P.L. Faulkner | Search USGS

USGS2001

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 differed from that of mixed-basin forests (>50 years old) with which they were once contiguous. However, the younger site (HC) was typical of natural, developing forests, whereas the older site (WS) was less well developed with low structural complexity. More stressful physicochemical conditions resulting from incomplete tidal flushing (elevated salinity) and variable topography (waterlogging) apparently affected plant survival and growth at the WS restoration site. Lower leaf fall and root production rates at the WS restoration site, compared with that at HC were partly attributable to differences 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. Macrofaunal consumption of tethered leaves also did not differ with site history, but was dramatically higher at HC compared with WS, reflecting local variation in leaf litter processing rates, primarily by snails (Melampus coffeus). Degradation of leaves and roots in mesh bags was slow overall at restoration sites, however, particularly at WS where aerobic decomposition may have been more limited. These findings indicate that local or regional factors such as salinity regime act together with site history to control primary production and turnover rates of organic matter in restoration sites. Species differences in senescent leaf nitrogen content and degradation rates further suggest that restoration sites dominated by Laguncularia racemosa and Rhizophora mangle should exhibit slower recycling of nutrients compared with natural basin forests where Avicennia germinans is more abundant. Structural development and biogeochemical functioning of restored mangrove forests thus 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 restoration projects.

Restoration Ecology↗

USGS2000

Mangrove peat analysis and reconstruction of vegetation history at the Pelican Cays, Belize

The substrate beneath mangrove forests in the Pelican Cays complex is predominately peat composed mainly of mangrove roots. Leaves and wood account for less than 20% of the peat mass. At Cat Cay, the depth of the peat ranges from 0.2 m along the shoreline to 1.65 m in the island center, indicating that the island has expanded horizontally as well as vertically through below-ground, biogenic processes. Mangrove roots thus play a critical role in the soil formation, vertical accretion, and stability of these mangrove cays. The species composition of fossil roots changes markedly with depth: Rhizophora mangle (red mangrove) was the initial colonizer on a coral base, followed by Avicennia germinans (black mangrove), which increased in abundance and expanded radially from the center of the island. The center of the Avicennia stand ultimately died, leaving an unvegetated, shallow pond. The peat thus retains a record of mangrove development, succession, and deterioration in response to sea-level change and concomitant hydroedaphic conditions controlling dispersal, establishment, growth, and mortality of mangroves on oceanic islands in Belize.

Pelican Cays↗