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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.

Wildfire-mediated changes to forests have prompted numerous studies on post-fire forest recovery of coniferous forests. Given climate change, a growing body of work demonstrates that conifer regeneration in temperate and boreal forests is declining, a phenomenon often termed 'regeneration failure.' However, the definition and parameters are numerous and variable. Characterization of drought also varies greatly, thus hindering the ability to compare results among areas. This review discusses new perspectives on conifer regeneration failure and places these studies into the context of drought and fire activity. We focus this review on three forest types where conifer regeneration failure is well documented: western boreal forests, cold mixed-conifer forests, and dry pine forests. To place the challenges to conifer tree regeneration in the context of regional climate trends, we present a novel regional analysis that summarizes drought conditions prior, during, and following the year of a large wildfire. We demonstrate the need to assess failure in the context of specific forest dynamics and well-defined metrics. For example, tree establishment may historically occur over longer periods, and current and future climate may exacerbate this and not promote pre-fire forest structure and composition. Many forests are undergoing rapid change and the type, magnitude, and causes of changes need to be compared among areas. As such, we should be cautious of quantifying 'regeneration failure' and drought without providing spatial and temporal context. Key words: tree regeneration, drought, climate change, forest conversion, regeneration failure, wildfires Les modifications des forets provoquees par les incendies de foret ont suscite de nombreuses etudes sur la regeneration des forets de coniferes apres un incendie. Compte tenu des changements climatiques, un nombre croissant de travaux demontre que la regeneration des coniferes dans les forets temperees et boreales est en declin, un phenomene souvent appele << accident de regeneration >>. Cependant, la definition et les parametres sont nombreux et variables. La caracterisation du phenomene de secheresse varie egalement beaucoup, ce qui entrave la capacite de comparer les resultats entre les regions. Cet article presente de nouvelles perspectives sur les accidents de regeneration des coniferes et place ces etudes dans le contexte des secheresses et des incendies. Nous nous concentrons sur trois types de forets ou les accidents de regeneration des coniferes sont bien documente : les forets boreales de l'Ouest, les forets mixtes froides de coniferes et les forets seches de pins. Pour placer les defis de la regeneration des coniferes dans le contexte des tendances climatiques regionales, nous presentons une nouvelle analyse regionale qui resume les conditions de secheresse avant, pendant et apres l'annee d'un grand incendie de foret. Nous demontrons la necessite d'evaluer l'accident de regeneration dans le contexte de dynamiques forestieres specifiques et de mesures bien definies. Par exemple, l'etablissement des arbres peut historiquement se produire sur des periodes plus longues, et le climat actuel et futur peut exacerber ce phenomene et ne pas favoriser la structure et la composition de la foret avant l'incendie. De nombreuses forets subissent des changements rapides et le type, l'ampleur et les causes de ces changements doivent etre compares entre les differentes regions. En tant que tel, nous devrions etre prudents dans la quantification des << accidents de regeneration >> et des secheresses sans fournir un contexte spatial et temporel. [Traduit par la Redaction] Mots-cles : regeneration des arbres, secheresse, changement climatique, conversion des forets, echec de la regeneration, incendies de foret, Introduction The composition, structure, and ecological function of forests, which occupy around 30% of the global landmass, are strongly patterned by landform and climate (Bond et al. 2005). Worldwide, forest [...]

Under climate change, ecosystems are experiencing novel drought regimes, often in combination with stressors that reduce resilience and amplify drought's impacts. Consequently, drought appears increasingly likely to push systems beyond important physiological and ecological thresholds, resulting in substantial changes in ecosystem characteristics persisting long after drought ends (i.e. ecological transformation). In the present article, we clarify how drought can lead to transformation across a wide variety of ecosystems including forests, woodlands, and grasslands. Specifically, we describe how climate change alters drought regimes and how this translates to impacts on plant population growth, either directly or through drought's interactions with factors such as land management, biotic interactions, and other disturbances. We emphasize how interactions among mechanisms can inhibit postdrought recovery and can shift trajectories toward alternate states. Providing a holistic picture of how drought initiates long-term change supports the development of risk assessments, predictive models, and management strategies, enhancing preparedness for a complex and growing challenge. [ABSTRACT FROM AUTHOR]

