Mangrove encroachment speeds the rate of belowground carbon cycling
The warming-induced coastal vegetation shift from salt marsh grasses to mangroves may alter processes that help sustain coastal soil elevation, especially rates of root decomposition and productivity. However, a mechanistic understanding of root dynamics has not been developed for ecotonal wetlands. Plant root structure and chemistry likely differ between salt marsh grasses such as Distichlis spicata and woody mangroves, but few studies have explored root inputs from these contrasting plant functional types. In this study, we examined root productivity in both salt-marsh and mangrove-dominated wetland zones (2 vegetation zones x 10). We also examined root litter decomposition of both mangrove and marsh roots in these same zones (2 root litter types x 2 vegetation zones x 10) . We collected and air-dried Avicennia germinans, Laguncularia racemosa, and D. spicata fine and coarse roots from our field site. In each plot, we buried three mixed bags of these mangrove roots and three bags of D. spicata roots. We also installed 4.5 x 30cm root ingrowth cores filled with milled Sphagnum peat. We collected the initial ingrowth core and one mangrove and D. spicatabag from each plot after four months, with following collections to occur at eight and twelve months.
Within plant type, mangroves lost 33.4% and D. spicata lost 13.3% of initial mass, but placement in the mangrove or marsh-dominated zones had no effect on mass loss (Two-way ANOVA: F=40.2445, p<.0001). Across all vegetative zones, mangroves lost 20.2% more mass than D. spicata roots. However, ingrowth cores in the mangrove zone showed four times more root growth than those in the salt marsh zone (Mann Whitney: W = 69, p = 0.01061). Taken together, these results suggest that mangrove encroachment increases the rate of carbon cycling in coastal wetlands through greater rates of both decomposition and belowground productivity. Predicting the subsurface expansion or subsidence of coastal wetlands under future conditions depends on accurate decomposition and productivity data such as those we developed. Our results on the dynamic belowground processes occurring due to a wetland plant range shift will be used to model coastal elevation under alternate sea level rise and warming scenarios.