Community and ecosystem effects of simulated and actual emerald ash borer outbreaks in ash-dominated forests
Invasive pests are among the greatest threats to Northeastern forests. The emerald ash borer (EAB, Agrilus planipennis) is one such pest and targets trees in the genus Fraxinus. EAB results in high rates of ash tree mortality and it is expected that the Great Lakes region will suffer the greatest ecological impact due to the prevalence of ash. The objectives of this research were to examine the community and ecosystem consequences of 1) simulated EAB outbreaks compared to intact ash-rich stands (n=6), and 2) EAB infested stands (n=3). All stands were located in western New York and were dominated by ash trees and thus expected to demonstrate the greatest impacts of EAB. EAB outbreaks were simulated in 400 m2 plots by girdling all ash trees > 2.5 cm dbh and then compared to an adjacent stand with intact and healthy ash trees over two growing seasons. The infested plots were of the same size and were in the early stages of infestation. We examined forest community composition and structure as well as carbon dynamics and microclimate by quantifying soil and air temperature, soil moisture, soil organic matter, decay rate, soil respiration, tree productivity, seedling photosynthesis, and the soil microbial community.
Ash was prevalent in all three woody strata (seedlings, saplings, and trees) and represented at least 50% of the forest canopy. In the simulated outbreaks, the microclimate shifted to warmer air temperatures but inconsistent soil moisture effects. Canopy leaf area declined considerably in these sites while the understory leaf area increased. Seedling photosynthesis increased and was linked to increased light availability. Although the understory varied between sites, tree diversity and abundance was generally low and likely to be outcompeted by exotic shrubs already present. Over the short term, sites impacted by EAB will become C sources as suggested by the slightly elevated soil CO2 efflux (one year after girdling) and decomposition rates. Additionally, there is a substantial loss of ash productivity in these stands that is not being fully compensated by other species. The EAB infested sites are showing many of the same signs but not as drastic given the early stage of infestation. In conclusion, in forests with abundant ash trees, EAB will have a considerable impact on forest community composition resulting in altered successional trajectories and ecosystem functioning including decreased C sink strength; however, these impacts are site-specific depending on environmental condition and initial species composition.