The invasive emerald ash borer (EAB, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae)) is a significant threat to biodiversity and ecosystem processes in North American forests. All native ash (genus: Fraxinus) trees greater than 2.5 cm in diameter are considered susceptible to EAB-induced mortality, with stand-level mortality rates of the genus frequently exceeding 99% within a few years following infestation. Of particular concern is the fate of Fraxinus nigra Marshall (black ash), a common component of forested wetlands that, unlike most members of the genus, is frequently a dominant canopy species across much of its range. To investigate the potential vegetation response to the loss of this foundational species, EAB infestation was simulated in black ash-dominated wetlands of Upper Michigan, USA. Nine uninfested wetlands were subjects in a manipulative study, in which early- and late-stage infestation impacts were emulated by either manually girdling (three ‘Girdle’ sites) all black ash stems greater than 2.5 cm, or felling all ash trees by chainsaw (three ‘Ash-Cut’ sites), while three unmanipulated wetlands (‘Control’) served as controls. Annual surveys of both woody and herbaceous species were conducted for one year prior to treatment application and three years post-treatment.
Results/Conclusions
Within the overstory, growth rates of residual non-ash stems were unaffected by treatment. However, sapling growth rates of non-ash species were significantly greater, averaging 275% higher in ‘Girdle’ sites compared to ‘Control’ sites. Species composition of the seedling layer is shifting to Acer rubrum L. (red maple) and Betula alleghaniensis Britton (yellow birch) in the treated sites, with increased densities of both species observed. Though black ash seedlings in the ‘Control’ sites increased significantly from 6000 to 63 000 stems ha-1 in the last year of the study, no increase was observed in either ‘Girdle’ or ‘Ash-Cut’ treatments. This suggests that as extant ash regeneration below the size infested by EAB recruits to upper vegetation layers, it may not be replaced by newly established seedlings. The most dramatic response to treatment occurred within the herbaceous strata, which nearly doubled in areal cover over the course of the study. Importantly, this expanded cover was not associated with decreased density of woody seedlings, suggesting that increased competition between these functional groups has not yet impacted the potential for recovery of woody vegetation in these forests. However, the interaction between these layers is likely to be among the most important factors in determining the future fate of woody species in these communities.