arbustif dans deux peuplements experimentaux etait de 38 et 59 % avant le brulage et de 36 et 45 % un an apres le brulage. Dans les deux peuplements, la densite des ilots d'arbustes a augmente tandis que la taille moyenne des ilots ponderee par la superficie et l'indice des plus grands ilots ont diminue. L'augmentation locale du pourcentage de couverture du materiel ligneux grossier etait associee a l'augmentation de la combustion des arbustes. Ces resultats fournissent de l'information aux gestionnaires des brulages diriges pour les aider a mieux anticiper la combustion et la repartition en ilots des arbustes dans des conditions similaires. [Traduit par la Redaction] Mots-cles: imagerie a haute resolution, foret experimentale de Teakettle, coniferes mixtes, risque d'incendie, traitements des combustibles, debris ligneux grossiers., Introduction Shrubs are an essential ecosystem component of forested environments in the western United States and are important for wildlife, nutrient cycling, and biodiversity (Hunter 1990; North et al. 2016). [...]

Changing climate and increasing area burned pose a challenge to forest carbon (C) storage, which is compounded by an elevated risk of high‐severity wildfire due to long‐term fire suppression in the westernUS. Restoration treatments that reduce tree density and reintroduce surface fire are effective at moderating fire effects and may help build adaptive capacity to changing environmental conditions. However, treatment implementation has been slow and spatially limited relative to the extent of the area affected by fire suppression. Using model simulations, we quantified how large‐scale restoration treatments in frequent‐fire forest types would influence C outcomes in the Sierra Nevada mountain range under projected climate–wildfire interactions. Our results indicate that large‐scale restoration treatments are an effective means of reducing fire hazard and increasing C storage and stability under future climate and wildfire conditions. The effects of implementation timing suggest that accelerated implementation of large‐scale restoration treatments may confer greater C‐storage benefits, supporting California's efforts to combat climate change.

Ecosystem carbon carrying capacity (CCC) is determined by prevailing climate and natural disturbance regimes, conditions that are projected to change significantly. The interaction of changing climate and its effects on disturbance regimes is expected to affect forest regeneration and growth, which may diminish forest carbon (C) stocks and uptake. We modeled landscape C dynamics over 590 years along the latitudinal gradient of the U.S. Sierra Nevada Mountains under climate and area burned by large wildfires projected by late 21st century. We assumed climate and wildfire stabilize at late-21st century conditions (2090-2100) to facilitate analysis of lags between warming and changing CCC. We show that compared with historical (1980-2010) climate and wildfire conditions, projected scenarios would drive a significant decrease of up to 73% in mean total ecosystem carbon (TEC) by the end of the 590-year simulation. Tree regeneration failure due to intensified growing season dryness and increased area burned would substantially decrease forested area, transitioning the system from C sink to source. Our results demonstrate the potential for a lower CCC in the system due to extensive vegetation type conversion from forest to non-forest types, and suggest a decline in the contribution of Sierra Nevada forests to U.S. C sink.

Climate influences forests directly and indirectly through disturbance. The interaction of climate change and increasing area burned has the potential to alter forest composition and community assembly. However, the overall forest response is likely to be influenced by species-specific responses to environmental change and the scale of change in overstory species cover. In this study, we sought to quantify how projected changes in climate and large wildfire size would alter forest communities and carbon (C) dynamics, irrespective of competition from nontree species and potential changes in other fire regimes, across the Sierra Nevada, USA. We used a species-specific, spatially explicit forest landscape model (LANDIS-II) to evaluate forest response to climate-wildfire interactions under historical (baseline) climate and climate projections from three climate models (GFDL, CCSM3, and CNRM) forced by a medium-high emission scenario (A2) in combination with corresponding climate-specific large wildfire projections. By late century, we found modest changes in the spatial distribution of dominant species by biomass relative to baseline, but extensive changes in recruitment distribution. Although forest recruitment declined across much of the Sierra, we found that projected climate and wildfire favored the recruitment of more drought-tolerant species over less drought-tolerant species relative to baseline, and this change was greatest at mid-elevations. We also found that projected climate and wildfire decreased tree species richness across a large proportion of the study area and transitioned more area to a C source, which reduced landscape-level C sequestration potential. Our study, although a conservative estimate, suggests that by late century, forest community distributions may not change as intact units as predicted by biome-based modeling, but are likely to trend toward simplified community composition as communities gradually disaggregate and the least tolerant species are no longer able to establish. The potential exists for substantial community composition change and forest simplification beyond this century.

