Topographic variation in tree group and gap structure in Sierra Nevada mixed-conifer forests with active fire regimes

Abstract

In Sierra Nevada mixed-conifer forests, heterogeneity in overstory tree spatial patterns is an important ecological characteristic associated with resilience to frequent fires. Regional managers often emphasize this heterogeneity as a key component of forest treatments. There is a lack of information about how the dimensions of fire-resilient tree spatial patterns might vary with topography, which is an important influence on forest structure. We mapped, analyzed, and compared overstory tree spatial patterns across three topographic slope positions (ridges, mid-slopes, and valleys) in unlogged stands with multiple recent lower/mixed-severity burns. Using analysis of Individual trees, Clumps of trees, and Openings (ICO), we found significant differences in a number of spatial pattern metrics, including stand-level tree density and maximum clump size (highest in valleys), forest opening size (largest on ridges), and species composition (percent shade-intolerant stems highest on ridges). We found notably non-significant differences in within-clump tree densities at all spatial scales. Additionally, stand-level averages for forest structure across topography did not consistently capture spatial patterns for individual trees or smaller clump sizes, but often matched trends for clumps and openings in the largest size classes. As “megafires” become more common, prescribed fire and thinning treatments will be used more often to increase forest resilience. By quantifying variable tree spatial patterns across topographic positions in frequent-fire forests, we provide general guidelines for marking prescriptions that may increase resilience of treated forests to increasing fire intensity.