OOS 26-6 - Giant sequoia wildfire mortality and recovery: A long-lived and highly fire-resistant species faces new threats in the 21st century

Giant sequoia (Sequoiadendron giganteum) groves are highly adapted to fire and represent one of the oldest forms of old-growth forest on the planet given the long-life spans of the individuals (some exceeding 3200 years). Modern wildfires, however, which are a product of a warming climate and fire-exclusion induced fuel buildup, present a new challenge to these very fire- resistant species. After three fires in sequoia groves in 2015 and 2017, we investigated the survival of large sequoias (> 1.2 m in diameter) and how fire damage corresponded to mortality. Within high -severity burned areas (as classified by remote sensing) we found that ~84% of large sequoias perished; in moderate severity areas ~28% of large sequoias were killed. We found that large giant sequoia mortality corresponded with significant damage to the lower tree bole, lower crown ratios and lower elevation areas. Within one grove where we tracked 204 trees over three years, we found total crown damage and the presence of fire scars were best predicted mortality by the third post-fire year, where large sequoias sustained up to ~ 85% crown damage before succumbing to injury. There, 24% trees were dead one-year post-fire, with 35% dead by year three. Our findings document significant and concerning large giant sequoia mortality across three wildfires, but these are surpassed by the total area of sequoia groves impacted by high severity fire (1442 ha) in even more recent 2020 and 2021 fires.

To assess potential post-fire recovery of giant sequoia groves we studied giant sequoia regeneration at 126 plots in three groves after the 2020 Castle Fire. Giant sequoia seedlings were patchily distributed and sparse in many places. More litter, and smaller-diameter surface wood (1-100 hour fuels) along with less ash appeared to be linked to higher sequoia seedling densities in areas burned at high severity, whereas no such relationship was present within portions of groves which burned at low severity. We hypothesize that lack of soil surface cover creates inhospitable conditions for regenerating seedlings, or that it signifies increased combustion of sequoia seeds and semi-serotinous cones in tree crowns. Given the observed extent of first-year sequoia regeneration failure (30.7% of plots), the likelihood of competing shrubs establishing quickly, and the lack of proximal future seed source in large high severity patch interiors, our findings are concerning for the future of giant sequoia populations impacted by recent fires.