Washington State University
The role of trees in managing stormwater runoff is an emerging area of interest in urbanizing landscapes across the world. Typically, stands of large residual native trees growing around urban areas face the threat of removal as residential and commercial development pushes outward from urban centers. Since the potential for native trees to mitigate stormwater runoff has not yet been thoroughly quantified in the Pacific Northwest region, we are currently studying tree water use at two sites near Olympia, WA in an effort to explain how mature native trees manage incident rainfall. To do this, 64 trees selected from four species common to the pacific northwest were chosen for instrumentation. The species that were selected were two evergreen species: douglas fir (Pseudotsuga menziesii) and western redcedar (Thuja plicata); and two deciduous species: red alder (Alnus rubra), and bigleaf maple (Acer macrophylum). Each tree was instrumented to measure sap flux, canopy throughfall, and stem flow. We are also measuring soil moisture and micro-climate in close proximity to the selected trees. Over two years, several rain events and intermittent dry periods will be targeted during which water budgets will be constructed for each tree. Data from these events will be used to quantify how much rainfall is prevented from becoming runoff through transpirative and canopy interception processes. These tree water budgets will provide critical information on how much urban stormwater volume can essentially be diverted away from stormwater collections systems by these four species of trees, thereby reducing downstream flooding and preventing toxic runoff from entering sensitive waters. Preliminary results from this work will be presented at the conference.