Ariel E. Lugo

Mangroves of Arid Environments in Puerto Rico and Adjacent Islands

The structure of mangrove vegetation, soil salinity, and topographic relief of the mangrove ecosystem were studied along the south coast of Puerto Rico, Culebra, and Mona Island. All systems, with the exception of the forest at Mona Island, were characterized by having a coastal fringe of live vegetation (usually dominated by the red mangrove), a zone of dead trees, and a hypersaline lagoon or dry salt flat on the landward side. Mean soil salinities were 44 0/00 for the zone of live trees, 72 0/00 for the zone of dead trees, and 87 0/00 for the salt flats. The Mona Island forest grew over a sandy hardpan that prevented mixing of sea water and the fresh water. Soil salinities were low in this forest, and trees reached a height of 15 m. Tree height was inversely proportional to soil salinity (r = 0.72) between 17 and 72 0/00. When soil salinities exceeded 65 0/00, dead tree basal area was higher than live tree basal area. It is suggested that mangrove growth is limited by soil salinities higher than 90 0/00. Mangrove succession in offshore overwash islands and arid coastlines is described. It is proposed that cyclic rainfall patterns and hurricanes act as regulators of speed and direction of succession. Rainy periods are associated with lower soil salinities and expansion of the red mangrove zone. Droughty periods result in high soil salinities, mangrove mortality, and expansion of salt flats. Hurricanes set back succession and reverse successional trends that reduce mangrove areas. A model of mangrove ecosystem function and implications for management are also discussed.