Emerald ash borer (EAB) (Agrilus planipennis Fairmaire) (Coleoptera: Buprestidae), a phloem-boring beetle native to Asia was first detected in the greater metropolitan area of Detroit, Michigan, USA in 2002. The range invaded by EAB has continued to progress and now encompasses much of the eastern U.S. and Canada. Previous studies have quantified mortality of ash (Fraxinus spp.) trees following EAB invasion, particularly in forests in southeast Michigan and Ohio, but little is known about the impacts of EAB in riparian forests bordering streams and rivers. In the northeastern U.S., black ash (F. nigra Marsh.) and green ash (F. pennsylvanica Marsh.) are abundant in riparian forests and are both highly preferred and vulnerable EAB hosts. In riparian forests where ash trees were abundant prior to the EAB invasion, mortality could result in cascading effects in both terrestrial and aquatic environments.My research, which is presented in three chapters of this thesis, focused on evaluating effects of EAB on forest overstories, regeneration dynamics, and associated woodboring cerambycid beetles in riparian forests of Michigan. Each chapter is intended to be submitted as a manuscript for publication in scientific journals.In Chapter One, I assessed the overstory trees, regeneration, shrubs, herbaceous plants, coarse woody debris, and photosynthetically active radiation in canopy gaps formed from EAB killed ash trees and in adjacent forests in riparian forests bordering first order streams. Sites were located in three distinct watersheds across southern Michigan and were selected to represent a temporal gradient of the EAB invasion. In canopy gaps along streams, green ash (F. pennsylvanica) and black ash (F. nigra) dominated the overstory before the EAB invasion but more than 85% were killed by EAB in all sites, resulting in canopy gaps. In southeastern sites with the longest EAB invasion history, dead ash trees had begun to fall and coarse woody debris (CWD) volumes were higher than in areas of more recent EAB invasion, where most of the dead ash remain standing. Regeneration in canopy gaps was dominated by ash but seedlings were rare and gap understories were dominated by dense sedge mats (Carex spp.).In Chapter Two, I captured cerambycid beetles (Coleoptera: Cerambycidae) in cross vane panel traps baited with (R) 3-hydroxyhexan-2-one, an aggregation pheromone attractive to several genera of cerambycids. Traps were deployed in the canopy and at ground level at the perimeter of the canopy gaps described in Chapter 1. Trapping took place during summer months of 2017 and 2018, and captured beetles were collected at 2-3 week intervals. During both trapping seasons, the majority of species were initially captured early in the summer. Beetle captures were similar among watersheds and between canopy and ground level traps, but canopy traps captured more species than ground traps. Several species captured in high abundance were notably more common in different watersheds, and are perhaps responding to forest changes in the aftermath of the EAB invasion.In Chapter Three, I evaluated riparian forests along three rivers in northwest lower Michigan that are economically important for recreation, provide habitat for spawning Great Lake trout and salmon and drain into reservoirs that empty into Lake Michigan. Forests were evaluated in a similar manner as in Chapter One. In these northern forests, canopy gaps from EAB killed ash trees comprised 15-20% of the forest area near river banks. Prior to the EAB invasion, green ash (F. pennsylvanica) and black ash (F. nigra) dominated overstories in all gaps, but >95% of those trees had died. Most dead ash trees remained standing and ash CWD was limited in all canopy gaps. Green ash saplings and recruits dominated the advance regeneration in canopy gaps, but black ash regeneration was minimal. As in southern Michigan, seedlings were rare in gaps, where understory vegetation was dominated by dense sedge mats.Overall, results from this research provide a baseline of the post-EAB invasion status of riparian forests across Michigan. Green ash and black ash have been functionally lost in the overstory, and canopy gaps now make up a large component of the riparian corridors. While ash regeneration is present in canopy gap understories, dense sedge mats may inhibit future seedling recruitment. If the remnant ash cohort is unable to persist, large areas where ash was abundant prior to the EAB invasion may transition away from forests towards sedge dominated meadows. This may present long term changes in terrestrial and aquatic conditions. Ash mortality appears to be altering assemblages of cerambycid species, indicating EAB can indirectly affect other taxa. Effects of EAB related changes in riparian forests on aquatic ecosystems are as of yet largely unknown.
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