Understanding invasion of stream restoration projects and the resulting impacts to the soundscape

Abstract

Stream restoration is an important tool to address stressors of urban streams, such as flashy flows and urban runoff, that lead to channel erosion and poor water quality. However, the disturbance associated with stream restoration can leave space and resources available for invasive plants to establish. To understand the dynamics between stream restoration and invasion, I examined the vegetation communities of 46 stream restoration projects in the Chesapeake Bay watershed and paired unrestored stream reaches. I found that restored stream reaches were more invaded than their unrestored pairs and the increased invasion, though related to increased resource availability (e.g., soil nutrients, photosynthetically active radiation), was not explained by differences in availability of those resources between reaches (i.e., restored vs. unrestored) or time since restoration. Utilizing the variation in restoration outcomes, I assessed the importance of resource availability, land use, project attributes, planting design, and project monitoring variables in predicting invasive plant cover. The most important variables were resource availability variables, such that increasing light or soil nutrient availability correlated with increased invasion. Therefore, recommendations to restoration practitioners on how to limit invasive plant establishment include preserving overstory trees and limiting use of fertilizer within the limits of disturbance. Additionally, projects that used a reference site, either in restoration design or for project monitoring post-restoration, had lower invasive cover than those that did not. Though important to ecosystem function and recovery, wildlife responses to restoration are rarely assessed as part of post-restoration monitoring. Therefore, I deployed autonomous recording units across 20 of the 46 paired streams for one year to examine the impacts of stream restoration and invasive plants on soundscapes, or the sum of all sounds in the environment (e.g., birds, frogs, insects). I observed significant seasonal variation between restored and unrestored streams and between high and low invasion streams, as determined through established soundscape indices, such as increased bioacoustic activity in the winter on high invasion streams. Soundscape differences between high and low invasion streams could be due to phenological differences between invasive and native plants whereas differences due to restoration could be due to changes in geomorphology and hydrology. Given the documented negative impacts of invasive plants on native ecosystems, my work provides an understanding of how stream restoration affects invasion, methods to limit invasion of stream restoration projects, and the first application of passive acoustic monitoring to assess effects of restoration and invasion on soundscapes.