Insights from Observations and Manipulative Experiments into Competition Between Mangroves and Salt Marsh Vegetation

Ecologists use multiple methods for studying interspecific competition, but different approaches may give different answers. We examined competitive interactions between Avicennia germinans (black mangroves) and salt marsh plants at their dynamic ecotone in Texas, USA. We compared four methods: two different methods of sampling a large (24 × 42 m) mangrove removal experiment, a transplant experiment conducted within the large experiment, and an observational study comparing sites naturally dominated by marsh or mangrove vegetation. In all of these approaches, mangroves strongly suppressed the cover and biomass of salt marsh plants. But our understanding of the strength of these interactions varied depending on the study method used, the plant species studied, and the spatial scale considered. The transplant experiment isolated the effects of competition driven only by the presence or absence of mangroves in the immediate (3 × 3 m plot) vicinity of the transplanted plants. In contrast, the observational study examined the combined effects of dispersal, abiotic suitability, and competition as a function of the cover of mangroves at the larger plot scale. Combining the approaches in areas with levels of mangrove cover varying from 45 to 97% provided insight into how results from the local scale could be extrapolated to the landscape. In this case, although mangroves may compete strongly with neighboring marsh plants growing in their immediate vicinity, marsh plants may not be totally eradicated from sites colonized by mangroves, but instead may persist on the landscape at low densities in stressful habitats that offer a refuge from competition.