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In some areas burned by recent wildfires, most or all giant sequoias were killed. Sequoia managers wish to know whether post-fire seedling establishment in those areas has been adequate to regenerate the locally extirpated sequoias. To provide a yardstick for interpreting sequoia seedling densities measured after the recent severe wildfires, here we calculate mean seedling densities measured one, two, and five years after several mixed-severity fires of the past. Our analyses are based on 42 sites in eight different sequoia groves in Sequoia and Kings Canyon national parks, California, which burned in 26 different fires spanning a 48-year period. Conservatively (i.e., without correcting probable errors of underestimated densities), mean sequoia seedling density the first summer following fire was 153,278/ha (Bayesian estimated median = 173,742/ha; 95% credible interval [CI] = 63,319/ha to 850,336/ha). Mean seedling densities the second and fifth summers following fire were, respectively, 34,870/ha (Bayesian estimated median = 39,562; 95% CI = 14,181/ha to 181,011/ha), and 8,601/ha (Bayesian estimated median = 9,513/ha; 95% CI = 3,827/ha to 34,057/ha). Case-study comparisons showed that measured post-fire seedling densities across the Board Camp Grove and in the severely burned portions of the Redwood Mountain Grove were significantly lower than our second-year reference seedling densities.

Predicting species response to climate change is a central challenge in ecology, particularly for species that inhabit large geographic areas. The mountain pine beetle (MPB) is a significant tree mortality agent in western North America with a distribution limited by climate. Recent warming has caused large‐scale MPB population outbreaks within its historical distribution, in addition to migration northward in western Canada. The relative roles of genetic and environmental sources of variation governing MPB capacity to persist in place in a changing climate, and the migratory potential at its southern range edge in the United States, have not been investigated. We reciprocally translocated MPB populations taken from the core and southern edge of their range, and simultaneously translocated both populations to a warmer, low‐elevation site near the southern range boundary where MPB activity has historically been absent despite suitable hosts. We found genetic variability and extensive plasticity in multiple fitness traits that would allow both populations to persist in a warming climate that resembles the thermal regime of our low‐elevation site. We demonstrate, for the first time, that supercooling points in MPBs are influenced both by genetic and environmental factors. Both populations reproduced with seasonally appropriate univoltine generation times at all translocated sites, and bivoltinism was not observed. The highest reproductive success occurred at the warmest, out‐of‐range low‐elevation site, suggesting that southward migration may not be temperature limited.

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.

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