Tree establishment following severe or stand-replacing disturbance is critical for achieving U.S. climate change mitigation goals and for maintaining the co-benefits of intact forest ecosystems. In many contexts, natural post-fire tree regeneration is sufficient to maintain forest cover and associated ecosystem services, but increasingly the pattern and scale of disturbance exceeds ecological thresholds and active reforestation may be warranted. Our capacity to plant trees, however, is not keeping pace with reforestation needs. This shortfall is uniquely apparent in the western U.S., where wildfire size and severity have increased in recent decades and long-term divestment in the reforestation supply chain has limited our ability to respond to existing needs. Here we present an analysis of key facets of both the supply and demand side of reforestation in the western U.S. and address six questions: (1) What is the current backlog of potential reforestation needs driven by high-severity wildfire?; (2) How will increasing wildfire activity through the end of the century affect potential reforestation needs?; (3) What is our capacity to meet current and future reforestation needs?; (4) How can we scale the reforestation supply chain to meet current and future demands?; (5) What approaches to reforestation can promote forest resilience to climate change and wildfire?; and (6) Where are opportunities emerging from recent policy initiatives, innovative public-private partnerships, and natural capital markets for scaling reforestation? Between 1984 and 2000, annual tree planting capacity met post-fire needs but cumulatively over the last two decades (2000 to 2021) it has fallen short of fire-driven needs by an estimated 1.5 million ha (ca. 3.8 million ac). We anticipate this gap will increase 2 to 3 fold by 2050. Scaling up reforestation efforts to close this gap will require increased investment across all facets of the reforestation supply chain, public-private partnerships, and novel approaches to reforestation that increase the resilience of western forests to drought and wildfire. We highlight emerging opportunities from recent policy initiatives and conservation finance for expanding reforestation efforts. [ABSTRACT FROM AUTHOR]

Changing climatic conditions are influencing large wildfire frequency, a globally widespread disturbance that affects both human and natural systems. Understanding how climate change, population growth, and development patterns will affect the area burned by and emissions from wildfires and how populations will in turn be exposed to emissions is critical for climate change adaptation and mitigation planning. We quantified the effects of a range of population growth and development patterns in California on emission projections from large wildfires under six future climate scenarios. Here we show that end-of-century wildfire emissions are projected to increase by 19-101% (median increase 56%) above the baseline period (1961-1990) in California for a medium-high temperature scenario, with the largest emissions increases concentrated in northern California. In contrast to other measures of wildfire impacts previously studied (e.g., structural loss), projected population growth and development patterns are unlikely to substantially influence the amount of projected statewide wildfire emissions. However, increases in wildfire emissions due to climate change may have detrimental impacts on air quality and, combined with a growing population, may result in increased population exposure to unhealthy air pollutants.

Climate change and disturbance are altering forests and the rates and locations of tree regeneration. In semi-arid forests of the southwestern USA, limitations imposed by hot and dry conditions are likely to influence seedling survival. We examined how the survival of 1-year seedlings of five southwestern US conifer species whose southwestern distributions range from warmer and drier woodlands and forests (Pinus edulis Engelm. Pinus ponderosa Douglas ex C. Lawson) to cooler and wetter subalpine forests (Pseudotsuga menziesii (Mirb.) Franco , Abies concolor (Gord. & Glend.) Lindl. Ex Hildebr. and Picea engelmannii Parry ex Engelm.) changed in response to low moisture availability, high temperatures and high vapor pressure deficit in incubators. We used a Bayesian framework to construct discrete-time proportional hazard models that explained 55–75% of the species-specific survival variability. We applied these to the recent climate (1980–2019) of the southwestern USA as well as 1980–2099 CMIP5 climate projections with the RCP8.5 emissions pathway. We found that the more mesic species (i.e. P. menziesii , A. concolor and P. engelmannii) were more susceptible to the effects of hot and dry periods. However, their existing ranges are not projected to experience the conditions we tested as early in the 21st century as the more xeric P. edulis and P. ponderosa , leading to lower percentages of their existing ranges predicted to experience seedling-killing conditions. By late-century, extensive areas of each species southwestern range could experience climate conditions that increase the likelihood of seedling mortality. These results demonstrate that empirically derived physiological limitations can be used to inform where species composition or vegetation type change are likely to occur in the southwestern USA. [ABSTRACT FROM AUTHOR]

