Functional Traits of Terrestrial Plants in the Intertidal: A Review on Mangrove Trees

True mangroves are vascular plants (Tracheophyta) that evolved into inhabiting the mid and upper intertidal zone of tropical and subtropical soft-sediment coasts around the world. While several dozens of species are known from the Indo-West Pacific region, the Atlantic-East Pacific region is home to only a mere dozen of true mangrove species, most of which are rare. Mangrove trees can form dense monospecific or multispecies stands that provide numerous ecosystem services. Despite their eminent socioecological and socioeconomic relevance and the plethora of studies on mangroves, many details of the ecology of mangrove ecosystems remain unknown; and our knowledge about general ecological principles in mangrove ecosystems is scarce. For instance, the functional trait concept has hardly been applied to mangroves. Here we provide an inventory of 28 quantitative and 8 qualitative functional traits of true mangrove species and stipulate some insight into how these traits may drive ecosystem structure and processes. The differentiation between true mangroves and mangrove associates, which can dwell inside as well as outside mangrove forests, is reflected by a number of leaf traits. Thus, true mangroves exhibit lower specific leaf area, lower leaf N content, and lower K:Na ratio, and higher leaf succulence, higher Na and Cl content, and higher osmolality than mangrove associates. True mangrove species that form pure stands produce larger leaves and exhibit higher N content per leaf area, higher leaf K and Ca content, greater maximum plant height, longer propagules, and lower root porosity than more sporadic species. The species-specific expression of most traits does not reflect the species' position along intertidal gradients, suggesting that adaptation to tidal inundation does not explain these traits. Rather, many of the traits studied herein exhibit strong phylogenetic signals in true mangroves. Thus, wood density is high in most species of the Rhizophoraceae, irrespective of their habitat or maximum height. On the other hand, species of the genus Sonneratia exhibit low wood density and do not grow taller than 20 m. Some leaf traits of true mangroves are more like those of plants from drier environments, reflecting the perception that a saline environment creates physiological drought stress. Along the same line, most true mangrove species exhibit sclerophyllous leaf traits. The few major mangrove tree species of the Atlantic-East Pacific are as distinct from each other, with regard to some traits, as are the many mangrove species of the Indo-West Pacific. We hypothesize that this phenomenon explains the similarly high biomass of mangrove forests in both the species-rich IndoWest Pacific and the species-poor Atlantic-East Pacific.