Effects of competition from shrubs on herbaceous wetland plants: a 4-year field experiment

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Abstract
While competition is known to occur among some species in some plant communities, we are not yet able to predict in which environments, or among which species, competitive interactions will be most intense. The objective of this study was to test for competition in a wetland plant community and then to determine which environments and which species were influenced by competition. The study site was the transition zone between shrubs and herbaceous plants on a lakeshore. To test for competitive release, shrubs were removed from treatment plots paired with controls in 25 sections of shoreline and cover of herbaceous species was monitored for 4 years. There were highly significant increases in cover, richness, and diversity in the removal plots, but less than one quarter of the individual species responded significantly. In general, these were small, partly evergreen species with high densities of buried seeds (e.g., Drosera intermedia, Hypericum boréale). On shores with frequent disturbance and low fertility there was no evidence for competitive release. The reduction in plant cover during a summer with high water levels suggests that lakeshores shift from abiotic to biotic structuring according to the water level in a particular year or series of years. Superimposed upon this is variation attributable to the type of shoreline and the type of species. Key words: competition, disturbance, exposure, lakeshores, Myrica gale, shrubs, wetlands, zonation.