Long-Term Impacts of Invasive Insects and Pathogens on Composition, Biomass, and Diversity of Forests in Virginia's Blue Ridge Mountains

Abstract

Exotic forest insects and pathogens (EFIP) have become regular features of temperate forest ecosystems, yet we lack a long-term perspective on their net impacts on tree mortality, carbon sequestration, and tree species diversity. Here, we analyze 3 decades (1987-2019) of forest monitoring data from the Blue Ridge Mountains ecoregion in eastern North America, including 67 plots totaling 29.4 ha, along with a historical survey from 1939. Over the past century, EFIP substantially affected at least eight tree genera. Tree host taxa had anomalously high mortality rates (>= 6% year(-1) from 2008 to 2019 vs 1.4% year(-1) for less-impacted taxa). Following the arrival of EFIP, affected taxa declined in abundance (- 25 to - 100%) and live aboveground biomass (AGB; - 13 to - 100%) within our monitoring plots. We estimate that EFIP were responsible for 21-29% of ecosystem AGB loss through mortality (- 87 g m(-2) year(-1)) from 1991 to 2013 across 66 sites. Over a century, net AGB loss among affected species totaled roughly 6.6-10 kg m(-2). The affected host taxa accounted for 23-29% of genera losses at the plot scale, with mixed net effects on alpha-diversity. Several taxa were lost from our monitoring plots but not completely extirpated from the region. Despite these losses, both total AGB and alpha-diversity were largely recovered through increases in sympatric genera. These results indicate that EFIP have been an important force shaping forest composition, carbon cycling, and diversity. At the same time, less-affected taxa in these relatively diverse temperate forests have conferred substantial resilience with regard to biomass and alpha-diversity.