Vegetation regime shift in coastal wetlands affects trapping of wrack subsidies from subtidal habitats
Mangrove vegetation is spreading to higher latitudes due to global warming, with mangroves now invading locations previously dominated by salt marsh vegetation. This constitutes a striking regime shift from grass and forb-dominated habitats to taller, woody vegetation. We studied how this regime shift affects trapping of wrack in a Texas coastal wetland. In 2012, we experimentally created ten 24 x 42 m plots that vary in nominal mangrove cover from 0 to 100 percent by thinning mangroves on Harbor Island, near Port Aransas. Marsh vegetation colonized areas where mangroves had been removed. Starting in 2014, we used these plots to study how mangrove density affects trapping of marine wrack.
At this site, wrack biomass is dominated by brown and red seaweeds and seagrasses. In plots with lower mangrove cover, wrack is transported by waves throughout most of the plot. In contrast, in plots with higher mangrove cover, almost all of the wrack is trapped by mangroves in the first 4 meters of the plot closest to the waterline. We are testing the hypotheses that these different patterns of wrack transport affect geomorphology (wrack trapping in the front of the plot promotes development of a levee), soil characteristics (wrack provides organic matter to sandy soils), plant nutrition (wrack fertilizes plants) and invertebrate communities (wrack supports a detritivore community dominated by amphipods and insects). In sum, we hypothesize that mangrove invasion makes coastal wetlands less permeable to wrack, and therefore concentrates wrack-mediated processes at the front edge of the wetland.