Rhizophora zonation, salinity, and nutrients in the western Atlantic

Rhizophora is the dominant genus of mangrove forests on the Atlantic coast of northern South America. What determines the zonation frequently observed in sympatric populations of the two neotropical species R. mangle, R. racemosa, and their hybrids, R. × harrisonii, is an open question. The most widely held hypothesis is that differences in salinity tolerance among the taxonomic groups explains the observed zonation. To address this question, we analyzed the elemental composition of soils and canopy leaves from 60 Rhizophora spp. trees distributed in different intertidal zones of an estuarine site of the Paria Gulf, Venezuela. The low intertidal zone showed lower salinity, organic matter, C, N, S, and Na, and higher bulk density, Al, Fe, and Mn concentrations compared with the higher intertidal zones. Using morphological characters and microsatellite molecular markers, we identified 39 pure R. mangle, 19 hybrids, and only two pure R. racemosa. We found that both intertidal position and taxonomic groups explained most of the differences in leaf variables measured across trees. The ratio Mg/Ca, however, was higher in R. mangle than in R. racemosa and hybrids regardless of intertidal position. Moreover, at some specific intertidal position, R. mangle differed from R. racemosa and hybrids in the values of C, N, K, Mg, Fe, Mn, C/N, K/Ca, S/Ca, and d13C. We conclude that despite the scarcity of R. racemosa and the absence of a clear species zonation, our results suggest that R. mangle copes with salinity differently than R. racemosa and R. × harrisonii.