Exotic plant communities shift water-use timing in a shrub-steppe ecosystem

Semiarid areas in the US have realized extensive and persistent exotic plant invasions. Exotics may succeed in and regions by extracting soil water at different times or from different depths than native plants, but little data is available to test this hypothesis. Using estimates of root mass, gravimetric soil water, soil-water potential, and stable isotope ratios in soil and plant tissues, we determined water-use patterns of exotic and native plant species in exotic- and native-dominated communities in Washington State, USA. Exotic and native communities both extracted 12 +/- 2 cm of water from the top 120 cm of soil during the growing season. Exotic communities, however, shifted the timing of water use by extracting surface (0-15 cm) soil water early in the growing season (i.e., April to May) before native plants were active, and by extracting deep (0-120 cm) soil water late in the growing season (i.e., June to July) after natives had undergone seasonal senescence. We found that 6180 values of water in exotic annuals (e.g., -11.8 +/- 0.4 parts per thousand for Bromus tectorum L.) were similar to 6180 values of surface soil water (e.g., -13.3 +/- 1.4 parts per thousand at -15 cm) suggesting that transpiration by these species explained early season, surface water use in exotic communities. We also found that delta(18)O values of water in taprooted exotics (e.g., -17.4 +/- 0.3 parts per thousand. for Centaurea diffusa Lam.) were similar to 6180 values of deep soil water (e.g., -18.4 +/- 0.1 parts per thousand at -120 cm) suggesting that transpiration by these species explained late season, deep water use. The combination of early-season, shallow water-use by exotic winter-actives and late-season, deep water-use by taprooted perennials potentially explains how exotic communities resist establishment of native species that largely extracted soil water only in the middle of the growing season (i.e., May to June). Early season irrigation or the planting of natives with established root systems may allow native plant restoration.