Anthropogenic climate change, timber harvest, livestock grazing, fire suppression, and changes in human ignitions have altered fire regimes in western North America. Wildland fire is increasingly causing abrupt changes in ecosystem structure and composition, especially where fire frequency or intensity mismatch species traits or moisture limitations constrain recovery. Feedbacks between vegetation and fire can alter fire-regime characteristics and ecosystem responses, but these dynamics are challenging to project. Understanding these feedbacks is crucial for projecting future fire activity (frequency and severity), as reduced burn severity could allow forests to reorganize as alternative forest types rather than transforming to non-forest. Transformations from forest to non-forest are expected to continue on climatic margins of forest cover, but the factors influencing forest persistence in altered states remain unresolved. Projections of future fire activity, ecosystem responses, and associated risks are vital for scientists and land managers. This study aims to quantify exposure of conifer forests to fire-regime change under a 2°C warming scenario, explore the direction and spatial distribution of vegetation feedbacks reflected by changes in burn severity, and estimate the vulnerability of conifer forests to fire-catalyzed ecological transformation by intersecting exposure with information about tree species composition and fire-resistance traits. Current approaches, including correlative and mechanistic models, have limitations in fully integrating fire occurrence, burn severity, and ecosystem responses. This study uses a multivariate approach, treating vegetation, fire, and their feedbacks as emergent properties of a complex system, constrained by climate but manifesting in non-deterministic ways due to local variation in ecosystem characteristics. The study defines exposure to fire-regime change as significant multivariate dissimilarity between fire regimes supported by contemporary and projected future climates, adaptive capacity as the ability of ecosystems to adjust to altered fire regimes, and vulnerability as the intersection of exposure and adaptive capacity.

The literature review examines existing research on altered fire regimes in western North America, highlighting the impacts of climate change and human activities. It discusses the complexities of modeling vegetation-fire interactions and the limitations of correlative and mechanistic models in fully capturing the dynamics of fire occurrence, burn severity, and ecosystem responses. Studies using correlative models, incorporating vegetation, suggest that negative feedbacks could dampen the rate of increase in annual area burned. Fine-scale correlative models indicate a reduced likelihood of fire within decades of an initial burn, consistent with short-term successional dynamics. Mechanistic models, while capable of simulating plausible futures, have not been widely applied at the scale of the western U.S. due to computational and ecological challenges. However, these models can reveal how persistent transformations might emerge, reducing projected area burned or altering the distribution of area burned across categories of fire-caused tree mortality. Existing research emphasizes the need for models that integrate fire occurrence, burn severity, and ecosystem responses, to better represent the complexity of these interactions. Different frameworks exist for understanding ecosystem vulnerability, with commonalities in recognizing that vulnerability is shaped by social-ecological elements beyond climate change alone. The authors utilize a simplified framework of vulnerability specific to their ecosystem and disturbance regime focus.

The study compared contemporary and projected future fire regimes across conifer forests of the western U.S. where conifers comprise >50% of basal area. Remotely sensed observations of fire-regime attributes were grouped based on similarity in 30-year climatological means of climatic water deficit (CWD) and actual evapotranspiration (AET). Climate analogs were used to project movements in climate space, assuming quasi-equilibrium between vegetation and climate over short timescales. Exposure to fire-regime change was defined as the multivariate dissimilarity between distributions of fire frequency, burn severity, and vegetation productivity supported by contemporary and projected future climates. The direction and magnitude of changes in fire frequency, burn severity, and productivity were quantified individually. The +2°C warming scenario was used, acknowledging uncertainties in the timing of temperature increases. Adaptive capacity was characterized by intersecting exposure with an atlas of community-weighted, fire-resistance trait scores, summarizing the relative abundance of traits enabling individual trees to survive low-severity burning. The study used firesheds, spatial containers delineated by U.S. federal fire managers, to summarize results. The analysis utilized a random sample of 5% of Landsat pixels that burned from 1984-2019 in flammable ecoregions. Data included fire rotation period (FRP), composite burn index (CBI), and normalized difference vegetation index (NDVI). The earth mover's distance (EMD) was used to measure multidimensional dissimilarity between contemporary and future fire regimes. Relative changes in FRP, CBI, and NDVI were calculated. A null distribution was used to assess whether projected changes in fire regimes differed from variability within current regimes. Fire resistance was incorporated using an atlas of community-weighted conifer fire-resistance trait scores. Finally, vulnerability among forest types was explored using FIA plot data to identify dominant forest types within each fireshed.

