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.红树属（Rhizophora）是南美洲北部大西洋沿岸红树林的优势属。针对该区域两种新热带物种角果木（R. mangle）、红海榄（R. racemosa）及其杂交种哈里逊红树（R. × harrisonii）的同域种群频繁出现的分带格局，其驱动机制仍是一个悬而未决的科学问题。当前学界主流假说认为，不同分类类群的盐度耐受性差异可解释观测到的分带现象。为解答这一科学问题，我们对委内瑞拉帕里亚湾某河口区域不同潮间带的60株红树属植物的土壤及冠层叶片元素组成展开分析。相较于高潮间带区域，低潮间带的盐度、有机质、碳（C）、氮（N）、硫（S）及钠（Na）含量更低，而容重、铝（Al）、铁（Fe）及锰（Mn）浓度更高。我们通过形态特征与微卫星分子标记，共鉴定出39株纯合角果木、19株杂交种，以及仅2株纯合红海榄。研究发现，潮间带位置与分类类群均可解释植株间绝大多数叶片测量变量的差异。不过，无论所处潮间带位置如何，角果木的镁钙比（Mg/Ca）均高于红海榄与杂交种。此外，在部分特定潮间带位置中，角果木在碳（C）、氮（N）、钾（K）、镁（Mg）、铁（Fe）、锰（Mn）、碳氮比（C/N）、钾钙比（K/Ca）、硫钙比（S/Ca）及碳同位素比值（δ¹³C）等指标上与红海榄及杂交种存在显著差异。综上，尽管红海榄样本量稀少且未观测到明确的物种分带格局，我们的研究结果仍表明，角果木与红海榄及哈里逊红树应对盐度胁迫的策略存在差异。