Climate, disturbance, vegetation response and their interaction are key factors in predicting the distribution and function of ecosystems across landscapes. A range of factors, operating through different pathways, are amplifying the feedbacks in this three‐way interaction.In the western USA, the relative strength of the influence of climate versus vegetation on fire activity varies spatially, realizing a diversity of fire regimes and fire‐selected species traits under historical conditions of climate–fire–vegetation interactions.Human intervention, via land use and fire‐exclusion, has homogenized frequent‐fire‐adapted forests in terms of structure and composition. Climate change is reinforcing the homogenization directly via increasing temperatures and drought and indirectly through climate‐driven tree mortality. The net effect will be forming novel climate–fire–vegetation interactions that act to homogenize fire regimes and catalyse large‐scale forest loss.While long‐term persistence of tree species in a given location may not be possible due to directional change of climate, slowing the rate of wildfire‐driven forest cover loss and maximizing the in‐situ persistence of a diversity of species will allow forest ecosystems to respond more incrementally to changing climate and provide an opportunity for ecosystem reassembly to occur from a large pool of species.As climate continues to change, management to resist wildfire‐driven forest cover loss may hinge on reducing forest density and creating a higher level of heterogeneity to reach the resistance and resilience exhibited by pre‐fire‐exclusion forests. Management operations should better leverage disturbance while strategically deploying silvicultural treatments to increase managed and prescribed fire.Further research is needed to improve our capacity for quantifying key mechanisms and system responses involved in the climate–fire–vegetation interactions and predicting how best to allocate resources to manage for functional forests. Read the free Plain Language Summary for this article on the Journal blog. [ABSTRACT FROM AUTHOR]

In forested systems throughout the world, climate influences tree growth and aboveground net primary productivity (ANPP). The effects of extreme climate events (i.e., drought) on ANPP can be compounded by biotic factors (e.g., insect outbreaks). Understanding the contribution of each of these influences on growth requires information at multiple spatial scales and is essential for understanding regional forest response to changing climate. The mixed conifer forests of the Lake Tahoe Basin, California and Nevada, provide an opportunity to analyze biotic and abiotic influences on ANPP. Our objective was to evaluate the influence of moisture stress (climatic water deficit, CWD) and bark beetles on basin-wide ANPP from 1987 to 2006, estimated through tree core increments and a landscape simulation model (LANDIS-II). Tree ring data revealed that ANPP increased throughout this period and had a nonlinear relationship to water demand. Simulation model results showed that despite increased complexity, simulations that include moderate moisture sensitivity and bark beetle outbreaks most closely approximated the field-derived ANPP--CWD relationship. Although bark beetle outbreaks and episodic drought-induced mortality events are often correlated, decoupling them within a simulation model offers insight into assessing model performance, as well as examining how each contributes to total declines in productivity. Key words: ANPP, net ecosystem production, increment cores, forest simulation model, LANDIS-II. Dans les ecosystemes forestiers partout dans le monde, le climat influence la croissance des arbres et la productivity primaire nette aerienne (PPNA). Les effets d'evenements climatiques extremes (c.-a-d. la secheresse) sur la PPNA peuvent etre aggraves par des facteurs biotiques (p. ex. des epidemies d'insectes). Pour comprendre la contribution de chacune de ces influences sur la croissance requiert de l'information a de multiples echelles spatiales et cela est essentiel pour comprendre la reaction regionale de la foret au changement climatique. Les forets melangees de coniferes du bassin du lac Tahoe, en Californie et au Nevada, fournissent une occasion d'analyser l'influence des facteurs biotiques et abiotiques sur la PPNA. Notre objectif consistait a evaluer l'influence du stress hydrique (deficit hydrique climatique, DHC) et des scolytes sur la PPNA a l'echelle d'un bassin de 1987 a 2006. La PPNA a ete estimee a l'aide de carotte prelevees sur des arbres et d'un modele de simulation du paysage (LANDIS-II). Les donnees dendrometriques revelent que la PPNA a augmente durant cette periode et qu'elle avait une relation non lineaire avec la demande en eau. Les resultats du modeles indiquent que, malgre l'augmentation de la complexite, les simulations qui incluent un niveau modere de sensibilite a l'humidite et les epidemies de scolytes produisent la plus proche approximation de la relation entre la PPNA et le DHC derivee a partir de donnees terrain. Bien que les epidemies de scolytes et la mortalite episodique induite par la secheresse soient souvent correlees, le fait de les decoupler dans un modele de simulation fournit des renseignements pour evaluer la performance du modele ainsi que pour examiner de quelle facon chaque facteur contribue au declin total de la productivite. [Traduit par la Redaction] Mots-cles: PPNA, production nette de l'ecosysteme, carottes de sondage, modele de simulation forestiere, LANDIS-II., Introduction Forests are an integral component of the global carbon (C) cycle, sequestering approximately 30% of annual anthropogenic C emissions (Pan et al. 2011a). Estimates of forest C dynamics are [...]