Cooperative and Competitive Interactions in the Recruitment of Marsh Elders

While most studies of plant recruitment focus on competitive interactions, recruitment can be influenced by both positive and negative interactions. We studied recruitment in the marsh elder, Iva frutescens, to examine the relationship between positive and negative forces in recruitment. Marsh elder seedling establishment was competitively precluded in undisturbed habitats by the dense perennial turfs that dominate marsh habitats. Adult marsh elders, however, passively trapped tidally transported plant debris that kills underlying vegetation. As a result, adult Iva created bare space and positively influenced seedling recruitment by facilitating the formation of seedling safe sites. Within bare patches, Iva seedling survivorship was high under adult conspecifics and at high seedling densities. In contrast, solitary Iva seedlings without adult neighbors suffered extremely high mortality. These positive associations among neighbors were experimentally shown to result from neighbor buffering of hypersaline soil conditions. Salinities in unshaded bare patches were elevated due to increased exposure to radiation and surface evaporation. Both adult nurse plants and high seedling densities shaded the soil, ameliorated high soil salinities, and thus had positive effects on seedling performance. Under adult canopies or when soil salinities were experimentally reduced by shading or watering, positive associations did not occur and all interactions among seedlings or between seedlings and adults were competitive. Our results indicate that marsh plant recruitment is dictated by a balance between positive and negative forces. Moreover, since positive interactions were only seen once densities and physical forces had been experimentally manipulated, our results when that positive interactions may be important forces in assemblages even where they are not conspicuous. These findings contribute to growing evidence that positive interactions, particularly those mediated by neighbor amelioration of harsh physical conditions, are important community level processes in a wide range of plant and animal assemblages.