An abiotic gradient in tidal influence and biotic interactions, competition and herbivory, work in concert and in conflict to set the distributional boundaries of an ecotonal salt marsh community sensitive to climate change

Background/Question/Methods

Understanding the ecological processes that determine the distribution of communities and species is a vital part of understanding how these systems will respond to changes that are the result of climate change. The high marsh-ecotonal community is limited to a narrow elevational zone across an intense environmental gradient caused by tides. The abrupt boundaries between the ecotone and the adjacent habitat types suggest strong ecological processes are structuring the distribution of this community. However, the ecological processes that are responsible for the maintenance of the distribution and abrupt boundaries of this community at the marine/terrestrial interface and therefore most likely to be disrupted in the face of climate change are unknown. To identify the salient ecological processes responsible both manipulative experiments and observational tools were used. The roles of competitive interactions and physiological tolerance limitation in setting the distribution of two dominant species of the ecotonal plant community, Jaumea carnosa and Distichlis spicata, were assessed through a neighbor removal experiment and an outplanting experiment, respectively. The results of these experiments were contrasted with the results of a multivariate analysis of soil parameters that characterized the gradient in tidal influence experienced across the distribution of this community.

Results/Conclusions

The results of these two manipulative experiments suggest that the upland boundary of both Distichlis and Jaumea are set by physiological intolerance to the underlying abiotic conditions, and competitive interactions play a lesser role in setting that boundary. In contrast, the results for the lower marsh boundary were less conclusive. It seems that this boundary is not set by physiological intolerance or competitive interactions.

Gradient analysis and multivariate analyses of several parameters defining the soil environment and tidal influence regime suggest that there are several threshold elevations where abrupt changes in soil chemistry may be integral in defining the boundaries of individual ecotonal species, though not in all cases. These results suggest a complex suite of ecological processes and environmental conditions are interacting at a very small spatial scale to define the limited distribution of this plant community. The complexity of these interacting processes suggest that even small changes in the abiotic environment where these processes occur may have large impacts on the structure and distribution of this community.