A forest reconstruction model to assess changes to Sierra Nevada mixed-conifer forest during the fire suppression era

Abstract

Fire suppression has resulted in dramatic changes to species composition and structural diversity in the Sierra Nevada mixed-conifer forests. We need a better understanding of how these forests have changed during the fire suppression era, but empirical historical datasets are rare and methodologies for developing new historical reference information are subject to limitations. We sought to develop historical reference information for the Yosemite Forest Dynamics Plot (YFDP), a research plot located in an old-growth mixed-conifer forest in Yosemite National Park. We performed a dendrochronologial fire history analysis to characterize the historical fire regime of the YFDP, resulting in an estimated pre-1900 point fire return interval of 29.5years. We then developed two different forest reconstruction models to estimate structural and compositional forest changes since 1900, the year the last widespread fire burned the YFDP, to the present. We explored the use of two different tree growth models—a regionally parameterized competition-dependent model and a parsimonious site-specific model—as well as a decay model based on published estimates of wood decay rates. The competition-dependent growth model predicted slightly higher stem densities (175 trees ha−1 vs. 112 trees ha−1 in 1900) and slightly lower basal area (20.9m2ha−1 vs. 24.1m2ha−1 in 1900) than the site-specific growth model. Predictions about dead trees, especially large diameter sugar pines, are potentially inaccurate, both in this study and other reconstruction studies in the Sierra Nevada, due to a lack of size-specific snag and log decay rate data. Our study highlights the need for more detailed decay data for Sierra Nevada mixed-conifer forests. While reconstruction models are constrained by the data used to parameterize them, they can still produce estimates of historical conditions that are useful for understanding the direction and magnitude of forest change, as well as for planning forest management. Our analysis demonstrates dramatic changes in forest conditions since fire-exclusion—a clear ecological basis for restoration. We encourage managers to focus on restoring key ecological processes that maintain ecosystem structure and function, in this case fire, rather than attempting tree-for-tree recreations of historical structure without reintroducing fire.