Restoration of biogeochemical function in mangrove forests

Forest structure of mangrove restoration sites (6 and 14 years old) at two locations (Henderson Creek [HC] and Windstar [WS]) in southwest Florida dif fered from that of mixed-basin forests (>50 years old) with which they were once contiguous. However, the younger site (HC) was typical of natural, de veloping forests, whereas the older site (WS) was less well developed with low structural complexity. More stressful physicochemical conditions result ing from incomplete tidal flushing (elevated salinity) and variable topogra phy (waterlogging) apparently affected plant survival and growth at the WS restoration site. Lower leaf fall and root production rates at the WS resto ration site, compared with that at HC were partly attributable to differenc es in hydroedaphic conditions and structural development. However, leaf and root inputs at each restoration site were not significantly different from that in reference forests within the same physiographic setting. Macrofaun al consumption of tethered leaves also did not differ with site history, bu t was dramatically higher at HC compared with WS, reflecting local variatio n in leaf litter processing rates, primarily by snails (Melampus coffeus). Degradation of leaves and roots in mesh bags was slow overall at restoratio n sites, however, particularly at WS where aerobic decomposition may have b een more limited. These findings indicate that local or regional factors su ch as salinity regime act together with site history to control primary pro duction and turnover rates of organic matter in restoration sites. Species differences in senescent leaf nitrogen content and degradation rates furthe r suggest that restoration sites dominated by Laguncularia racemosa and Rhi zophora mangle should exhibit slower recycling of nutrients compared with n atural basin forests where Avicennia germinans is more abundant. Structural development and biogeochemical functioning of restored mangrove forests th us depend on a number of factors, but site-specific as well as regional or local differences in hydrology and concomitant factors such as salinity and soil waterlogging will have a strong influence over the outcome of restora tion projects.