Environmental effects of canopy gap formation in high-rainfall mangrove forests

This study investigated the importance of gap formation in mangrove swamps on the island of Kosrae, Federated States of Micronesia, in order to unders tand better both natural processes of forest development and the effects of harvesting trees for firewood in these wetlands. Measurements were concent rated in seven plots located near four rivers: three in fringe zones and fo ur in basin zones. Each plot was a cluster of five points and covered an ar ea of ca 1.3 ha. From every point in each of the seven plots, the nearest c anopy gap greater than or equal to 10 m(2) was located; 25 of the 35 gaps w ere formed by harvesting. Porewater salinity was significantly higher under the canopy in fringe mangrove forests than in basin mangrove forests. Alth ough gaps were small (mean gap size = 158 m(2); median gap size = 92 m(2)), soil temperatures were significantly higher in gaps of both zones. Soil re dox potential was significantly lower and porewater salinity significantly higher in the gaps than under the canopy in the basin zone only. Higher por ewater salinity may be attributed to high evaporation rates from the soil a nd high transpiration rates from trees surrounding gaps. There were signifi cantly more seedlings in gaps than under the canopy only in the fringe zone s. Although gap formation alters the soil environment of Kosraean mangrove swamps, high freshwater input may buffer these effects in basin mangrove sw amps by reducing porewater salinity. Current harvesting rates do not appear to be changing canopy species composition, but large gaps, especially in m angrove forests in more arid areas, may lead to major changes.