Fire & Climate
Recovery after fire
Climate warming and increased frequency and severity of wildfires have the potential to undermine forest resilience to wildfires. Species demography implies that vegetation responses to fires depend on a series of population filters, including adult survival, seed availability, germination, establishment, and survival; the impacts of these varying filters will ultimately determine post-fire forest trajectories. The objectives of this study were to (1) assess how fires affect biologically-relevant microclimate conditions; and (2) evaluate the relative influence of seed availability, microclimate, and fire effects (on overstory, understory, and soil conditions) on post-fire seedling demography. This study was conducted in two large, lightning-ignited wildfires from the regionally extensive fire season of 2017 in northern Rocky Mountain mixed- conifer and subalpine forests. Conifer seedling density, survival, and growth were tracked in the first three years post-fire in 69 plots spanning environmental and fire severity gradients, and microclimate conditions were measured in a subset of those plots. We found that sites burned at high severity had, on average, 3.7 °C higher daily maximum temperatures and 0.81 kPa higher daily maximum VPD compared with adjacent unburned forest. In addition, we found that seedling regeneration was abundant, with a median density of 2,633 ha-1. Variables describing seed availability, microclimate, overstory fire severity, understory vegetation, and soil nitrogen availability accounted for 75% of the variability in seedling density among plots. The robust regeneration observed in this case is likely due, in part, to moderate post-fire climate conditions, which supported high annual survivorship rates exceeding 50%. Our results highlight the importance of fine-scale heterogeneity in fire effects for supporting post-fire tree regeneration, which alter microclimate conditions and create diverse microsite environments for seedlings. These results reveal the importance of residual structures in burned areas to buffer microclimate conditions, and they highlight future research needs to identify favorable microsites for reforestation efforts, particularly as the climate warms. This study contributes to an improved understanding of the mechanisms of forest resilience to wildfires and demonstrates the utility of a demographic perspective for anticipating forest responses to future wildfires under changing environmental conditions.