Use of a forest reconstruction model to assess changes to Sierra Nevada mixed-conifer forest during the fire suppression era
Fire suppression has resulted in dramatic changes to species composition and structural diversity in the Sierra Nevada mixed-conifer forests of California. Advancing the ecological understanding and management of these forests requires a better understanding of changes that occurred during the fire suppression era, but empirical historical datasets are rare and methodologies for developing new historical reference information are subject to many limitations. I sought to develop historical reference information for the Yosemite Forest Dynamics Plot (YFDP), a research plot located in an old-growth mixed-conifer stand in Yosemite National Park. I performed a dendrochronological fire history analysis to characterize the historical fire regime of the YFDP (pre-1900 fire return interval: 29.5 years). I then developed two different forest reconstruction models to estimate pre-suppression forest conditions. Two alternative tree growth models, one regionally-parameterized and one locallyparameterized, and a decay model based on published estimates of tree decay rates were evaluated. Limited tree decay data available in the literature is a source of uncertainty in forest reconstructions, both for this study and other studies in the Sierra Nevada. Model analysis demonstrated that the regionally-parameterized growth model resulted in unreasonably fast tree growth rates. The site-specific growth model produced results similar to empirical historical datasets (84.5 trees ha-1 and 25.7 m 2 ha-1 in 1900) – I utilized these results to investigate patterns of tree establishment during the fire suppression era. I found evidence for spatial attraction between early ingrowth (trees that established between 1930 and 1970) sugar pine and legacy trees (trees established before 1930) and spatial repulsion between late ingrowth (trees that established after 1970) sugar pine and legacy trees. This indicates that fire suppression is driving changes to intertree relationships, causing current tree spatial patterns to be outside of their historical range and variability. These results highlight the need for a substantial increase in research efforts regarding tree decay data for Sierra Nevada mixed-conifer forests. When developing restoration targets, managers should avoid a “one size fits all” approach and consider site-specific factors, such as parent material and the historical fire regime, which have influenced changes to forest conditions during the fire suppression era.