How to catch the patch? A dendrometer study of the radial increment through successive cambia in the mangrove Avicennia

摘要

Successive vascular cambia are involved in the secondary growth of at least 200 woody species from >30 plant families. In the mangrove these successive cambia are organized in patches, creating stems with non-concentric xylem tissue surrounded by internal phloem tissue. Little is known about radial growth and tree stem dynamics in trees with this type of anatomy. This study aims to (1) clarify the process of secondary growth of trees by studying its patchiness; and (2) study the radial increment of stems, both temporary and permanent, in relation to local climatic and environmental conditions. A test is made of the hypothesis that patchy radial growth and stem dynamics enable trees to better survive conditions of extreme physiological drought. Stem variations were monitored by automatic point dendrometers at four different positions around and along the stem of two trees in the mangrove forest of Gazi Bay (Kenya) during 1 year. Patchiness was found in the radial growth and shrinkage and swelling patterns of stems. It was, however, potentially rather than systematically present, i.e. stems reacted either concentrically or patchily to environment triggers, and it was fresh water availability and not tidal inundation that affected radial increment. It is concluded that the ability to develop successive cambia in a patchy way enables trees to adapt to changes in the prevailing environmental conditions, enhancing its survival in the highly dynamic mangrove environment. Limited water could be used in a more directive way, investing all the attainable resources in only some locations of the tree stem so that at least at these locations there is enough water to, for example, overcome vessel embolisms or create new cells. As these locations change with time, the overall functioning of the tree can be maintained.