BARK AND FOLIAR WATER UPTAKE CAPACITY OF TROPICAL TREE SPECIES IN RWANDA

Abstract

Water is essential for tree function and survival, yet climate warming poses increasing threats to tree health, particularly in tropical regions where droughts are intensifying. While foliar water uptake (FWU) and bark water uptake (BWU), described as the direct uptake of water through leaf and bark surfaces, are increasingly recognized as important supplementary water sources, both processes remain understudied— especially in tropical forests. Here, we investigate species- and climate-related variation in foliar and bark water uptake capacity in 11 tropical tree species (8 for BWU) growing along an elevational gradient in Rwanda. All trees were subjected to a standardized wetting treatment, where stems segments and leaves were fully submerged in water for two hours under controlled conditions. Ten species demonstrated the capacity to absorb water through foliage and all eight species through bark, with substantial interspecific variation. In addition, we examined the influence of a range of physical traits on water uptake. The ability to retain water on surfaces—especially via trichomes on leaves and rough bark on stems—emerged as key factors influencing bark and foliar water uptake. This is the first study, to our knowledge, to assess bark and foliar water uptake in African trees as well as simultaneously on the same individual plants, offering novel insights into the role of external water uptake in tropical tree ecophysiology.