Old-Growth Forest Dynamics After Fire and Drought in the Sierra Nevada, California, USA

Understanding forest ecosystems is important because forests cover approximately one-third of Earth’s land area, store half of Earth’s carbon, shelter half of Earth’s species, and absorb a quarter of new anthropogenic carbon emissions, slowing climate change. This dissertation provides insight into future forest habitat, fuels, species composition, and structure by investigating what happens to snags, seedlings, and trees in an old-growth forest after a low- to moderate-severity fire. Chapter II explores how low- to moderate-severity fire changes snag fall rates. Predicting how long snags will remain standing after fire is essential for managing habitat, understanding chemical cycling in forests, and modeling forest succession and fuels. Pre-fire snags––which tend to be preferred habitat because they include more large-diameter snags in advanced stages of decay––were at least twice as likely to fall as new snags within 3–5 years after fire. Pre-existing snags were most likely to persist five years after fire if they were > 50 cm in diameter, > 20 m tall, and charred on the trunk to heights above 3.7 m. Chapter III examines the effects of fire severity and microclimate on conifer regeneration after fire. Available seed, lower burn severity on the forest floor, more fire-caused tree mortality, and earlier snowmelt during the germination year gave Pinus lambertiana seedlings an advantage over Abies concolor seedlings, suggesting that lower-severity fire could naturally shift forest species composition toward Pinus species, which are more resistant to fire and drought. Chapter IV investigates the effects of lower-severity fire on tree growth by analyzing the tree-ring widths of seven mixed-conifer species throughout the Sierra Nevada. Post-fire growth patterns were not substantially different from growth fluctuations at adjacent unburned plots, suggesting that reintroducing lower-severity fire to forests where fire has been excluded over the last century will not prevent surviving trees from attaining pre-fire growth rates within five years after fire. Chapter V focuses on recruitment of large-diameter trees after fire, analyzing how local post-fire mortality within tree neighborhoods impacts post-fire radial growth of surviving trees. Cause of mortality influenced the relationship between neighborhood change and the growth of surviving trees, and this relationship was different for A. concolor compared to P. lambertiana, suggesting that species differences in cause of mortality could affect the species composition of future large-diameter tree populations. These findings demonstrate that low- to moderate-severity fire can promote Pinus seedlings and trees, exemplifying the concept that ecosystems shift toward species composition and structure that maximize resilience to challenging climate and disturbance regimes. This research was possible because of the existence of a long-term, spatially explicit, observational old-growth forest dataset with annual resolution.