The safer, the better? A safe hydraulic architecture as wood anatomical explanation for the difference in distribution of the mangroves Avicennia and Rhizophora.

Although mangroves have been extensively studied, little is known about their functional wood anatomy so far. The aim of this study is to measure and to make an ecological interpretation of different wood characteristics (vessel density, vessel grouping and vessel diameters) of Avicennia marina (FORSSK.) VIERH. (Avicenniaceae), a species with a broad local distribution and a representative of the genus with the largest latitudinal range. Relationship between these wood characteristics, measured in 35 trees from seven different sites in Gazi Bay on the east coast of Kenya, and two environmental factors (salinity and inundation frequency) as well as different tree characteristics (Leaf Area Index, tree height and tree diameter) were investigated. A comparison with Rhizophora mucronata LAMK. (Rhizophoraceae), another mangrove species with a different ecology and habit but also growing in Gazi Bay, was made.
Results have shown different ways in which the hydraulic system of A. marina seems to be adapted to local environmental conditions. Higher vessel density was observed with higher soil water salinity and lower inundation frequency. Higher soil water salinity was also related to higher vessel grouping. More stressful environmental conditions were related to smaller vessel diameters as confirmed by the observation of bigger embolised and smaller functional vessels. Mean vessel density was higher in A. marina compared to R. mucronata. Furthermore A. marina had smaller vessel diameters and vessels of mainly horizontally elliptic, almost round shape.
We can conclude that the plasticity in vessel characteristics of A. marina towards environmental factors at least contributes to the capacity of A. marina to grow in sites with of diverse environmental conditions. It seems that this species is better adapted than other mangrove species to a large range of stressful environmental conditions by a safe hydraulic system (high mean vessel density and small, almost round vessels). This is a possible explanation of the ecologically broad local distribution and the wide latitudinal range of the species.