Nitrogen cycling responses to simulated emerald ash borer infestation in Fraxinus nigra-dominated wetlands

Understanding short- and long-term responses of forest nutrient cycling to disturbance is vital to predicting future forest function. Mortality of ash trees (Fraxinus spp.) due to emerald ash borer [EAB, Agrilus planipennis (Coleoptera: Buprestidae)] invasion is likely to alter ecosystem processes within infested stands throughout North America. In particular, the loss of Fraxinus nigra (black ash) from F. nigra-dominated swamps may significantly impact the biogeochemical cycles within these ecologically important wetlands. A multiyear manipulative study of nine F. nigra-dominated wetlands in Michigan, USA was undertaken to investigate the potential response of above- and belowground biogeochemical processes to EAB. Short- and long-term changes to site conditions following infestation were emulated by respectively girdling or felling F. nigra saplings and overstory trees. Following disturbance, a short-term reduction in demand for soil nitrogen (N) by dominant canopy species was hypothesized to result in increased soil N availability and a subsequent increase in N uptake by retained species. Though reduced total N return via litterfall indicated decreased demand, this resulted in minimal impacts to soil N availability following treatment. Additionally, increased N uptake by co-dominant Acer rubrum (red maple) and Betula alleghaniensis (yellow birch) was not observed; these combined responses may be attributable to increased immobilization of N by soil microbes. In the 3 years following treatment, the response of foliar characteristics of residual stems—including decreased N concentrations and increased leaf mass per area—appeared to be driven primarily by aboveground conditions and a change from shade- to sun-acclimated leaves. While increased microbial immobilization of N may reduce long-term changes in site fertility, these responses may also limit the potential for short-term positive growth responses of extant woody vegetation. In the longer term, replacing N-rich F. nigra leaf litter with that of A. rubrum and B. alleghaniensis, which have lower N content, is likely to have important feedback effects on soil processes