Facilitation of survival and growth of Baccharis halimifolia L. by Spartina alterniflora Loisel. In a created Louisiana salt marsh

Abstract

Coastal wetland loss is a major environmental issue in the Mississippi Delta region of the southern United States, where grasses such as Spartina alterniflora may play a critical role both as early colonizers on created sites and as facilitators of other marsh grasses and shrubs, particularly at high intertidal elevations. We explored the potential role of S. alterniflora as a facilitator of the colonization and growth of the shrub Baccharis halimifolia at two created wetlands in southwestern Louisiana through a combination of plant surveys and experimentation. Surveys for the presence of B. halimifolia inside and outside the bare centers of S. alterniflora clones that had begun to senesce inwardly were conducted at a 4-yr-old site originally created in 1993. The percent of clones containing B. halimifolia and the number of individual B. halimifolia plants per clone increased with increasing S. alterniflora clone size and decreased with increasing distance from an adjacent containment berm. Two experiments conducted at a second 4-yr-old site that was originally created in 1996 were designed to assess seed capture and growth of seedlings of B. halimifolia inside S. alterniflora clones. These experiments revealed that while significantly fewer seeds settle inside clones, those seedlings that grow in such areas gain a clear advantage in terms of both survival and rate of growth, which helps to explain the results of the initial survey and testifies to the role of S. alterniflora as a nurse plant in newly-created high intertidal marshes.