Ideas and perspectives: Tree–atmosphere interaction responds to water-related stem variations

Land–atmosphere interactions depend on momentum transfer from the atmosphere to the canopy, which in turn depends on the tree drag coefficient. It is known that the drag coefficient, and thus tree–atmosphere momentum transfer, can vary strongly within a canopy. Yet, only few measurements are available to study the variation of tree–atmosphere momentum transfer in time and space, and in response to tree water deficit. In this paper we use accelerometers to estimate tree–atmosphere momentum transfer for 19 individual trees of 7 different species in the Brazilian Amazon. The 5-month monitoring period included the transition from wet to dry months. Here, we demonstrate that, under field conditions, tree–atmosphere momentum transfer can vary considerably in time and space (up to a factor of 2.5). Increased water-related stem variations during the dry months are related to observed changes in tree–atmosphere momentum transfer, which is hypothesized to be caused by tree-water-deficit-induced changes in tree mass.