The widespread invasion in California grasslands has led to the loss of millions of hectares of native grasslands. In spite of this large invasion, remnant stands of native perennials still persist and are able to resist invasion even when in close proximity to exotic annual grasslands. In this study we investigate three potential mechanisms that could help these native grasslands resist invasion. We predicted that 1) if resistance was maintained through resource interactions, resource availability and above- and belowground competitive abilities would differ among the grassland types. We also predicted 2) if herbivory conferred invasion resistance, herbivory of native seedlings would be greater in exotic grasslands than native grasslands and vice versa for exotic seedlings. Moreover, we predicted 3) if resistance were maintained via plant-soil feedbacks, indicator species of each community would grow better in their resident soil than foreign soil. In 10 sites each containing paired patches of exotic and native grassland communities in Orange County California, we measured resource dynamics (inorganic nitrogen (N) pools, soil moisture, light availability, and productivity three times throughout the growing season) and the effects of increased light availability (with vegetation tie-downs) and herbivory (with exclosures) on seedling performance.

Results/Conclusions

While soil moisture did not differ between the vegetation types, soil inorganic N pools, aboveground productivity, and light interception were greater in exotic patches than native patches.  Increasing light availability improved the performance of native seedlings relative to exotic seedlings in exotic patches. Reducing herbivory pressure did not affect native or exotic seedling performance and species grew equally well in soils cultured with resident and foreign microbial communities. These results indicate a trade-off between above- and belowground competition, where natives may be better competitors for belowground soil resources, reducing N availability to the detriment of the exotic annual grasses, and exotics may be better competitors for light, reducing light availability to the detriment of the native species. The results also suggest that competitive interactions may play a larger role in invasion dynamics than herbivory or microbial feedbacks. Attention to mechanisms related to resource availability (litter accumulation, effects of fire and nitrogen deposition) will be important to consider in order to maintain the resistance of the native grasslands and facilitate native recovery and re-invasion into exotic annual grasslands.