Foliar water uptake changes the world of tree hydraulics

Climate change-induced drought events have become more prevalent during recent years, resulting in an increasing number of high-impact studies focussing on this important topic. In their recent review, Choat et al. provide a thorough assessment of the triggers of tree mortality under drought, but foliar water uptake is only mentioned as a side-note important for arid environments rather than a generic mechanism of general importance. Even worse, most papers focussing on tree hydraulics during drought ignore this mechanism entirely. However, the positive effects of foliar water uptake on tree hydraulics during drought may be substantial and this for a variety of ecological communities ranging from tropical montane cloud forests to mangrove and dryland ecosystems. Not only may foliar water uptake rehydrate leaves in all environments where leaf wetting events occur, also stems and roots could rehydrate by the redistribution of water absorbed by the leaves. It has been shown that this redistributed water can release tension on the water column, enable turgor-driven growth, may lead to embolism repair and could delay the increased probability of reaching the critical or lethal water potential threshold leading to tree mortality. However, direct evidence for the latter is still scarce. If occurring, tree survival might be substantially prolonged by tissue rehydration and protection of root structures during fog, dew or rainfall events that do not substantially wet the soil. As such, foliar water uptake is likely essential for the growth and survival of seedlings and trees with limited access to soil water, such as trees growing in forests with steep slopes, saline conditions and shallow rooting species.