Stand structure and productivity of the introduced Rhizophora mangle in Hawaii

Since its introduction in the early part of this century, Rhizophora mangle L. has spread extensively through most of the main islands of the Hawaiian Archipelago. We investigated the structural properties and estimated productivity of a R. mangle population at Nuupia Ponds Wildlife Management Area (NPWMA), on windward Oahu, where the mangroves were being controlled due to their propensity to overgrow archaeological sites and the habitat of endangered Hawaiian waterbirds. Mangroves within NPWMA were very dense (> 24,000 trees ha^-1) and most were relatively small (only 3.3% of the trees were ≥ 10 cm DBH). Mean basal area, aboveground biomass, and number of seedlings were all high, at 37.2 m² ha^-1, 279 t (dry wt) ha^-1, and 121 m^-2, respectively. The seedling density may be particularly unusual and appears to be due to extremely high rates of propagule production coupled with low rates of propagule predation. Stand productivity was estimated by stem growth (allometry), litterfall, and a light attenuation approach to determining net canopy photosynthetic production. All three methods yielded estimates that are higher than previously reported for R. mangle and comparable with estimates of highly productive Rhizophora spp.-dominated stands in Australia and Asia. The high density, biomass, and productivity of this stand relative to stands within the species' native range may be due to a combination of favorable site conditions, lack of competition from other woody plants, and very low rates of herbivory and propagule predation.