Coexistence and interference between a native perennial grass and non-native annual grasses in California

摘要

Using the example of native perennial bunchgrasses and non-native annual grasses in California grasslands, the effects of interference from non-native grasses (primarily Avena barbata, Bromus hordeaceus and Vulpia myuros) on the different life stages of the native perennial bunchgrass Nassella pulchra were investigated in relation to competition for water. In a series of field and greenhouse experiments involving removal of neighbouring plants and additions of water, it was found that seedling recruitment of N. pulchra was strongly limited by availability of seed. In both field and greenhouse experiments, natural recruitment of N. pulchra seedlings from grassland soil was extremely low. In field plots where seeds were added, addition of water to field plots increased density of N. pulchra seedlings by 88% and increased total aboveground N. pulchra seedling biomass by almost 90%, suggesting that water was the primary limiting resource. In the greenhouse, simulated drought early in the growing season had a greater negative effect on the biomass of annual grass seedlings than on the seedlings of N. pulchra. In the field, presence of annuals reduced growth and seed production of all sizes of N. pulchra, and these effects did not decrease as N. pulchra individuals increased in size. These negative effects appeared to be due to competition for water, because N. pulchra plants had less negative pre-dawn leaf water potentials when adjacent annual grasses were removed. Simply adding water caused the same increases in aboveground biomass and seed production of N. pulchra plants as removing all annual grasses. There was no evidence that established N. pulchra plants were able to suppress non-native annual grasses. Removing large N. pulchra individuals did not affect peak biomass per unit area of annuals.