Abstract
BackgroundInformation 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.ResultsWhile 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.ConclusionsOur 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.
Highlights
Information about fire and historical forest structure and composition in fir-dominated mixed conifer forests is lacking, especially at the landscape scale
By comparing current forest conditions to those based on direct measurements from early inventories, we provide information on if or how potential changes in these forests differ from those in pine-dominated mixed conifer (MC) forests
Precipitation was the primary driver of density, with drier sites averaging less than 1299 mm of precipitation annually having average tree densities of 102 trees ha−1
Summary
Information about fire and historical forest structure and composition in fir-dominated mixed conifer forests is lacking, especially at the landscape scale. Overall tree density (especially small- to medium-sized shade-tolerant trees) has increased in many areas in the Sierra Nevada from fire suppression (e.g., Vankat and Major 1978; Parsons and Debendetti 1979; Beaty and Taylor 2008; Dolanc et al 2014a, 2014b; Levine et al 2017; Easterday et al 2018), leading to an expansion of MC forests at the expense of ponderosa pine forests in this mountain range (Dolanc et al 2014a; Safford and Stevens 2017). Higher small-tree densities and greater surface fuel loads have increased fire hazards in many Sierra Nevada forests, especially those that once burned frequently with low-moderate intensity fire regimes (Biswell 1989; Agee and Skinner 2005; Stephens et al 2009)
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