Modelling impacts to water quality in salmonid-bearing waterways following the introduction of emerald ash borer in the Pacific Northwest, USA

Abstract

Oregon ash (Fraxinus latifolia Benth.) wetlands and riparian forests are an important economic, cultural, and ecological resource in the Pacific Northwest, USA, and are threatened by the invasive insect, emerald ash borer (Agrilus planipennis Fairmmaire) (EAB). Following the discovery of EAB in Forest Grove, Oregon in June of 2022, it is predicted that EAB-induced ash mortality will alter vegetation communities and modify hydrology of wetlands and streams in the Pacific Northwest. Of primary concern is an increase in solar loading of already degraded waterways and wetlands that support Endangered Species Act-listed salmonids and other cold-water species following canopy dieback of Oregon ash. Our study, which began before the first detection of EAB on the West Coast, models potential impacts of EAB-mediated Oregon ash canopy loss to temperature-related water quality on two waterways in the vicinity of Portland, Oregon. Our results indicate a significant increase in solar loading with likely negative impacts to Endangered Species Act-listed salmonids, other aquatic wildlife, and associated habitat. We forecast greater impacts to these resources outside our study scope and include considerations for entities with water quality-related regulatory obligations.