With longer and more severe fire seasons predicted, the incidence and extent of fires are expected to increase in western North America. As more area is burned, past wildfires may influence the spread and burn severity of subsequent fires, with implications for ecosystem resilience and fire management. We examined how previous burn severity, topography, vegetation, and weather influenced burn severity on four wildfires, two in Idaho, one in Washington, and one in British Columbia. These were large fire events, together burning 330 000 ha and cost $165 million USD in fire suppression expenditures. Collectively, these four study fires reburned over 50 000 ha previously burned between 1984 and 2006. We used sequential autoregression to analyze how past fires, topography, vegetation, and weather influenced burn severity. We found that areas burned in the last three decades, at any severity, had significantly lower severity in the subsequent fire. Final models included maximum temperature, vegetation cover type, slope, and elevation as common predictors. Across all study fires and burning conditions within them, burn severity was reduced in previously burned areas, suggesting that burned landscapes mitigate subsequent fire effects even with the extreme fire weather under which these fires burned. Key words: fire weather, reburn, repeated wildfires, sequential autoregression, self-regulation. Prevoyant que la saison des feux sera plus longue et plus severe, on anticipe une augmentation de l'incidence et de l'etendue des feux dans l'ouest de l'Amerique du Nord. A mesure que davantage de superficie est brulee, les feux de foret passes peuvent influencer la propagation et la severite des feux subsequents, ce qui a des implications pour la resilience des ecosystemes et la gestion du feu. Nous avons etudie de quelle facon la severite des feux passes, la topographie, la vegetation et les conditions meteorologiques ont influence la severite de quatre feux de foret : deux en Idaho, un dans l'Etat de Washington et un en Colombie-Britannique. Il s'agit de feux majeurs qui ensemble ont brule 330 000 ha et engendre des couts de suppression de 165 millions de dollars US. Collectivement, les quatre feux a l'etude ont brule a nouveau 50 000 ha deja brules entre 1984 et 2006. Nous avons utilise l'autoregression sequentielle pour analyser comment les feux passes, la topographie, la vegetation et les conditions meteorologiques ont influence la severite des feux plus recents. Nous avons trouve que les zones brulees au cours des trois dernieres decennies, peu importe la severite, avaient une severite significativement plus faible lors de feux subsequents. Les modeles finaux incluaient la temperature maximum, le type de couvert vegetal, la pente et l'altitude en tant que predicteurs communs. Parmi tous les feux etudies et les conditions de brulage associees a ces feux, la severite du feu etait reduite dans les endroits qui avaient brule anterieurement, indiquant que les paysages ayant deja brules attenuent les effets des feux subsequents, meme si ces feux surviennent dans des conditions meteorologiques extremes. [Traduit par la Redaction] Mots-cles : conditions meteorologiques propices aux feux de foret, rebrulage, feux de foret repetes, autoregression sequentielle, autoregulation., Introduction As a self-regulating process, the pattern of previous fires may limit the progression and burn severity of subsequent wildfires for some time due to limited burnable fuels and changes [...]

Earth is experiencing widespread ecological transformation in terrestrial, freshwater, and marine ecosystems that is attributable to directional environmental changes, especially intensifying climate change. To better steward ecosystems facing unprecedented and lasting change, a new management paradigm is forming, supported by a decision-oriented framework that presents three distinct management choices: resist, accept, or direct the ecological trajectory. To make these choices strategically, managers seek to understand the nature of the transformation that could occur if change is accepted while identifying opportunities to intervene to resist or direct change. In this article, we seek to inspire a research agenda for transformation science that is focused on ecological and social science and based on five central questions that align with the resist–accept–direct (RAD) framework. Development of transformation science is needed to apply the RAD framework and support natural resource management and conservation on our rapidly changing planet. [ABSTRACT FROM AUTHOR]

Assessing resilience of wildfire-prone social-ecological systems (SES) can provide a baseline for decision makers and inform future management actions. In this study, we measure stakeholder perceptions of how SES resilience was influenced by historic 2020-wildfire events in northern Colorado and what actions they think are needed to enhance future resilience to wildfire. We developed a contextualized set of resilience principles and indicators based on the literature and expert opinion that included ecological, ecosystem service, and social categories. We conducted 15 stakeholder interviews and 37 surveys to characterize subjective measures of SES resilience. Stakeholders perceived that the resilience of ecological indicators pre- and post-wildfires remained mostly unchanged, but that ecosystem service indicators were negatively affected and social indicators positively affected. Related to future resilience, stakeholders underscored a need for more strategic fuel treatments, especially prescribed fire, and more collaboration and engagement across landowner types. Our findings highlight that wildfire events can have negative and positive effects on components of SES resilience, underscoring the need to disaggregate resilience components. Additionally, we find that stakeholder perceptions of resilience are in line with the literature on objective measures of post-wildfire resilience, suggesting that expert opinion can be used as a reliable method to measure resilience indicators in some contexts. [ABSTRACT FROM AUTHOR]

The 2002 Hayman Fire burned with mixed-severity across a 400-ha dry conifer study site in Colorado, USA, where overstory tree and surface cover attributes had been recently measured on 20 0.1-ha permanent plots. We remeasured these plots repeatedly during the first post-fire decade to examine how the attributes changed through time and whether changes were influenced by fire severity. We found that most attributes were temporally dynamic and that fire severity shaped their dynamics. For example, low-severity plots experienced a modest reduction in live overstory density due to both immediate and delayed tree mortality, and no change in live overstory basal area through time; in contrast, high-severity plots experienced an immediate and total loss of live overstory density and basal area. Large snag density in low-severity plots did not vary temporally because snag recruitment balanced snag loss; however, in high-severity plots large snag density increased markedly immediately post-fire and then declined by about half by post-fire year ten as snags fell. Mineral soil cover increased modestly immediately post-fire in low-severity plots and substantially immediately post-fire in high-severity plots, but changed little in ensuing years for either severity class. By incorporating pre-fire and repeatedly-measured post-fire data for a range of severities, our study uniquely contributes to the current understanding of wildfire effects in dry conifer forests and should be of interest to managers, researchers, and others. [ABSTRACT FROM AUTHOR]

Abstract: Forest resilience to climate change is a global concern given the potential effects of increased disturbance activity, warming temperatures and increased moisture stress on plants. We used a multi‐regional dataset of 1485 sites across 52 wildfires from the US Rocky Mountains to ask if and how changing climate over the last several decades impacted post‐fire tree regeneration, a key indicator of forest resilience. Results highlight significant decreases in tree regeneration in the 21st century. Annual moisture deficits were significantly greater from 2000 to 2015 as compared to 1985–1999, suggesting increasingly unfavourable post‐fire growing conditions, corresponding to significantly lower seedling densities and increased regeneration failure. Dry forests that already occur at the edge of their climatic tolerance are most prone to conversion to non‐forests after wildfires. Major climate‐induced reduction in forest density and extent has important consequences for a myriad of ecosystem services now and in the future. [ABSTRACT FROM AUTHOR]