The heartbeat of mangrove seedlings: dendrometry and anatomy of on-tree developing and establishing Bruguiera gymnorrhiza seedlings.

Mangrove trees are remarkably well adapted to the tropical intertidal environment. Aerial roots and salt excluding mechanisms at the root and leaf level are one of the important mechanisms that help them surviving the highly dynamic mangrove environment. The formation of viviparous hydrochorous seedlings is another adaptation. All mangrove species belonging to the pantropical family Rhizophoraceae are viviparous. This means that their seeds
germinate without a dormancy period, while they are still attached to the parent tree. The resulting mangrove seedlings form the mangrove dispersal units or propagules. Like most plants, adult mangrove trees exhibit diurnal reversible stem diameter patterns, which are caused by diurnal fluctuations in internal water storage tissues. During on-tree development, viviparous Rhizophoraceae mangrove seedlings receive water, minerals and carbohydrates from the parent trees. Yet, little is known on the diameter dynamics of these developing conspicuous structures (reflecting fluctuations in internal water content), let alone on the same plants after abscission during establishment. We registered the diameter dynamics of on-tree developing and post-abscission establishing
Bruguiera gymnorrhiza (L.) Sav. seedlings at high spatial (micrometer) and temporal (minutes) resolution by use of point dendrometers, and simultaneously recorded the variation in environmental conditions. The developing and maturing seedlings elongated, but lost in diameter in a stepwise manner towards abscission. Complementary anatomical observations, based on the combination of several techniques, provided context for understanding the seedling diameter fluctuations and contribution of the seedling to its own carbon balance. Establishing seedlings started exhibiting diurnal diameter fluctuations, typical of adult trees, upon the opening of their first leaf pair.