Invasive species and non-native pests, in particular, are altering the composition and functioning of forests worldwide and have been called the greatest threat to Northeastern forests. The prevalence of ash trees (Fraxinus species) in the Great Lakes region is concerning given the spreading population of the emerald ash borer (EAB, Agrilus planipennis) which targets ash trees. Our focus here was in forests of western New York in the Lake Ontario plain that are dominated by ash. Our objectives were to evaluate both community and ecosystem-level consequences of simulated EAB outbreaks as well as in actual infestations. Six simulated outbreaks were conducted by girdling all ash trees > 2.5 cm dbh in 20 x 20m plots and then comparing these to adjacent control plots in which the ash remained intact. Three infested plots of the same size were established to follow the progression of the EAB infestation, currently still in relatively early stages with ash trees damaged but with relatively little mortality. In each plot, we have examined the forest community composition and structure as well as various ecosystem-level variables including C dynamics and microclimate.
Results/Conclusions
Ash trees were indeed dominant in all plots and in all strata (seedlings, saplings, and canopy) but especially so in the canopy representing 85% of canopy stems and 80% by dominance. Two years following the outbreak simulation and 100% mortality of the girdled trees, the canopy leaf area index of the control plots averaged 4.64 compared to 2.19 in the girdled plots. The understory LAI demonstrated that the understory is responding favorably to the canopy reduction with a nearly 70% increase in the girdled plots. Non-native shrubs and forbs are the primary drivers of this understory increase and represent a major shift in community composition, pushing the system to a novel state. The analysis of the forest C dynamics suggest that EAB outbreaks will result in shifts of the stands to C sources via a substantial reduction in canopy tree wood production (75%) and litter production (70%) while there were modest increases in decay rates and soil respiration. Despite increased cover and productivity in the understory, it remained much less than that of the uninfested plots. In sum, EAB has had a tremendous impact on forest successional trajectory and ecosystem functioning but results varied by site depending on initial condition.