Liquid-phase water permeation pathways on outer-bark surfaces of teak (Tectona grandis): a tropical deciduous hardwood

It has recently been suggested that processes related to water uptake from the tree-stem surface, such as water vapor exchange and rainwater permeation, have implications for forest hydrology. However, few studies have investigated stem surface water uptake itself, and the water permeation pathway has not been elucidated in detail. Based on previous findings that trees with outer bark composed mainly of rhytidome have a high frequency of exfoliation and greater water uptake ability, we hypothesized that exfoliation of the outer bark is the main pathway for water uptake from the outer bark surface to the innermost periderm in these species. We tested this hypothesis in teak, a tropical deciduous hardwood with a high frequency of outer-bark exfoliation. We conducted laboratory experiments using dried bark pieces with different outer-bark shapes to evaluate the rate, amount, and pathway of liquid-phase water permeation of outer-bark surfaces. The rate and amount of water absorption differed markedly among samples. The permeation rate was correlated with the degree of outer-bark exfoliation, and water permeation was observed to begin at exfoliated surfaces. These results support our hypothesis. In addition, the increased water content of bark pieces immediately after the start of the experiment was strongly correlated with the surface roughness of the outer bark, implying that roughness may indicate the water-retention capacity of a given tree species.