Do Forest Gaps Influence the Population Structure and Species Composition of Mangrove Stands in Northern Australia?1

Abstract

ABSTRACTGap‐phase regeneration of tropical mangroves was studied to test if there were significant environmental, structural, and floristic differences between canopy gaps and adjacent forests with intact canopies. Twenty‐five canopy gaps in the low to mid intertidal zone were sampled at two study areas in northern Australia. Canopy gaps at the more equatorial study area were larger and had a higher surrounding canopy. Soil analyses showed that sediment texture exerts strong control on conductivity, pH, organic matter, total P, total N and total S. Some soil factors differed between gaps and intact forest, and PAR was significantly higher in gaps. Despite this, propagule and young seedling abundances did not differ between gaps and adjacent forests due to the dominance of viviparous species. Enhanced recruitment of seedlings to the sapling stage was detected in many canopy gaps, although saplings were also present under mangrove forest canopies. Nevertheless, the overall floristic composition, species richness, and species rank abundance were similar between canopy gaps and forests with intact canopies. There appears to be little evidence for specialized gap colonists in tropical mangrove forest as distinct from other tropical forests. Instead, gap‐phase regeneration of mangroves appears to follow a simple “direct replacement” model in which a species population is replaced by members of the same species in forest gaps. The scope for gap‐dependent specialization in mangroves via seed dormancy and fast growth rates, common in rain forest pioneers, may be constrained by the intertidal environment in which mangroves occur.