Patterns and Drivers of Recent Tree Mortality in Diverse Conifer Forests of the Klamath Mountains, California

Tree mortality rates are increasing in North America, yet our understanding of the processes affecting mortality across different tree species is limited. We assessed drivers of recent mortality in the Russian Wilderness, an area in the Klamath Mountains (California, USA) with the highest conifer diversity on record. We surveyed 3446 trees across 142 plots and document widespread die-off of Shasta red fir (Abies magnifica var. shastensis), a common species across this area, and notably high levels of mortality for subalpine fir (A. lasiocarpa) and lodgepole pine (Pinus contorta var. murrayana). Both fir species were susceptible to Wien's dwarf mistletoe (Arceuthobium abietinum subsp. wiensii), and for Shasta red fir, mistletoe rating was positively correlated with the proportion of trees with fir engraver beetle (Scolytus ventralis). Shasta red fir mortality increased with stand density, and in plots with larger recent increases in minimum winter temperature. We also document mortality and levels of pests and pathogens for nine additional conifer taxa, including several species with limited distributions. Our study highlights the complexity of climate, stand density, pathogens, and insects as they relate to tree mortality across different taxa, and is the first to investigate the current die-off of Shasta red fir.