Wildfire interactions with recruitment of giant sequoia in experimental canopy gaps

The century-long era of fire suppression has resulted in dramatic decreases in giant sequoia regeneration and recruitment. A 2001 mechanical disturbance in a native grove created experimental canopy gaps ranging in size from 0.05 to 0.4 ha, where factors of giant sequoia recruitment were monitored. The patches of recruitment all burned in a 2021 wildfire, creating an opportunity to study fire-gap recruitment dynamics. The study questions were (1) what was the relationship between gap size and tree height up to the point of the fire, (2) which tree- and gap-level variables explained mortality amongst the planted trees, and (3) did a pre-fire thin-and-prune treatment increase fire-related crown damage? There was an asymptotic relationship between average tree height and gap size. Increases in gap size beyond 0.2 ha led to only modest increases in average tree height. The overall mortality rate of study trees, averaged across gaps, was 26 %, ranging from 5.0 % to 52 %. A generalized linear mixed-effects model (GLMM) was fit to determine factors of mortality. Taller trees had lower probability of mortality, followed by trees closer to the centers of gaps. The pre-fire thin-and-prune treatment did not have a statistically significant effect on mortality or crown damage. A key finding was that canopy gaps facilitated giant sequoia recruitment by altering the resource environment while also altering patterns of fuel accumulation. Tree height was maximized in gaps greater than 0.2 ha, but gaps as small as 0.05 ha protected interior trees from fire. • Generalized linear mixed model evaluated factors of wildfire mortality. • Wildfire generally killed edge trees and spared interior trees. • A thin-and-prune treatment did not predict mortality or crown scorch. • Asymptotic relationship between tree growth and gap size.