Early seral mixed-conifer forest structure and composition following a wildfire reburn in the Sierra Nevada

Before the era of modern fire suppression, California’s northern Sierra Nevada mixed-conifer and yellow pine forests were self-regulating; recurring short-interval, low-mixed severity wildfires maintained forest structure and composition, which in turn exerted bottom-up controls on subsequent wildfires. As a result of fire suppression, and coupled with the effects of climate warming and other anthropogenic disturbances, the fundamental structure of mixed-conifer and yellow pine forests has shifted. Wildfires may now be increasing in size, severity, and frequency across western North America. However, little is known about the post-fire impacts of repeat wildfire on a forest after a long era of suppression. In this study, I report findings regarding early successional vegetation of Sierra Nevada mixed conifer forests that experienced two large wildfires, the Storrie Fire (in 2000) and the Chips Fire (in 2012). These wildfires burned within the historic fire frequency window for this ecosystem, but much of the forest within their fire footprints had not burned for at least 100 years beforehand. I addressed three questions: (1) how does wildfire affect plant community structure and composition among yellow pine and mixed-conifer forests?; (2) do fire severity and fire frequency interact to influence post-fire vegetation conditions?; and (3) are post-fire responses similar between forests that have burned once, twice, or have not burned in the past century, or that have burned at high, moderate, or low severity? In 2014, I sampled 74 plots in the Plumas and Lassen National Forests. Of these plots, 50 plots were sampled from three fire severity classes and two fire frequencies in and around the Chips Fire (2012). A portion of the Chips Fire had reburned the Storrie Fire (2000), affording the opportunity to compare them to post-fire effects of a single burn on fire-suppressed forests at the same stage of post-fire succession. I also collected data in 24 unburned plots to contrast fire-suppressed plots with plots that experienced wildfire. Wildfire decreased tree density but also decreased available seed sources, which can limit tree regeneration in high severity fire or reburns. Increased tree mortality also produced greater fuel loading in reburns compared to single burns, though burned plots exhibited less fuel loading and fuel connectivity than unburned plots. I also observed that wildfire diversified species composition in single burns, increasing species richness, evenness, and diversity. However, reburning plots appeared to reduce species richness, causing reburns to exhibit richness similar to unburned plots. Still, reburn plots only shared about half of its species with unburned plots, and 13% of species were exclusive to reburns. My study was limited to a particular time (two years post-fire), and post-fire effects may become more pronounced as early seral communities continue to respond to the effects of the wildfire. Nonetheless, my results indicate that wildfire can produce forest structure and composition that is dramatically different from fire-suppressed mixed-conifer forests. Though it is unknown whether ecological processes can be restored by just one or two wildfire events within a short time-span in fire-suppressed landscapes, the post-fire conditions observed in my study have begun to resemble pre-suppression conditions by exhibiting reduced tree densities, lower fuel loads, and enhanced species diversity, especially at low to moderate fire severities. Because post-fire vegetation response is a stochastic and long-term process, understanding the effects of wildfire reintroduction and reburn will likely take multiple observations