A safe hydraulic architecture as wood anatomical explanation for the difference in distribution of the mangroves Avicennia and Rhizophora

P>Wood anatomical characteristics are an important source of information about how trees are coping with the conflicting requirements of optimal hydraulic conductivity and safety. In this study we compared the vessel characteristics of Avicennia marina and Rhizophora mucronata to contribute to a better understanding of the difference in distribution of these two mangrove species. Density, grouping and diameters of vessels together with vessel element length of A. marina trees growing on seven different study sites in Gazi Bay (Kenya) were measured and compared with the vessel characteristics of R. mucronata of the same research area. Furthermore, the relation of environmental factors related to the water relations of mangrove trees to the wood anatomy of both species was tested. Vessel density of both species and vessel grouping of A. marina are higher with higher salinity. In addition, A. marina was shown to have a higher vessel density, a higher vessel grouping, smaller vessel diameters and shorter vessel element lengths as compared to R. mucronata. As the vessel characteristics of A. marina are related to protection against cavitation or the effects of cavitation, we can infer that this species has a water transport system which can be considered safer under water limiting conditions than that of R. mucronata. The contrast in the safety of the water transport system between the two mangrove species studied is reflected in their distribution at local scale, over the land-sea ecotone, and it may also explain the wide latitudinal distribution of A. marina as compared to R mucronata. This conclusion emphasizes the importance of the anatomy of the water transport system for survival under diverse environmental conditions. In general it is shown that dominant species of a single ecosystem can deal with the stressful conditions they live in by contrasting ecological strategies.