Postfire futures in southwestern forests: Climate and landscape influences on trajectories of recovery and conversion.

Southwestern ponderosa pine forests are vulnerable to fire-driven conversion in a warming and drying climate, yet little is known about what kinds of ecological communities may replace them. To characterize postfire vegetation trajectories and their environmental determinants, plant assemblages (361 sample plots including 229 vascular plant species, surveyed in 2017) were sampled within eight burns that occurred between 2000 and 2003. I used nonmetric multidimensional scaling, k-means clustering, principal component analysis, and random forest models to assess relationships between vegetation pattern, topographic and landscape factors, and gridded climate data. I describe seven postfire community types, including regenerating forests of ponderosa pine, aspen, and mixed conifers, shrub-dominated communities of Gambel oak and mixed species, and herb-dominated communities of native bunchgrasses and mixtures of ruderal, native, and nonnative species. Forest recovery was generally associated with cooler, mesic sites in proximity to forested refugia; shifts toward scrub and grassland types were most common in warmer, dryer locations distant from forested refugia. Under future climate scenarios, models project decreases in postfire forest recovery and increases in nonforest vegetation. However, forest to nonforest conversion was partially offset under a scenario of reduced burn severity and increased retention of forested refugia, highlighting important management opportunities. Burning trends in the southwestern United States suggest that postfire vegetation will occupy a growing landscape fraction, compelling renewed management focus on these areas and paradigm shifts that accommodate ecological change. I illustrate how management decisions around resisting, accepting, or directing change could be informed by an understanding of processes and patterns of postfire community variation and likely future trajectories.

Fire and land use impact soil properties in a Mediterranean dry sclerophyll woodland.

Fire directly impacts soil properties responsible for soil function and can result in soil degradation. Across the globe, climate change-induced droughts and elevated temperatures are exacerbating fire regime severity, breadth, and frequency, thus posing a threat to soil function and dependent ecosystem services. In Australia, the 2019-2020 fire season consumed nearly 50% of Kangaroo Island, South Australia, burning both dry sclerophyll woodland and adjacent historically cleared and grazed pastureland. Due to exacerbated fire regime elements, e.g., intensity and area affected, and interactions with historical land use, post-fire recovery of soil function was uncertain. This study assessed the impacts of a) the 2019-2020 fire event in Western River, Kangaroo Island on dry sclerophyll woodland and b) the interaction between this fire event and historical clearing and grazing on post-fire function of the soil. To do so, the following physicochemical and biological soil properties were analysed: labile active carbon, total carbon, total nitrogen, carbon to nitrogen ratio (C/N), pH, electrical conductivity, soil water repellency, aggregate stability, microbial community composition, and microbial diversity. Our results showed that the fire was of high severity, causing a reduction in nutrient content, an extreme rise in pH, and significant modifications to fungal communities in burnt compared to unburnt dry sclerophyll woodland. Furthermore, clearing and grazing raised post-fire soil nutrient levels and soil microbial diversity but reduced soil C/N and the abundance of ectomycorrhizal fungi in burnt pastureland compared to burnt woodland soils. This study highlights the role of management and fire severity in post-fire outcomes and emphasizes the need for comprehensive soil function assessments to evaluate the impacts of disturbance on soil. Taking direct measure of soil properties, as done here, will improve future assessments of fire season impacts and post-fire recovery in fire-prone landscapes.

Mountain lions avoid burned areas and increase risky behavior after wildfire in a fragmented urban landscape.

Urban environments are high risk areas for large carnivores, where anthropogenic disturbances can reduce fitness and increase mortality risk.1 When catastrophic events like large wildfires occur, trade-offs between acquiring resources and avoiding risks of the urban environment are intensified. This landscape context could lead to an increase in risk-taking behavior by carnivores if burned areas do not allow them to meet their energetic needs, potentially leading to human-wildlife conflict.2,3 We studied mountain lion behavior using GPS location and accelerometer data from 17 individuals tracked before and after a large wildfire (the 2018 Woolsey Fire) within a highly urbanized area (Los Angeles, California, USA). After the wildfire, mountain lions avoided burned areas and increased behaviors associated with anthropogenic risk, including more frequent road and freeway crossings (mean crossings increased from 3 to 5 per month) and greater activity during the daytime (means from increased 10% to 16% of daytime active), a time when they are most likely to encounter humans. Mountain lions also increased their amount of space used, distance traveled (mean distances increased from 250 to 390 km per month), and intrasexual overlap, potentially putting them at risk of intraspecific conflict. Joint pressures from urbanization and severe wildfire, alongside resulting risk-taking, could thus increase mortality and extinction risk for populations already suffering from low genetic diversity, necessitating increased connectivity in fire-prone areas.

Evidence for widespread changes in the structure, composition, and fire regimes of western North American forests.

Implementation of wildfire- and climate-adaptation strategies in seasonally dry forests of western North America is impeded by numerous constraints and uncertainties. After more than a century of resource and land use change, some question the need for proactive management, particularly given novel social, ecological, and climatic conditions. To address this question, we first provide a framework for assessing changes in landscape conditions and fire regimes. Using this framework, we then evaluate evidence of change in contemporary conditions relative to those maintained by active fire regimes, i.e., those uninterrupted by a century or more of human-induced fire exclusion. The cumulative results of more than a century of research document a persistent and substantial fire deficit and widespread alterations to ecological structures and functions. These changes are not necessarily apparent at all spatial scales or in all dimensions of fire regimes and forest and nonforest conditions. Nonetheless, loss of the once abundant influence of low- and moderate-severity fires suggests that even the least fire-prone ecosystems may be affected by alteration of the surrounding landscape and, consequently, ecosystem functions. Vegetation spatial patterns in fire-excluded forested landscapes no longer reflect the heterogeneity maintained by interacting fires of active fire regimes. Live and dead vegetation (surface and canopy fuels) is generally more abundant and continuous than before European colonization. As a result, current conditions are more vulnerable to the direct and indirect effects of seasonal and episodic increases in drought and fire, especially under a rapidly warming climate. Long-term fire exclusion and contemporaneous social-ecological influences continue to extensively modify seasonally dry forested landscapes. Management that realigns or adapts fire-excluded conditions to seasonal and episodic increases in drought and fire can moderate ecosystem transitions as forests and human communities adapt to changing climatic and disturbance regimes. As adaptation strategies are developed, evaluated, and implemented, objective scientific evaluation of ongoing research and monitoring can aid differentiation of warranted and unwarranted uncertainties.

Wildfire-Driven Forest Conversion in Western North American Landscapes.

Changing disturbance regimes and climate can overcome forest ecosystem resilience. Following high-severity fire, forest recovery may be compromised by lack of tree seed sources, warmer and drier postfire climate, or short-interval reburning. A potential outcome of the loss of resilience is the conversion of the prefire forest to a different forest type or nonforest vegetation. Conversion implies major, extensive, and enduring changes in dominant species, life forms, or functions, with impacts on ecosystem services. In the present article, we synthesize a growing body of evidence of fire-driven conversion and our understanding of its causes across western North America. We assess our capacity to predict conversion and highlight important uncertainties. Increasing forest vulnerability to changing fire activity and climate compels shifts in management approaches, and we propose key themes for applied research coproduced by scientists and managers to support decision-making in an era when the prefire forest may not return.