Historical and modern landscape forest structure in fir ( Abies )-dominated mixed conifer forests in the northern Sierra Nevada, USA

摘要

Information about fire and historical forest structure and composition in fir-dominated mixed conifer forests is lacking, especially at the landscape scale. This study used historical timber survey data to characterize early forest conditions in a large fir-dominated northern Sierra Nevada landscape (area 10 000 ha). We used automated model selection to determine the suite of environmental variables that best predicted forest structure, and regression tree analysis to identify environmental settings associated with historical forest conditions and to provide a comparison to contemporary forest data. While changes at our study site were more subtle when compared to those in more xeric, pine-dominated mixed conifer areas in the central and southern Sierra Nevada, there were significant increases in small-tree density and a decrease in large-tree density in the current forest. The loss of large trees was most pronounced in environments with relatively low summer water deficit and shallow slopes, which is where the highest large-tree densities were observed historically. Within the historical forest, tree density and basal area were greater in areas with higher precipitation and actual evapotranspiration, along with lower snowpack. Our results indicated that restoration plans should differ depending on the type of mixed conifer forest encountered. Whereas low tree densities and basal areas produced open stand conditions in many pine-dominated regions of this forest type, areas that were dominated by fir species with longer growing seasons, high water availability, and low water demand were denser and likely provided important habit for late seral species such as the California spotted owl, Strix occidentalis occidentalis (Xántus de Vesey , 1860). Working to develop forest restoration and adaptation plans to account for these differences is an important strategy to restore resiliency and encourage adaption in fire-excluded mixed conifer forests.