Ryuji Ichihashi, Chen-Wey Chiu, Hikaru Komatsu, Tomonori Kume, Yoshinori Shinohara, Makiko Tateishi, Kenji Tsuruta, Kyoichi Otsuki.

Plants absorb water from the ground and release it into the air from the leaves (transpiration) in exchange for CO2 uptake. The amount of water transpired from the vegetation is an important factor of the water cycle, affecting, for example, local climate and the water resources available to neighbouring societies.

Lianas (woody vines) are a peculiar yet important component of forests. They climb neighbouring trees to attain higher positions, and display their leaves in the forest canopy, yet their basal stems remain quite thin. Lianas have a much greater amount of leaves relative to their basal stem area (BA, through which water flows), compared with trees. The top-heavy architecture implies that lianas possibly consume more water through transpiration than expected from their BA. Here, we evaluated the contribution of lianas to community-level canopy transpiration in a warm temperate forest stand, and discussed the potential importance of lianas to forest water dynamics.

We measured the rates of water ascent in the stems (sap flux) for 1 year for four species each of lianas and trees using a thermal dissipation method (based on empirical relationships between the extent of heat dissipation from the sensor and the water-flow rates surrounding the sensor). Lianas showed an average of 2–4 times greater sap flux per unit stem area than trees throughout the year. By extrapolating the sap flux data to the inventory data of the study plot, we estimated that lianas contributed 12.8% to the annual stand transpiration while comprising only 2.3% of stand BA.

Our results indicate that the contribution of lianas to forest water dynamics may be several times greater than their contribution to forest BA. This implies that a slight increase of liana abundance might have considerable effects on water dynamics and, through competition with trees for limited water, the capacity of forests to store carbon. The enlargement of liana communities has already been observed across neotropical forests. This study underlines the necessity of evaluating the relative importance of lianas to forest water dynamics in forests worldwide.

Image caption: A liana in the forest canopy. Photo by Ryuji Ichihashi.
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