Summer water in a restored native grassland flushes annual grass seed bank but fails to increase native perennial cover

Abstract

AbstractNatural and artificial flushing of the weed seed bank, followed by killing of seedlings, is a common practice in agricultural and prerestoration settings, but its application for post‐restoration management has not been tested in the context of restored native plants. Summer watering at already restored sites could reduce exotic annual grass cover by decreasing the seed bank, thereby increasing native perennial success in subsequent growing seasons. Five replicated watering treatments of 11.35 L per watering event were applied for up to a 4 week period, with watering events ranging from once daily for a minimum or four and maximum of 16 d, or twice daily for 4 d, for a total of 45.4–181.7 L water applied in each 1‐m2 plot. Two of the watering treatments triggered significant flushing of annual grasses: watering for a total of 16 d once per day, and 4 d twice per day. Although this conferred a short‐term reduction in annual grass emergence at the start of the subsequent rainy season, it did not reduce total annual cover at peak flowering in the growing season, or provide a longer term advantage to native perennial grasses. It is possible that there are sufficient seeds in the seed bank that this at least partly compensated for the seed bank reduction and did not result in a reduction in final cover, or that lower densities of seedlings are sufficient to achieve “total” cover. Perennial grass cover increased in response to all watering treatments, but this did not result in significantly increased cover the following year. These results suggest that even watering treatments that produce large flushes of exotic annual grasses are insufficient to reduce exotic cover longer term. It should be explored whether more water, applied more frequently, could be effective in reducing the exotic grass seed bank enough to confer a long‐term reduction in exotic cover and a benefit to native perennial grasses.