Changing forest structure across the landscape of the Sierra Nevada, CA, USA, since the 1930s

Abstract

Understanding the dynamics of forest structure aids inference regarding future forests and their distributions around the world. Over the last few decades, several papers have addressed changing forest structure in the Sierra Nevada, CA, USA, but these studies were limited in scope. We carried out a broad comparison of forest density and composition in the 1930s versus the 2000s for the west slope of the central and northern Sierra Nevada, using the two most extensive data sets available. Forests in this region have endured a long, complex history of human disturbance, and are now experiencing climatic shifts. We subdivided the landscape into elevation and latitude zones and compared historical and modern tree densities within each zone. We compared densities in historical plots to burned and unburned modern plots, as well as densities of individual tree species in historical vs. modern plots for their entire elevational distribution. Density of small trees (10.2–30.4 cm dbh) was significantly higher in the modern data set for all elevations and all latitudes, ranging from 20 to 148% higher. However, density of large trees (≥61.0 cm) was lower in the modern data set for most elevations and latitudes, ranging from 41% to 60% lower in most zones. Density difference of mid‐sized trees (30.5–60.9 cm) was mixed, but was generally higher in modern plots. The pattern of more small trees but fewer large trees held for most individual species as well, but with notable exceptions. Our comparison of burned and unburned plots strongly implicates fire suppression as a driver of increased density of small trees in low‐ to mid‐elevation forests. However, modern high‐elevation (>2500 m) forests, where fire suppression impacts should be minimal, were also significantly denser than historical plots. Changing climatic conditions may be driving increased densities of small trees in high elevations, as well as decreased densities of large trees across the region.

Historical and current landscape‐scale ponderosa pine and mixed conifer forest structure in the Southern Sierra Nevada

“…The present forest in the western portions of Sequoia and Sierra National Forests is characterized by much higher overall tree densities, shifted species dominance from pine to fir (especially in mixed conifer forests), lower density of large trees, and higher canopy cover. These findings are consistent with those from previous studies in the Sierra Nevada (Parsons and Debenedetti 1979, Ansley and Battles 1998, Scholl and Taylor 2010, Collins et al 2011, Dolanc et al 2014.…”

Rescuing and Sharing Historical Vegetation Data for Ecological Analysis: The California Vegetation Type Mapping Project

“…They attributed some of these changes to fire suppression, although not all, as alpine forests, which do not experience fire, also showed increases in density. Dolanc et al (2014) conducted a similar study of the entire elevational range of west-facing slopes in the Central and South Sierra Nevada, further confirming significant patterns of change in forest structure and composition. McIntyre et al (2015) used both VTM plot data (n = 6,572) from the entire state, and compared baselines in forest structure and composition in oaks (Quercus) and pines (Pinus) with those derived from FIA data (n = 1909).…”

“…Changes to structure and composition of California forests.-Comparison of the VTM plots with current vegetation (using both relocated plots and contemporary vegetation maps) revealed consistent evidence of an increase in young-growth and small-diameter trees across the state and decreases in large trees (Minnich et al 1995;Fellows and Goulden 2008;Goforth and Minnich 2008;Lutz et al 2009;Dolanc et al 2013aDolanc et al , 2014McIntyre et al 2015), as well as changes in forest composition (Minnich et al 1995;Dolanc et al 2013a;McIntyre et al 2015). Much of this evidence of change in forest age structures derives from resurveyed VTM plots (Table 1).…”

“…The plots cover a gradient of vegetation types and include data regarding tree stand structure (number per diameter class), percent cover of dominant overstory and understory vegetation by species, soil type, parent material, leaf litter, elevation, slope, aspect, parent material, and other environmental variables. All plot data were stored on paper data sheets and individual plots were numbered according to map name, quad section Goforth and Minnich 2008;Fellows and Goulden 2008;Lutz et al 2009;Lutz et al 2010;Swanson et al 2013;Dolanc et al 2013a;Dolanc et al 2013b;Dolanc et al 2014;Maxwell et al 2014Walker 2000* Dodge 1975Taylor 2000 Conifer and hardwood forest Crimmins et al 2011;Dobrowski et al 2011;Crimmins et al 2013;Crimmins et al 2014 None None Land cover 6 Plots: Maps:…”

Governance of Land Use Planning to Reduce Fire Risk to Homes Mediterranean France and California

“…Despite these changes, the area remains overwhelmingly wildland, with only 6% designated as WUI [47,48]. Over 40% is forested, much of it with second growth forests between 100 and 150 years old with heavy fuel loads [49] ( Table 1). The recent increase in high severity fires has created large patches of landscape that are recently burned where few trees have survived [50].…”