Reduced fire severity offers near-term buffer to climate-driven declines in conifer resilience across the western United States

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. © 2023 the Author(s)

Forest Composition Change After a Mountain Pine Beetle Outbreak, Rocky Mountain National Park

Recent severe and extensive mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have created novel conditions in Southern Rocky Mountain lodgepole pine forests which historically had disturbance regimes dominated by extensive, stand-replacing fires. The goal of this study is to investigate patterns of and potential mechanisms in post-outbreak forest change in order to better understand the ecological legacy of the recent outbreak in the context of its implications for resilience to future disturbances and adaptation to climate change. To this end, we collected field data on forest structure and species composition in 2012 in lodgepole pine dominant forests in Rocky Mountain National Park. We then used a combination of modeling and statistical methods to identify possible mechanisms in post-outbreak forest conditions and evaluate the effect of the MPB outbreak on forest heterogeneity. We found that the outbreak initiated a shift in forest structure from single-cohort lodgepole pine stands to stands with greater diversity in age classes and species composition. This increase in landscape asynchrony may increase resiliency to future disturbances. However, this heterogeneity is a result of more spruce and fir on the landscape, species which are less adapted to projected future climate conditions. Our results indicate that disturbances do not necessarily increase the rate at which vegetation adapts to a changing climate, and that it is essential to consider disturbance type and available seed sources when predicting future forest conditions

Evidence of compounded disturbance effects on vegetation recovery following high-severity wildfire and spruce beetle outbreak.

Spruce beetle (Dendroctonus rufipennis) outbreaks are rapidly spreading throughout subalpine forests of the Rocky Mountains, raising concerns that altered fuel structures may increase the ecological severity of wildfires. Although many recent studies have found no conclusive link between beetle outbreaks and increased fire size or canopy mortality, few studies have addressed whether these combined disturbances produce compounded effects on short-term vegetation recovery. We tested for an effect of spruce beetle outbreak severity on vegetation recovery in the West Fork Complex fire in southwestern Colorado, USA, where much of the burn area had been affected by severe spruce beetle outbreaks in the decade prior to the fire. Vegetation recovery was assessed using the Landsat-derived Normalized Difference Vegetation Index (NDVI) two years after the fire, which occurred in 2013. Beetle outbreak severity, defined as the basal area of beetle-killed trees within Landsat pixels, was estimated using vegetation index differences (dVIs) derived from pre-outbreak and post-outbreak Landsat images. Of the seven dVIs tested, the change in Normalized Difference Moisture Index (dNDMI) was most strongly correlated with field measurements of beetle-killed basal area (R2 = 0.66). dNDMI was included as an explanatory variable in sequential autoregressive (SAR) models of NDVI2015. Models also included pre-disturbance NDVI, topography, and weather conditions at the time of burning as covariates. SAR results showed a significant correlation between NDVI2015 and dNDMI, with more severe spruce beetle outbreaks corresponding to reduced post-fire vegetation cover. The correlation was stronger for models which were limited to locations in the red stage of outbreak (outbreak ≤ 5 years old at the time of fire) than for models of gray-stage locations (outbreak > 5 years old at the time of fire). These results indicate that vegetation recovery processes may be negatively impacted by severe spruce beetle outbreaks occurring within a decade of stand-replacing wildfire

Wildfire Catalyzed Shift from Conifer to Aspen Dominance in Montane Zone, Colorado

Climate change impacts on forest systems will likely be concentrated through influences on climate-sensitive ecological disturbances, such as wildfire. Over the last two decades, a shift towards megafires is evident and projected warming this century will continue to increase the frequency of these events. Forest recovery following megafires is challenging due to increased seed dispersal distances, harsh post-fire environment, and weather conditions. Species such as Quaking aspen (Populus tremuloides) with long seed dispersal distances and the ability to establish or resprout in harsh sites should be favored to expand their range in the context of wildfire trends. Aspen are well adapted to wildfire, however they are not drought-adapted. Unusually rapid and widespread mortality in aspen has been documented throughout its range during the 21st century, primarily as a result of warm, dry weather conditions. Increases in wildfire severity and extent caused by climate change may provide opportunities for aspen regeneration, especially at cooler, high- elevation sites. Aspen\u27s sensitivity to drought suggests that regeneration following fire might be constrained to cooler and wetter topographic locations on the landscape that reduce drought stress on vegetation. Aspen establishment and persistence are known to occur at high elevation sites due to cooler, wetter conditions, while aspen mortality is demonstrated to occur at low elevation sites. Low- and high-elevation aspen persistence is well-understood; however, patterns of aspen regeneration and persistence at mid-elevation sites is still relatively unexplored. We studied the 2002 Hayman fire (Colorado, USA) to explore whether high-severity wildfire has provided opportunities for aspen regeneration at mid-elevations in which aspen was not observed before the fire. If regeneration had occurred, we asked if regeneration is contingent on topographic conditions. Cool, wet microclimates created by fine-scale topography at mid-elevations may allow for increased aspen regeneration, however this is unexplored in the Hayman landscape. Our findings demonstrate that the Hayman fire provided opportunities for aspen regeneration at mid-elevation where aspen were not observed before the fire and that the density of regeneration is contingent on topography. Specifically, aspen regeneration is most dense at mid-elevations on steep slopes. Forest management may focus on threats to aspen health and vigor (i.e., ungulate herbivory) on steep slopes at mid-elevations rather than at low-elevation sites for efficiency

Date and sensor type for Landsat scenes used in analysis.

<p>Date and sensor type for Landsat scenes used in analysis.</p

Growing-season NDVI for the Papoose burn area (top row), and West Fork and Windy Pass burn areas (bottom row).

<p>NDVI images are clipped to spruce/fir forest cover type. Left images are from 2002 (pre-disturbance) and right images are from 2015 (2 years post-fire recovery).</p

Comparison between spruce beetle outbreak extent detected by ADS from 2002 to the indicated year (left; ADS polygons shown in orange) and dNDMI (right).

<p>Top dNDMI = NDMI₂₀₀₆ –NDMI₂₀₀₂; bottom dNDMI = NDMI₂₀₁₂ –NDMI₂₀₀₂. Color scales for dNDMI are based on standard deviations within images.</p

R² values from OLS regression tests comparing 2002–2015 dVIs to the beetle-killed basal area of spruce in field plots measured in 2015.

<p>R² values from OLS regression tests comparing 2002–2015 dVIs to the beetle-killed basal area of spruce in field plots measured in 2015.</p

Top-performing multivariate SAR models predicting 2015 NDVI for point locations in red-stage and gray-stage of spruce beetle outbreak prior to fire.

<p>Top-performing multivariate SAR models predicting 2015 NDVI for point locations in red-stage and gray-stage of spruce beetle outbreak prior to fire.</p

Pattern in NDVI change for locations which burned at high severity compared to undisturbed 2002 conditions.

<p>Solid line represents a 1:1 relationship.</p