Predicting the effects of emerald ash borer on hardwood swamp forest structure and composition in southern Michigan

Background/Question/Methods

The study of Emerald Ash Borer (EAB) in urban and upland habitats containing white and green ash has been well documented in invasive literature.  EAB within hardwood swamps containing black and green ash (Fraxinus nigra and F. pennsylvanica), however, has received much less attention.  Black and green ash often occur as dominant components of hardwood swamp communities, unlike ash in upland communities.  Therefore EAB is likely to have a dramatic effect on the structure and composition of hardwood swamps due to a large amount of dieback.  The objective of this study was to survey three hardwood swamps with ash and model the future forest structure and composition of each stand.  Three hardwood swamps in southern Michigan were surveyed during the summer of 2014.  We used a nested plot design, with four 400 m² plots at each site to measure trees, 200 m² plots to measure saplings and shrubs, and four 4 m² microlots in each plot to measure seedlings and herbaceous cover.  All ash trees were rated using a 1-5 health scale to determine the status of ash death. We used the Forest Vegetation Simulator (US Forest Service software) to project future forest structure and composition. 

Results/Conclusions

The three sites differ slightly in composition, but not structure.  While all sites contained both black and green ash, they varied widely on both the percentage of ash (12.6-61.8% by basal area) and other dominant species present.   In addition, the species in the canopy, such as ash and red maple, tend not to be the principal species in the sapling layer, where musclewood or poison sumac dominate.  All mature ash trees have died, while the majority (73.4-84.2%) of ash saplings are alive but declining.  Current results show that the three sites are expected to respond in slightly different ways, with two showing a continuing prevalence of other current dominants, such as red maple and yellow birch, while one shows a transition to a forest dominated by small trees and shrubs, such as musclewood and buckthorn.  This study will provide insight into the current status of EAB affected swamps, of which little is known, and illustrate the community scale impacts of a selective pest.