Latitudinal and climate-driven variation in the strength and nature of biological interactions in New England salt marshes.

We examined the linkage between climate and interspecific plant interactions in New England salt marshes. Because harsh edaphic conditions in marshes can be ameliorated by neighboring plants, plant neighbors can have net competitive or facilitative interactions, depending on ambient physical stresses. In particular, high soil salinities, which are largely controlled by solar radiation and the evaporation of marsh porewater, can be ameliorated by plant neighbors under stressful conditions leading to facilitative interactions. Under less stressful edaphic conditions, these same neighbors may be competitors. In this paper, we use this mechanistic understanding of marsh plant interactions to examine the hypothesis that latitudinal and inter-annual variation in climate can influence the nature and strength of marsh plant species interactions. We quantified the relationship between climate and species interactions by transplanting marsh plants into ambient vegetation and unvegetated bare patches at sites north and south of Cape Cod, a major biogeographic barrier on the east coast of North America. We hypothesized that the cooler climate north of Cape Cod would lead to fewer positive interactions among marsh plants. We found both latitudinal and inter-annual variation in the neighbor relations of marsh plants that paralleled latitudinal differences in temperature and salinity. South of Cape Cod, plant neighbor interactions tended to be more facilitative, whereas north of Cape Cod, plant neighbor interactions were more competitive. At all sites, soil salinity increased and plant neighbor interactions were more facilitative in warmer versus cooler years. Our results show that interspecific interactions can be strikingly linked to climate, but also reveal that because the sensitivity of specific species interactions to climatic variation is highly variable, predicting how entire communities will respond to climate change will be difficult, even in relatively simple, well-studied systems.