Dwarfing in Mangroves: A Review

Mangroves are dynamic ecosystems that occur in intertidal areas of tropical and temperate regions. These woody trees or shrubs are important because of their global extensive area coverage and high primary productivity. Mangroves that fringe coastlines are tall (> 4 m) and well-developed, while those occurring inland are dwarfed (< 1.5 m) and sparse. Dwarf mangroves are not limited to particular latitudes or arid environments but occur worldwide at high elevations in the intertidal zone where soils are hypersaline. Dwarfing in mangroves has been attributed to edaphic factors such as salinity, tidal frequency, surface hydrology, and nutrient limitation. This review gives an overview of the current information on dwarf mangroves and their ecophysiological characteristics. It compares soil and plant characteristics, soil salinity, plant water relations, nutrient availability, photosynthesis, and biological nitrogen fixation between fringe and dwarf mangroves. It investigates the nature and cause of dwarfing in mangroves and determines the ecophysiological mechanisms underlying this phenomenon. Overwhelming evidence from various sources indicates that dwarfing in mangroves is due to nitrogen limitation. Fringe trees have higher nitrogen concentrations than dwarf trees due to the regular tidal influence, higher organic matter, and well-developed aerial roots inhabited by diazotrophs, which are microorganisms that contribute significantly to the nitrogen budget by biological nitrogen fixation. This review concludes by making recommendations for the management and conservation of dwarf mangroves.