Successive cambia: useful to survive in water stressed environments?

The mangrove Avicennia, the only mangrove with successive cambia, has the broadest distribution of all mangrove genera. The distribution of Avicennia at local scale also shows its ecological flexibility: trees can grow more landward and at places with more stressful environmental conditions if compared to other mangroves. We discovered that the reticulate internal phloem and xylem structure of Avicennia, generated from a network of successive cambia, is more complex (i.e. multiple branched) with higher soil water salinity levels. The higher complexity is concomitant with a higher amount of internal phloem tissue. This indicates a functional advantage from the internal phloem tissue in harsh environmental conditions. As internal phloem is proposed (i) to be responsible for a higher water storage potential and (ii) to play a role in the safety of the water transport system through embolism repair, we would expect species with successive cambia and internal phloem tissue to have a functional advantage. This allows them to grow in water stressed environments as we could observe for the mangrove Avicennia. In this study we compiled data for about 200 woody species with concentric internal phloem tissue in order to test the hypothesis that species with successive cambia and internal phloem mainly occur in water stressed environments. We can observe that most of the trees and shrubs with concentric internal phloem are growing in dry or saline habitats. Vines and lianas with this wood anatomical characteristic are far more distributed in moist and wet habitats. We can therefore conclude that growth via successive cambia is a beneficial ecological strategy under two contrasting environmental conditions: under humid-wet conditions it offers ecological advantage for woody plants with scandent growth, while under drought and high salt conditions it can be beneficial for trees and shrubs. Successive cambia thus seem to be an ecologically important wood anatomical characteristic explaining species distribution at least partially, not only for the mangrove genus Avicennia, but for many phylogenetically distant genera.