Resource competition, not facilitation, structures gravel beach plant communities

Questions: How are plant–plant interactions mediated by environmental factors to influence plant community structure on gravel beaches? Location: Two coastal gravel beaches on the eastern coast of New Zealand. Methods: We surveyed plant co-occurrence patterns on the beaches and performed plant addition experiments, varying nutrients, water, and presence of potential nurse plants. We used C-score analysis to test whether observed individual and species co-occurrence patterns deviated from random expectation. Generalised linear modelling of survival within the experiments tested the importance of potential nurse plants vs both resource- and non-resource-based stress. Results: Despite the harsh conditions, species co-occurred less frequently than expected if distributions were random. The presence of adult nurse plants had a negative, rather than facilitative effect on growth and survival of planted shrub seedlings. Abiotic factors also influenced seedling survival, although responses were species-specific. Conclusions: We found no evidence that facilitation plays a role in structuring plant communities in the highly stressed environment of New Zealand gravel beaches. Rather, our results support the prediction that resource competition is the dominant factor in determining community structure in highly stressed environments and is inconsistent with the stress gradient hypothesis