Assessing giant sequoia mortality and regeneration following high severity wildfire

Abstract

Fire is a critical driver of giant sequoia (Sequoiadendron giganteum [Lindl.] Buchholz) regeneration. However, fire suppression combined with the effects of increased temperature and severe drought have resulted in fires of an intensity and size outside of the historical norm. As a result, recent mega-fires have killed a significant portion of the world’s sequoia population (13 to 19%), and uncertainty surrounds whether severely affected groves will be able to recover naturally, potentially leading to a loss of grove area. To assess the likelihood of natural recovery, we collected spatially explicit data assessing mortality, crown condition, and regeneration within four giant sequoia groves that were severely impacted by the SQF- (2020) and KNP-Complex (2021) fires within Sequoia and Kings Canyon national parks. In total, we surveyed 5.9 ha for seedlings and assessed the crown condition of 1140 trees. To inform management, we used a statistical methodology that robustly quantifies the uncertainty in inherently ‘noisy’ seedling data and takes advantage of readily available remote sensing metrics that would make our findings applicable to other burned groves. A loss of giant sequoia grove area would be a consequence of giant sequoia tree mortality followed by a failure of natural regeneration. We found that areas that experienced high severity fire (above ~800 RdNBR) are at substantial risk for loss of grove area, with tree mortality rapidly increasing and giant sequoia seedling density simultaneously decreasing with fire severity. Such high severity areas comprised 17.8, 142.0, 14.6, 1.6 hectares and ~90%, ~14%, ~53%, and ~27% of Board Camp, Redwood Mountain, Suwanee, and New Oriole Lake groves, respectively. In all sampling areas, we found that seedling densities fell far below the average density measured after prescribed fires, where seedling numbers were almost certainly adequate to maintain giant sequoia populations and postfire conditions were more in keeping with historical norms. Importantly, spatial pattern is also important in assessing risk of grove loss, and in two groves, Suwanee and New Oriole Lake, the high severity patches were not always contiguous, potentially making some areas more resilient to regeneration failure due to the proximity of surviving trees.

Postfire reproduction of a serotinous conifer, the giant sequoia, in the Nelder Grove, California

“…In our Nelder Grove data, median sequoia seedling density for areas with RdNBR values <477 was 255/ha, while areas with RdNBR values 477-899 had a median sequoia seedling density of 2165/ha, and areas with RdNBR ≥900 had a median density of 2674/ha (nearly 17 times higher than the median sequoia seedling density at 5 years after prescribed fires in Stephenson et al, 2023). As Soderberg et al (2024) reported results 1 year postfire, our results reported 6 years postfire, may not be inconsistent with theirs. Sequoia seedling density may be relatively lower in areas with RdNBR values ≥900 at 1 or 2 years postfire, and such areas may provide good conditions for sequoia seedling survival, growth, and perhaps additional germination and establishment, which could shift the relative seedling density higher in such very high-severity areas by several years postfire.…”

“…Thus, important management decisions, with potentially long-term effects, are being made in the absence of scientific evidentiary support and essential knowledge. Soderberg et al (2024) reported lower postfire giant sequoia seedling density at the upper end of the high-severity fire spectrum (particularly ≥900 RdNBR) at 1 year postfire. Our results, at 6 years postfire, do not suggest a decrease.…”

“…Statements made in a pre-print study (Stephenson et al, 2023) and (Soderberg et al, 2024) are being used to guide a large U.S. National Park Service project implementing expansive sequoia seedling planting of nursery-grown stock into high-severity fire patches within designated Wilderness Areas (USDOI, 2023). Both studies state that natural postfire reproduction in high-severity areas would not be adequate to restore high-severity burned sequoia groves, leading to regeneration failure and loss to portions of sequoia groves (Soderberg et al, 2024;Stephenson et al, 2023).…”

“…Statements made in a pre-print study (Stephenson et al, 2023) and (Soderberg et al, 2024) are being used to guide a large U.S. National Park Service project implementing expansive sequoia seedling planting of nursery-grown stock into high-severity fire patches within designated Wilderness Areas (USDOI, 2023). Both studies state that natural postfire reproduction in high-severity areas would not be adequate to restore high-severity burned sequoia groves, leading to regeneration failure and loss to portions of sequoia groves (Soderberg et al, 2024;Stephenson et al, 2023). Neither of the studies investigated whether there is a postfire sequoia seedling density threshold below which regeneration failure will likely occur, and neither presented data on postfire sequoia reproduction in high-severity fire areas compared to low-/moderate-severity areas.…”

“…Soderberg et al. (2024) reported lower postfire giant sequoia seedling density at the upper end of the high‐severity fire spectrum (particularly ≥900 RdNBR) at 1 year postfire. Our results, at 6 years postfire, do not suggest a decrease.…”