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.