Wind Speed Response of Sap Flow in Five Subtropical Trees Based on Wind Tunnel Experiments

Aims: We evaluated the responses of tree sap flow to wind speeds in coniferous and broad-leaved plants under steady and unsteady wind conditions. Study Design: We performed sap flow and micro-meteorological measurements on two conifers, Chamaecyparis obtusa var. formosana and Araucaria cunninghamii, and three broadleaved species, Swietenia mahagoni, Michelia formosana and Plumeria acutifolia in a wind tunnel. Place and Duration of Study: Civil Engineering Department, National Central University, Zhongli, Taiwan, China, between May and July 2011. Methodology: In a wind tunnel, wind speed was increased incrementally from 0 to 2, 4, 6, and 8 m s–1, then decreased from 8 to 0 m s–1. To examine how sap flow responded to unsteady wind, we estimated the time constant of each individual for each step change in wind speed. Moreover, we examined differences in transpiration rate and leaf conductance among individuals under stable wind speeds. Results: The time constant was generally about 30 min in both conifers and broadleaved individuals. Interestingly, under steady winds, transpiration rate showed two different response patterns to increased wind speed: linear and saturated. The two patterns may be a consequence of different stomatal conductance values, but not of different leaf shapes. Conclusion: Our results suggested that neither hydraulic system nor leaf shape differences between coniferous and broad-leaved trees was an ultimate factor affecting the transient response of tree sap flux and transpiration to wind speed, and that stomatal conductance played an important role in transpiration regulation in response to wind speed