Tree mortality across competitive and geographic gradients in Northern California

Sophia Lemmo, Forestry & Wildland Resources, Humboldt State University, Arcata, CA, Lucy P. Kerhoulas, Forestry and Wildland Resources, Humboldt State University, Arcata, CA and Rosemary L. Sherriff, Geography Department and Forest, Watershed, Wildland Science Graduate Program, Humboldt State University, Arcata, CA

Background/Question/Methods
The 2012-2015 California drought was unprecedented and contributed to the death of millions of trees. While the effects of the 2012-2015 drought on forests are relatively well studied in the central and southern Sierra Nevada, less is known about its effects on the heavily forested areas of Northern California. This study addressed how recent tree mortality in Northern California forests vary with climate, species, stand density, and habitat, and if the rates differed before, during, and after the 2012-2015 drought. The study area included the forested region of Northern California, centered on the Klamath ecoregion. The Klamath ecoregion is unique for its highly diverse plant taxa; especially notable is the presence of over 30 conifer species. Total number of dead and live trees were counted on 54 (0.25 ha) plots located at Calflora locations targeted for six canopy-dominant conifer species. Non-metric Multidimensional Scaling (NMDS) was used to examine associations between mortality distributions and explanatory parameters including climate gradient, topographic position, damage agents, stand density and composition, and species. Additionally, standard dendrochronological techniques were used to establish the years of tree death to explore how mortality trends relate to climate.

Results/Conclusions
Preliminary results indicate that true firs (Abies) had higher mortality rates in the wetter parts of their range, while Brewer spruce (Picea breweriana) had no observed mortality in the wetter part of their range, demonstrating that tree mortality by moisture regime is species-dependent. Across plots, Shasta red fir (A.magnifica var. shastensis) and white fir (A. concolor) had 17% and 13% mortality rates, respectively, while Brewer spruce had a 3% mortality rate, and most of the pine species (Pinus) had mortality rates between 21-36%. Mortality was more prevalent in large diameter white fir trees, while in Shasta red fir mortality was more evenly distributed across size classes. Field observations identify the fir engraver beetle as the predominant damage agent on dead firs. Analyses on mortality trends in other tree species were highly variable. Understanding forest mortality trends by tree species enables targeted management to conserve the diversity of Northern California forests and reduce mortality on the landscape. These findings also allow for enhanced forest modeling and inform future forest climate research needs.