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

Abstract

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.

Keywords: climate change; ecological transformation; post-fire regeneration; vegetation transition; wildfire.

Fig. 1.

Characteristics of the 10,230 plots utilized in this study. (A) Four study regions (gray outlines), study sites from which postfire tree regeneration was sampled (black points), and forest area that contains at least one of the eight study species within each region (colored by number of study species), hereafter “study area”. The four study regions (INW: interior Northwest; NR: northern Rockies; CAK: California and the Klamath; SRSW: southern Rockies and AZ/NM mountains) were defined by aggregating level 3 US Environmental Protection Agency ecoregions that contained field sites. Across the study area (A), the eight study species account for 89% of the conifer basal area (based on values from ref. 34). (B) 30-y mean annual climatic water deficit (1981 to 2010) of the study area compared with that of the sampled study sites for each region.

Fig. 2.

Regional variability in postfire conifer recruitment under past and future climate and fire-severity scenarios. Postfire conifer recruitment probability from the all-species model under past and current climate, a future climate scenario (Representative Concentration Pathway (RCP) 4.5), and (A) low- and (B) high-severity fire scenarios. Bars beneath maps show the proportion of the study area that falls into each category. Shades of blue represent areas where recruitment is likely, whereas warm colors represent areas where recruitment is unlikely. Areas in gray highlight the range of threshold probabilities above which recruitment is likely (see Methods). (C) Differences in recruitment probability between fire severity scenarios. Map shows where recruitment is unlikely under both fire-severity scenarios (orange), likely under only the low severity scenario (yellow), or likely under both severity scenarios (blue; SI Appendix, Table S18). Results for future climate under the RCP 8.5 scenario shown in SI Appendix, Fig. S10.

Fig. 3.

Changing postfire tree recruitment under past and future climate and fire-severity scenarios. Distribution of recruitment probability projected by individual species models across the range of each species within each region (see Fig. 1 caption for region abbreviations). Species columns are ordered from lower elevation species (Left) to higher elevation species (Right). Different colors represent the different time periods. Rows represent fire-severity scenario and region combinations. The gray vertical band highlights the range of threshold probabilities above which recruitment is likely (see Methods). “PIPO/PIJE” is P. ponderosa/P. jeffreyi; “PSME” is P. menziesii; “ABCO/ABGR” is A. concolor/A. grandis; “PICO” is P. contorta; “PIEN” is P. engelmannii; “ABLA” is A. lasiocarpa.

Fig. 4.

Climatic and nonclimatic controls of postfire tree regeneration. Partial dependence plots for the all-species model showing the relationship between model predictors and postfire recruitment probability while holding other variables at their median values. Mean annual def. is the 30-y mean climatic water deficit from 1981 to 2010. “Max. (Min.) postfire growing season def.” is the maximum (minimum) growing season (April to September) climatic water deficit anomaly experienced in the first 5 y postfire. Where interactions were significant, they are shown by plotting blue, green, and orange lines for the 10th, 50th, and 90th percentiles, respectively, of the interacting variables from our dataset. Bands in A–H and boxes in I are 95% CIs. “*” indicates significantly different (P < 0.05) than no prefire disturbance. Rug plot on the x-axis in A–H show the distribution of data. Numbers above x-axis in I show sample size for each group. Partial dependence plots for individual species models shown in SI Appendix, Figs. S2–S7.