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

Abstract

Background: 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.Results: 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. Conclusions: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.

Evidence for widespread changes in the structure, composition, and fire regimes of western North American forests

“…This article is protected by copyright. All rights reserved Collins et al 2015, 2018cBattaglia et al 2018Johnston et al 2018 Consistent with the finding that Williams and Baker (2011) methods overestimate tree density (Levine et al 2017, Johnston et al 2018, Knight et al 2020, early timber inventory records and treering reconstructions for the same study areas documented substantially lower tree densities than those estimated using Williams and Baker (2011) methods…”

Mid-Scale Drivers of Variability in Dry Mixed-Conifer Forests of the Mogollon Rim, Arizona

“…When compared to mixed-conifer reference conditions outside the southwest, our results are similar to those found on the Front Range of northern Colorado and southern Wyoming [74,75], and considerably less dense than the results reported from some parts of the Sierra Nevada in California [76]. Historical conditions from other parts of the Sierra Nevada [33,77,78] and mountains in Oregon [79][80][81] had higher basal area and lower tree density than in our study area, suggesting that those forests had fewer and larger trees than those found on the Mogollon Rim.…”

“…Historical logging targeted large, commercially valuable trees, which may have been concentrated on lower, more productive microsites. While density has increased significantly across the entire study area, Rodman et al [23] and Stephens et al [33] both found that density increases were greatest on mesic sites such as valleys and lower slopes. The increase in density on the Mogollon Rim sites was also related to compositional changes.…”

“…Microsites with higher water availability had higher tree densities, and sites with cold, dry winters were associated with larger trees. Winter temperatures influence snowpack duration into the spring and may in turn determine forest density [33]. Spring precipitation is important for ponderosa pine seedling establishment in some forests [31], while fall temperatures may influence fuel moisture and wildfire behavior.…”

Mixed‐conifer forest reference conditions for privately owned timberland in the southern Cascade Range

“…2015, Stephens et al. 2015, 2018). The inventories were largely conducted prior to the major structural and compositional changes associated with later 20th‐century land use practices including widespread grazing, logging, and fire suppression (Safford and Stevens 2017), supporting their use as reference conditions for forest restoration.…”