Legacy effects of emerald ash borer on riparian forest vegetation and structure

Abstract

High levels of ash (Fraxinus spp.) mortality have been reported in areas of North America following invasion of emerald ash borer (EAB) (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae), but little is known about the impacts of EAB in riparian forests, particularly along first order streams. We delineated canopy gaps resulting from EAB-caused ash mortality along four first order streams in each of three distinct watersheds, which represented a temporal gradient of the EAB invasion across southern Michigan. Time lapsed aerial imagery showed canopy gaps were apparent by 2005, 2010, and 2012 in southeast, central and southwest sites, respectively. We assessed overstory trees, regeneration, shrubs, herbaceous plants, coarse woody debris, and photosynthetically active radiation in each canopy gap and in forests surrounding the gap. Canopy gaps were originally dominated by green ash (F. pennsylvanica) and black ash (F. nigra) but more than 85% of ash basal area was killed by EAB across all sites. Live basal area was greater in forests than in gaps, but was similar among all three watersheds. A lower percentage of dead ash trees remained standing and volume of ash coarse woody debris was higher in southeastern sites, with the longest history of EAB invasion, than in central and southwestern sites. Species richness of overstory trees was higher in forests (34 species) than in gaps (27 species). Photosynthetically active radiation, measured 1.5 m above ground, was 55% higher in canopy gaps than in adjacent forests during summer, but was similar during winter. Species composition of pre-EAB forest overstories and current saplings were similar among canopy gaps; overall, 66.3% of saplings were ash. Sapling species in forested areas adjacent to gaps were often dominated by Acer rubrum, while Quercus rubra, Prunus serotina, Acer saccharum and A. rubrum were common in overstories. Seedlings were rare within gaps now dominated by dense mats of sedges (Carex spp.). Although ash has been functionally lost from riparian forest overstories in most of southern Michigan, consistently high sapling densities, combined with reduced EAB densities, could facilitate ash persistence in post-invasion areas. If EAB densities increase, however, recruitment of ash saplings into the overstory will be limited. Continued monitoring will be important to assess the future status of ash and overstory dynamics in of post-EAB riparian forests.