Wildfires and climate change push low-elevation forests across a critical climate threshold for tree regeneration

Climate change is increasing fire activity in the western United States, which has the potential to accelerate climate-induced shifts in vegetation communities. Wildfire can catalyze vegetation change by killing adult trees that could otherwise persist in climate conditions no longer suitable for seedling establishment and survival. Recently documented declines in postfire conifer recruitment in the western United States may be an example of this phenomenon. However, the role of annual climate variation and its interaction with long-term climate trends in driving these changes is poorly resolved. Here we examine the relationship between annual climate and postfire tree regeneration of two dominant, low-elevation conifers (ponderosa pine and Douglas-fir) using annually resolved establishment dates from 2,935 destructively sampled trees from 33 wildfires across four regions in the western United States. We show that regeneration had a nonlinear response to annual climate conditions, with distinct thresholds for recruitment based on vapor pressure deficit, soil moisture, and maximum surface temperature. At dry sites across our study region, seasonal to annual climate conditions over the past 20 years have crossed these thresholds, such that conditions have become increasingly unsuitable for regeneration. High fire severity and low seed availability further reduced the probability of postfire regeneration. Together, our results demonstrate that climate change combined with high severity fire is leading to increasingly fewer opportunities for seedlings to establish after wildfires and may lead to ecosystem transitions in low-elevation ponderosa pine and Douglas-fir forests across the western United States.Plot data from: Wildfires and climate change push low-elevation forests across a critical climate threshold for tree regeneration This file contains annual recruitment rates for post-fire sites in Montana, Idaho, Colorado, New Mexico, Arizona, and California. The rates were determined by destructively sampling trees to age precisely at the root-shoot boundary (see manuscript for more details). Also included in this file is relevant site information. Davis_et_al_recruitment_plot_data.csv Davis_et_al_climate_timeseries This file contains annual climate timeseries for each site for the climate variables used in the boosted regression tree models. See manuscript for detailed information about how climate data was obtained and/or modeled. Davis_et_al_code This file contains the R code used in the analysis. Davis_et_al_figure_code This file contains the R code used to make the figures in the manuscript. Funding provided by: Joint Fire Science ProgramCrossref Funder Registry ID: http://dx.doi.org/10.13039/100015231Award Number: 16-1-01-15Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: OISE-0966472Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: BCS-1232997Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: BCS-1461576Funding provided by: National Institute of Food and AgricultureCrossref Funder Registry ID: http://dx.doi.org/10.13039/100005825Award Number: 1012438Funding provided by: National Aeronautics and Space AdministrationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000104Award Number: NNH11ZDA001N-FIRE