The Role of Seed Banks, Disturbance, and Sea Level Rise in Determining the Plant Community Structure of Oligohaline Coastal Marshes.

Abstract

Coastal wetlands worldwide are threatened by sea level rise, which is expected to affect the growth and survival of wetland vegetation by increasing water level and salinity. While the effects of salinity and inundation on adult vegetation of oligohaline marshes have been widely studied, the species composition of the seed bank and its response to elevated salinity and water level have not been examined. Additionally, the role of disturbances of different intensities in structuring marsh plant communities and possible interactions between disturbance and rising sea level have received little attention. Disturbances in coastal marshes include nonlethal disturbances such as fire and herbivory (which can remove aboveground vegetation but leave rhizomes intact), and lethal disturbances such as wrack deposition, sedimentation, scouring, or fire or herbivory followed by flooding. I conducted a series of experiments to examine the role of seed banks, disturbance, and sea level rise in structuring the plant communities of oligohaline marshes in coastal Louisiana, USA. I found that seed banks of three oligohaline marsh communities were similar despite differences in the adult vegetation, and that higher salinity or water level reduced the germination of most species. In a field experiment lethal disturbances promoted the establishment of species via seedling recruitment, leading to persistent changes in community composition. Regeneration following less intense nonlethal disturbances, however, was primarily via vegetative growth which quickly restored the pre-disturbance community structure. The effects of salinity and inundation on community structure in greenhouse mesocosms were greater and occurred more rapidly following disturbance than occurred in the absence of disturbance. Disturbance coupled with both higher water level and salinity killed almost all vegetation. These results suggest that regeneration patterns in oligohaline marshes are a function of seed banks, disturbance intensity, and salinity and water level. Higher salinity and water level associated with sea level rise may inhibit seedling recruitment or vegetative growth following disturbance, resulting in shifts in species composition or wetland loss. Understanding the effects of salinity and inundation on patterns of regeneration following disturbance is critical to predicting the responses of coastal marsh communities to rising sea level.