Significant multivariate dissimilarity (exposure to fire-regime change) was projected across 65% of western US conifer forests. Median dissimilarity was significant for >95% of conifer-dominated firesheds. Projected changes in fire frequency, burn severity, and productivity were heterogeneous. Reduced burn severity and productivity were dominant patterns. Firesheds and forest types vulnerable to ecological transformations exhibited high exposure and low adaptive capacity across a broad climatic gradient. Exposure varied substantially across geographic space, reflecting differences in projected changes in mean AET and CWD and observed fire activity. Areas along steep climatic gradients with large projected increases in AET faced high exposure, while areas with low projected change in AET and CWD showed low exposure. Arid regions tended to have higher exposure than mesic regions. Changes in burn severity were linked to changes in fire frequency and vegetation productivity. Future burn severity was projected to be lower than the contemporary period for 63% of conifer forests, higher for 32%, and unchanged for 5%. Widespread declines in productivity were projected, particularly in eastern Oregon, Utah, Colorado, New Mexico, and Arizona. Vulnerable firesheds (high exposure, low fire resistance) were identified, including pinyon-juniper, spruce-fir, and some lodgepole pine and Douglas-fir forests. High exposure was common across Utah and the southern Rocky Mountains, while low exposure was observed in coastal California and Oregon and central Washington. Mesic forest types tended to cluster around intermediate exposure and low fire resistance. Douglas-fir forests exhibited a wide range of exposure and fire-resistance scores. Arid forest types (junipers, pinyon pines, ponderosa pine) were projected to face high exposure, varying substantially in fire resistance.

The study's findings suggest that a majority of western US conifer forests are projected to face exposure to fire-regime change under a 2°C warming scenario. However, vulnerability to transformation depends on adaptive capacity, specifically shifts in forest composition towards species with fire-resistant traits. The results indicate that climate shifts altering vegetation productivity and fire frequency will drive changes in species composition and burn severity, potentially leading to ecological transformations. The combined metrics of exposure and fire resistance identify areas of high vulnerability to transformation. The projections are consistent with expectations of changes in fire frequency and severity during the 21st century. The study differs from previous research by characterizing fire regimes using recent remotely sensed observations rather than historical archetypes. Low exposure in areas with substantial departures from 20th-century conditions suggests that future climates will support fire activity similar to the recent past. Projected changes in fire frequency were spatially heterogeneous, consistent with the idea that relationships between aridity and fire frequency are strongest at intermediate climatic water deficits. The analysis suggests that negative feedbacks between fire and vegetation may emerge under 2°C warming, leading to declines in burn severity independently of changes in area burned. This is partially consistent with previous projections showing weak negative vegetation feedbacks to annual area burned after decades of increased burning. The study underscores that exposure to climate change alone may not result in system transformation and that ecosystems possess the capacity to adapt and persist through shifts in structure and composition. The capacity for species with fire-resistant traits to expand is viewed as essential for resilient forests. Areas with insufficient or misaligned traits are more vulnerable to transformation. The analysis reveals potential transformations in forests at both warm-dry and cool-wet margins of climate space, exhibiting low individual fire resistance.

This study projects widespread exposure to altered fire regimes in western U.S. conifer forests under a 2°C warming scenario, highlighting the importance of considering both exposure and adaptive capacity for assessing vulnerability. The findings reveal potential for both adaptation and transformation, emphasizing the need for active management strategies that align with projected future conditions. Future research should focus on understanding the transient dynamics between contemporary and future fire regimes, and how extreme fire weather events might impact forest responses. Further investigation is needed into the role of dispersal limitations and competition in shaping ecosystem adaptation and transformation pathways.

The study's approach compares fire regimes supported by a single reference condition to those supported by a single future condition, without fully capturing transient dynamics between time points. The analysis may not fully capture the consequences of extreme fire weather events. Dispersal limitations and competition from alternative plant functional types could influence long transient dynamics, potentially hindering ecosystem adaptation. The impact of fire suppression and other management legacies is not explicitly accounted for. The model does not explicitly account for ignitions. The study focuses on conifer forests and doesn't fully consider transformations to other ecosystem types.