Effects of varying soil and atmospheric water deficit on water use characteristics of tropical street tree species

Urban trees provide several ecosystem services, yet they are less studied compared to trees in natural ecosystems. Investigating ecophysiological responses of different tree species to seasonal conditions and drought will help determine which would succeed in urban conditions. Here, we examined water use characteristics of common species in Bangkok, Thailand: Pterocarpus indicus (Pi), Swietenia macrophylla (Sm) and Lagerstroemia speciosa (Ls), with different phenology, under seasonal variations and soil drying. Thirty small trees were potted and irrigated to ≥80% of the field capacity (θFC) of the soil. Granier-type sensors were used to measure sap flux density from 23 August to 18 December 2017. Drought treatments were imposed on five trees of each species by withholding irrigation until θ reached ∼50% θFC. Results suggested that water use patterns of semi-evergreen and evergreen species (Pi and Sm) were not sensitive to either seasonal or soil moisture variations while deciduous species (Ls) exhibited decreased water use and earlier stomatal closure upon soil drying in the dry season. These findings suggested that water use characteristics of the evergreen species may conserve water use regardless of atmospheric and soil moisture conditions while those of the deciduous species may result in high cost for irrigation in the wet season. Nevertheless, we believe that both evergreen and deciduous species may be selected for planting to maximize greening areas in cities throughout the year. However, knowledge of different water use characteristics of street tree species should be applied to devise strategic planning for optimized irrigation in urban greening.