A gap dynamic model of mangrove forest development along gradients of soil salinity and nutrient resources

1 A gap dynamic model (FORMAN) was developed as a first synthesis of available data for three Caribbean mangrove species into an individual-based model that simulates the long-term dynamics of mangrove forest development. 2 Field observations at three sites along the Shark River estuary were compared with simulation results, assuming development following Hurricane Donna in 1960. Total basal area simulated for each site was within +/-10% of that observed, although species-specific basal area predictions were less accurate. 3 A decrease in nutrient availability from marine to mesohaline sites modelled the reduced basal area of Avicennia germinans and Laguncularia racemosa. However, at the lower nutrient site a 83% reduction in maximum sapling recruitment of both A. germinans and L. racemosa was required to fit field results. Increased basal area of A. germinans and L. racemosa limited the development of Rhizophora mangle at higher nutrient downstream sites, apparently 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 sites. 4 Succession was projected for the next 500 years at a site in the lower estuary. Longterm forest dynamics were sensitive to species-specific maximum sapling recruitment rates. In the absence of large-scale disturbance, initial dominance by L. racemosa was predicted to be replaced 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 resource and salinity illustrated their relative competitive balance over time (up to 300 years). Laguncularia racemosa dominated in fertile soils with low salinity at early stages of recovery, but its abundance decreased over time while A. germinans increased. The dominance of R. mangle was limited to regions with low nutrient availability and low salinity. Avicennia germinans dominated at higher salinities, where the effect of nutrient availability was overwhelmed by the tolerance of individual species to salt stress.