Manipulation experiment overstates the competitive interactions occurring between mangroves and salt marsh vegetation

Ecologists use multiple methods for studying community-level interspecific competition, but different approaches may give different answers. We compared four methods to quantify the competitive interactions between Avicennia germinans (black mangroves) and salt marsh vegetation in Texas, USA. We compared four methods to quantify the competitive interactions between mangroves and marsh vegetation: two different methods of sampling a large (24 x 42 m) mangrove removal experiment, a transplant experiment conducted within the large experiment, and a natural experiment comparing sites naturally dominated by marsh or mangrove vegetation. We found stronger competition in the mangrove removal experiment than in the natural experiment. This was likely because the site chosen for the mangrove removal experiment had higher densities of mangroves than did the sites chosen for the natural experiment. Outcomes also differed among marsh plant species, and also differed as a function of spatial scale: the strength of competition in the transplant experiment was driven only by the presence or absence of mangroves in the immediate (3x3 m cell) vicinity of the transplanted plants, but natural colonization of 3x3 m cells within the 24 x 42 m plots was also a function of the cover of mangroves at the plot scale. Our findings suggest that manipulation experiments can give results that do not reflect patterns at the landscape scale if study sites are not representative of the landscape. Although global warming is likely to facilitate continued spread of mangroves, marsh plants are likely to persist on the landscape in areas where mangroves do not attain high cover.