Biotic and abiotic factors constraining the distribution and abundance of the rare mangrove Pelliciera rhizophorae in Panama

Pelliciera rhizophorae is a Neotropical mangrove species with a narrow geographic distribution. Despite having a wide fossil distribution around the Caribbean Sea, extant populations of P. rhizophorae are rare on the Caribbean coast of Central and South America. The current extent of the distribution of P. rhizophorae on the Caribbean coast of Panama was investigated. Populations are small and isolated, with seven populations occurring on freshwater streams in Bocas del Toro and Colon provinces. A shadehouse experiment was conducted to test the shade and salinity tolerances of P. rhizophorae compared to three widespread mangrove species: Avicennia germinans, Rhizophora mangle, and Lumnitzera racemosa. Pelliciera rhizophorae is more sensitive to soil salinity and high light conditions than other, more common mangroves but its salinity tolerance does not explain its narrow distribution. Obstacles to recruitment for P. rhizophorae were examined by quantifying propagule predation by crabs and seedling survival in different forest types. Crab predation of P. rhizophorae propagules was much higher in mangrove forests where P. rhizophorae does not occur then where it does occur. Seedlings were planted that were protected from predation by crabs, and seedling survival did not differ between forest types, suggesting that crab predation limits recruitment in areas dominated by other mangrove species. Nutrient dynamics and biomass allocation ratios were studied in P. rhizophorae from populations on the Pacific and Caribbean coasts of Panama. There are two growth forms that occur on each coast: a stunted "dwarf" form and a taller "fringe" form. In other species the stunted form is caused by nutrient limitation, but the nutrient status of P. rhizophorae had never been studied. Dwarf trees had lower nutrient content than fringe trees, and also allocated more biomass to stems than leaves relative to fringe trees. A combination of abiotic (e.g., shade availability) and biotic factors (e.g., propagule predation by crabs) are responsible for the narrow distribution of P. rhizophorae.!...