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

Little is known about the potential for coexistence between native and non-native plants after large-scale biological invasions. Using the example of native perennial bunchgrasses and non-native annual grasses in California grasslands, we sought to determine the effects of interference from non-native grasses on the different life stages of the native perennial bunchgrass Nassella pulchra. Further, we asked whether N. pulchra interferes with non-native annual grasses, and whether competition for water is an important component of these interspecific interactions in this water-limited system. In a series of field and greenhouse experiments employing neighbor removals and additions of water, we found that seedling recruitment of N. pulchra was strongly seed-limited. In both field and greenhouse, natural recruitment of N. pulchra seedlings from grassland soil was extremely low. In field plots where we added seeds, 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 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 showed less negative pre-dawn leaf water potentials when annual neighbors were removed. Also, simply adding water caused the same increases in aboveground biomass and seed production of N. pulchra plants as removing all annual neighbors. We found 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. We conclude that effects of interference from non native annuals are important through all life stages of the native perennial N. pulchra. Our results suggest that persistence of native bunchgrasses may be enhanced by greater mortality of annual than perennial seedlings during drought, and possibly by reduced competition for water in wet years because of increased resource availability.