The importance of competition in regulating plant species abundance along a salinity gradient

Current theories differ in their predictions concerning the effects of inte rspecific interactions on species growth and distribution along environment al gradients. In this study, we examined the influence of competition on sp ecies composition across a salinity gradient. This work involved three comm on fresh and brackish marsh species. The three species, Spartina patens, Sa gittaria lancifolia, and Panicum hernitomon, differ in salt tolerances but are commonly found in overlapping zones across coastal marsh gradients in t he southeastern United States. Plants were grown in a greenhouse under four salinity treatments (0, 2, 4, and 8 g/kg), in monocultures and in three-sp ecies mixtures. Data from the monocultures and mixtures were used to examin e the importance of competition in regulating community composition and spe cies' growth. Results for individual species were significantly different w hen grown in monoculture vs, the three-species mixture. While with increasi ng salinity community dominance shifted toward the most salt-tolerant speci es, Spartina parens, competition was found to alter community composition e qually over all salinities. However, the responses of individual species de monstrated a very different pattern; the relative importance of competition in influencing individual growth differed substantially depending on the s alinity tolerances of species. These results support the contention that, a s abiotic stress increases, competition becomes less of a limiting factor i n regulating the abundance of a species. However, the role of competition a t the community level is the sum of the individual species' responses and m ay or may not vary with the level of abiotic stress.