Invasion of salt marsh by mangroves: Understanding how facilitation promotes recruitment under environmental change

Background/Question/Methods   Surveys in estuaries of southeast Australia have revealed a recent decline in the area occupied by salt marsh and a concurrent “invasion” of these marshes by the native mangrove Avicennia marina. Coastal salt marsh is listed as an Endangered Ecological Community in this temperate region and thus the observed landward encroachment of mangroves into salt marshes is a cause for concern. A leading explanation for this invasion is that environmental change, particularly increased run-off of nutrients and freshwater due to human modification of the coastal landscape, has reduced stressful abiotic conditions of salt marshes and facilitated the recruitment of A. marina, allowing competitive displacement of the species of plants that characteristically dominate the marsh. To examine this explanation, in a series of field experiments we manipulated salt marsh vegetation and nutrients along tidal elevation gradients dominated by different species of plants, supplied mangrove propagules to these areas, and monitored the establishment and growth of seedlings. Results/Conclusions   Seedling establishment was universally poor in areas without marsh vegetation, but varied in the presence of marsh vegetation. Marsh vegetation facilitated mangrove recruitment at low tidal elevations dominated by the chenopod Sarcocornia quinqueflora, but largely precluded recruitment at high tidal elevations dominated by the larger rush Juncus kraussii. Nutrients had no influence on seedling establishment. Nevertheless, a multiyear, field experiment where we manipulated nutrient load surrounding mangrove seedlings already established in the marsh revealed that nutrients reduced the physiological stress (as determined by chlorophyll fluorescence) of seedlings and catalyzed the demographic transition from seedling to sapling, with some individuals flowering after only two years. The amelioration of harsh abiotic conditions by the presence of marsh vegetation and nutrients clearly has the potential to facilitate the establishment and spread of mangroves within coastal salt marshes. The key to predicting future patterns of mangrove invasion will be an understanding of the processes that promote recruitment, particularly those that break down barriers to propagule dispersal and facilitate seedling establishment and early growth. Given that the southeast coast of Australia is predicted to be one of the most influenced by environmental change over the next century, due to a combination of increased population growth and warming under climate change, understanding the ecological processes responsible for mangrove invasion will help to predict distributional shifts from seaward to landward and with latitude.