Tree mortality in western U.S. forests forecasted using forest inventory and Random Forest classification

Abstract

Climate change is projected to significantly affect the vulnerability of forests across the western United States to wildfires, insects, disease, and droughts. Here, we provide recent mortality estimates for large trees for 53 species across 48 ecological sections using an analysis of 23,215 Forest Inventory plots and a Random Forest classification model. Models were also used to predict mortality in future FIA inventories under the RCP 4.5 emissions scenario. Model performance indicated species identity as the most important predictor of mortality under both current and future scenarios, with contributions from climate and soil variables. Our results show relatively high levels of recent mortality in the Middle and Southern Rocky Mountains driven by high mortality in Populus tremuloides, Pinus contorta, Pinus albicaulis, and Abies lasiocarpa. Low levels of mortality were observed in several species, with <1% annual mortality observed throughout all other sections. Future mortality was predicted to increase significantly for most species and correlated well with recent mortality at the species level, but not at the plot level. These results suggest that future attempts to model or predict widespread forest mortality would benefit from more research on vulnerable species and that significant mortality events in some species may not be important for dynamics