Transpiration in response to wind speed: can apparent leaf-type differences between conifer and broadleaf trees be a practical indicator?

Key message: Transpiration in conifer species was controlled by wind speed, similar to broadleaf species, indicating that caution is required when using the simplified Penman–Monteith equation. The significance of aerodynamic controls on tree transpiration (E) is critical when modeling E using the simplified Penman–Monteith equation. However, few studies have explicitly compared the significance of aerodynamic controls on E between conifer-type needles and broadleaf leaves. Although previous studies assumed that the response of E to wind speed (w) could be insignificant in conifers, we hypothesized based on the evapotranspiration theory that the significance of E regulation by w is difficult to determine based on apparent such leaf-type differences. We performed sap flux measurements using a wind tunnel on two conifer species, Chamaecyparis obtusa var. formosana and Araucaria cunninghamii, and three broadleaf species, Swietenia mahagoni, Michelia compressa, and Plumeria rubra. Chamaecyparis obtusa and P. rubra showed no distinctive E response to w, while A. cunninghamii, S. mahagoni, and M. compressa had significant increases in E due to w, supporting our hypothesis. Our experimental and model calculation data suggested that the aerodynamic controls on E by w probably result from differences in stomatal conductance rather than leaf type. We emphasize the need to confirm the significance of aerodynamic controls on E using both direct and indirect measurements for coupling conditions between the leaf surface and atmosphere when modeling E, irrespective of leaf type.