Species difference of transpiration in three urban coniferous forests in a semiarid region of China

Abstract

Tree transpiration and associated evaporative cooling play a critical role in water cycles and combating ‘Urban Heat Islands’ effects. However, species differences in transpiration magnitude, seasonality, and biophysical regulation in dryland urban regions are not well understood. We measured the sap flow of three widely planted coniferous species, Pinus tabulaeformis, Pinus sylvestris, and Picea meyeri, for urban greening, and concurrent soil moisture and meteorological variables in three pure urban forest plantations during three successive growing seasons (May to September during 2017–2019) under a semiarid urban environment in northern China. Stand-level transpiration (Ec) differed significantly among the three species (p < 0.001), where the mean daily Ec of P. tabulaeformis, P. sylvestris, and P. meyeri plantations were 0.78 ± 0.02 mm d–1, 0.68 ± 0.01 mm d–1, and 0.20 ± 0.01 mm d–1, respectively. Additionally, distinct differences in the seasonality of Ec were observed among tree species. Except for 2017, the Ec of P. tabulaeformis and P. sylvestris maintained at low levels in May and June and increased significantly with precipitation in July and August (p < 0.05) when soil volumetric water content (VWC) increased, but the Ec of P. meyeri remained relatively stable throughout the growing season. Responses of Ec in P. tabulaeformis and P. sylvestris plantations to meteorological factors can be changed by soil water availability. Under relatively higher water availability, the Ec of P. tabulaeformis and P. sylvestris were primarily controlled by vapor pressure deficit (VPD) and solar radiation (Rs). Under soil water stress, the Ec of P. tabulaeformis and P. sylvestris were more affected by VWC than P. meyeri. Biologically, P. tabulaeformis had a higher stomatal sensitivity to VPD than P. sylvestris and P. meyeri. At the beginning of growing seasons, the high water use in P. tabulaeformis and P. sylvestris plantations could lead to soil water depletion. Therefore, the species-specific biophysical regulations of tree water use should be carefully considered for urban forest management to achieve ecosystem services by balancing the tradeoff between water consumption and its cooling effects (through evapotranspiration) in semiarid cities.