Background/Question/Methods Fire suppression policies enacted in the early 1900s have led to changes in forest structure and composition throughout California. California forests are facing increased threats, including climate change and more severe wildfires. Thus, understanding shifts in forest composition is important for successful forest management and future resilience. We used 1930s Vegetation Type Mapping (VTM) data and 2010 Forest Inventory Analysis (FIA) data to compare changes in yellow pine mixed conifer forest density and size class in southern California. We examined changes by National Forest, species, shade and fire tolerance, and elevation. This study is one of the first in-depth compositional analyses of southern California forests. Results/Conclusions Our analysis shows that southern California yellow pine mixed conifer forests are denser today than they were in the early 1900s. Across all of southern California, average tree density (trees/ha) has increased by approximately 64.3%. This increase in density is primarily due to a 202.6% increase in a shade-tolerant oak species, canyon live oak (Quercus chrysolepis). Results from our size class distribution analysis show that there is an increase in density across all size classes. Small diameter oaks are increasing at all elevations and across all size classes. Small diameter conifers have decreased at lower elevations yet are increasing at higher elevations (>2000m). However, large diameter conifers have increased at all elevations. Our results suggest that southern California forests may be experiencing a compositional shift to more oak-dominated forests at lower elevations.
