Influence of pocket gopher mounds on nonnative plant establishment in a shrubsteppe ecosystem

Soil disturbances across a wide range of spatial scales have been found to promote the establishment of invasive plant species. This study addresses whether mounds built by northern pocket gophers (Thomomys talpoides) in the shrubsteppe environment of north central Washington are facilitating plant invasions into native-dominated fields. Research was conducted in native-dominated plant communities adjacent to ex-arable, nonnative-dominated fields. To determine the effect of mounds on plant growth, we recorded new establishment and persistence of all plant species over 2 growing seasons on 10-19 mound and intermound areas in 10 fields. Nonnative plant establishment was not affected by mounds, but native plant establishment, particularly of the dominant native Pseudoroegneria spicata was lower on mounds than on intermounds. Early in the growing season, mounds had reduced soil moisture, bulk density, soil strength, N mineralization rates, and total N and C concentrations, and similar extractable NO concentrations relative to intermound soils. Our results did not suggest that soil disturbance improved nonnative growth resulting in competitive suppression of natives; rather, our results suggested that low soil moisture and slow N mineralization rates on mounds in this ecosystem present relatively stressful conditions for native plant growth.