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 g rasslands, we sought to determine the effects of interference from non-nati ve 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 co mponent of these interspecific interactions in this water-limited system. I n a series of field and greenhouse experiments employing neighbor removals and additions of water, we found that seedling recruitment of N. pulchra wa s strongly seed-limited. In both field and greenhouse, natural recruitment of N. pulchra seedlings from grassland soil was extremely low. In field plo ts 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 limiti ng resource. In the greenhouse, simulated drought early in the growing seas on 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 gro wth and seed production of all sizes of N. pulchra, and these effects did n ot decrease as N. pulchra individuals increased in size. These negative eff ects 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 above- ground biomass and seed production of N. pulchra plants as removing all ann ual neighbors. We found no evidence that established N. pulchra plants were able to suppress non-native annual grasses. Removing large N. pulchra indi viduals did not affect peak biomass per unit area of annuals. We conclude t hat effects of interference from non native annuals are important through a ll 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 co mpetition for water in wet years because of increased resource availability .