Effects of an Invasive Grass (Phalaris Arundinacea) on Water Availability in Semi-Arid Riparian Zones

Abstract

In much of the western United States, riparian zones are shifting from native woody and herbaceous species to invasive grasses, potentially altering plant water use and availability. Compared to the woody species they often replace, dense grass stands may have higher productivity and transpiration, causing them to use more water. We investigated evidence of this effect in semiarid eastern Washington, where many streams experience low flow that degrades water quality and habitat. Most of these riparian zones have extensive stands of invasive reed canary grass (Phalaris arundinacea), which could reduce stream flow through high transpiration rates. We tested how growing season, transpiration, and specific leaf area of reed canary grass compared to native riparian plant species to assess its effects on plant water use and water availability. Reed canary grass had a longer growing season by two weeks compared to eight other common riparian species. Although its transpiration rates per leaf area are not exceptional, its high specific leaf area and ability to produce dense stands of biomass resulted in more photosynthetic surface through which to transpire water than any other species tested, and substantially higher estimates of transpiration per ground surface area. We present strong evidence that reed canary grass is altering the patterns of water availability and use in these systems.