COS 124-1
Effects of local adaptation on California salt marsh plant interactions and responses to precipitation
Species that occur along large environmental gradients can display corresponding variation in traits that make them better adapted to local conditions. In stressful environments, the Stress-Gradient Hypothesis predicts that interactions will be more facilitative, but it is unclear whether interactions differ along environmental gradients due to variation among populations or differing environments. In California salt marshes, a number of species occur across large gradients in precipitation and are likely to be locally adapted. We established a common garden experiment with two common salt marsh plants, Sarcocornia pacifica and Jaumea carnosa, to test three hypotheses: (1) populations are adapted to precipitation conditions at their site of origin, (2) interactions are more facilitative under low precipitation conditions, and (3) population differences affect species interactions resulting in more competition between plants from arid than wet sites under low precipitation conditions. Plants were collected from six sites between San Diego and Eureka, CA and grown in pots in San Diego under low (San Diego), medium, or high (Eureka) precipitation conditions. Plants were grown alone or with a neighbor of the other species from the same site. We measured plant growth weekly to determine the effects of precipitation and source on plants and their interactions.
Results/Conclusions
Growth of both species differed by source. We found an interactive effect of source, precipitation and neighbor presence on J. carnosa growth. When J. carnosa was grown alone, plants from the southernmost sites were unaffected by precipitation. Plants from all other sites grew best under medium and high precipitation suggesting that J. carnosa is adapted to precipitation conditions at its original source. When either plant was grown with a neighbor, growth was generally reduced. However, under low precipitation conditions at some sites, J. carnosa growth was similar regardless of neighbor presence, indicating that there was reduced competition under stressful conditions as the Stress-Gradient Hypothesis would predict. These results suggest that populations of these common west coast marsh species have geographical trait variation that may be driven by adaptation to local conditions. They also indicate that the strength of interactions between species differs due to variation among populations. These trends are likely to become stronger as the rainy season ends and water stress increases, and they suggest that species interactions may become more facilitative as precipitation regimes change in the future.