A GAP DYNAMIC-MODEL OF MANGROVE FOREST DEVELOPMENT ALONG GRADIENTS OFSOIL-SALINITY AND NUTRIENT RESOURCES

1 A gap dynamic model (FORMAN) was developed as a first synthesis of a vailable data for three Caribbean mangrove species into an individual- based model that simulates the long-term dynamics of mangrove forest d evelopment. 2 Field observations at three sites along the Shark River estuary were compared with simulation results, assuming development fo llowing Hurricane Donna in 1960. Total basal area simulated for each s ite was within +/-10% of that observed, although species-specific basa l area predictions were less accurate. 3 A decrease in nutrient availa bility from marine to mesohaline sites modelled the reduced basal area of Avicennia germinans and Laguncularia racemosa. However, at the low er nutrient site a 83% reduction in maximum sapling recruitment of bot h A. germinans and L. racemosa was required to fit field results. Incr eased basal area of A. germinans and L. racemosa limited the developme nt of Rhizophora mangle at higher nutrient downstream sites, apparentl y due to competition for light resources. Both observed and simulated results indicated that R. Mangle had higher frequencies in the smaller size classes at all three sites, compared to a bell-shaped size-class distribution of L. racemosa, particularly at the higher nutrient site s. 4 Succession was projected for the next 500 years at a site in the lower estuary. Longterm forest dynamics were sensitive to species-spec ific maximum sapling recruitment rates. In the absence of large-scale disturbance, initial dominance by L. racemosa was predicted to be repl aced eventually by A. germinans, even when maximum sapling recruitment rate of A. germinans was set at half of L. racemosa and R. mangle. 5 Response curves for each species along gradients of soil nutrient reso urce and salinity illustrated their relative competitive balance over time (up to 300 years). Laguncularia racemosa dominated in fertile soi ls with low salinity at early stages of recovery, but its abundance de creased over time while A. germinans increased. The dominance of R. ma ngle was limited to regions with low nutrient availability and low sal inity. Avicennia germinans dominated at higher salinities, where the e ffect of nutrient availability was overwhelmed by the tolerance of ind ividual species to salt stress.