The Relationship Between Forest Structure, Composition, and Landscape on Bark Beetle Related Tree Mortality in Sequoia and Kings Canyon National Parks, California

Abstract

Outbreaks of bark beetles (Dendroctonus spp., Ips spp., Scolytus spp.) have resulted in widespread tree mortality throughout the western United States, with the area of forest die-off in California quadrupling to over 4 million acres between 2014 and 2016 during a period of intense drought (California Forest Pest Council, 2016). Scientists are concerned that the rate of tree mortality is exceeding the rate at which forests can regenerate, which could result in landscape level changes and have long-term implications for species, climate, and water supply (Bentz et al., 2009). While research indicates that prolonged drought and crowded, homogeneous tree stands have made forests susceptible to bark beetle attacks, very few studies have looked comprehensively at what specific tree characteristics, stand conditions, and landscape factors are most significant in mediating these outbreaks. To better understand the key factors influencing the bark beetle outbreak in the Sierra Nevada, my research focused on two heavily impacted tree species in Sequoia and Kings Canyon National Parks, white fir and sugar pine. The primary research question addressed in this study was: What are the main ecological drivers mediating bark beetle infestations in white fir and sugar pine? I hypothesized that DBH, slope, and time since last fire would have a positive correlation and aspect, elevation, species diversity, and stand density would have a negative correlation with bark beetle related tree mortality in both white firs and sugar pines. To test these hypotheses, I surveyed 72 red fir-white fir forest plots in Sequoia and Kings Canyon National Parks in 2018. Within each plot, trees were evaluated to determine species, size, and individual mortality factors. These data were then used to calculate stand characteristics such as stand density and species diversity within each plot. Time since last fire and other landscape factors, such as elevation, slope, and aspect, were evaluated at the plot level based on field measurements and existing Geographic Information Systems data. In addition, mortality data collected by U.S. Geological Survey in 2016 and 2018 from mixed conifer forests were also analyzed for this study. Mixed effects logistic regression models were then run for each individual dataset as well as in a combined model to determine which factors were correlated with bark beetle related mortality in white fir and sugar pine. The results indicate that bark beetle related mortality was positively correlated with DBH and negatively correlated with slope in sugar pine. In white fir, the results indicate that bark beetle related mortality had a negative correlation with DBH and elevation and a positive correlation with time since last fire. I concluded that these differences in mortality factors could be attributed to differences in host preferences for the main pests found in each tree species, including fir engraver (Scolytus ventralis), Scolytus subscaber, and Scolytus praeceps for white fir and mountain pine beetle (Dendroctonus ponderosae) for sugar pine. While this analysis is limited to white fir and sugar pine in Sequoia and Kings Canyon National Parks, the results further our understanding of bark beetle epidemiology and tree mortality factors in the southern Sierra Nevada mountain range of California. Understanding these drivers are key not only to identify which forest stands are most susceptible to future bark beetle attacks, but also to determine if any conditions can be managed to reduce the risk of tree mortality. The results suggest that management measures such as prescribed fire may be more effective in lower elevational forests comprised of trees targeted by “secondary” pests, which generally attack stands stressed by a wider range of stand characteristics and landscape factors.