Applied Environmental Research

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Studying tree water relations in response to soil drought improves our understanding of hydrological processes that are central to most ecosystem modelling. Urban greening is an approach to ameliorate rising carbon dioxide emissions whose effects are intensified in cities. Thus, an investigation of water relations with respect to drought of urban trees is of interest for effective watermanagement in urbangreening. This study examined responses of sap flux density, water use and stomatal conductance of potted saplings of three common street trees in Bangkok: Pterocarpus indicus (Pi), Swietenia macrophylla (Sm) and Lagerstroemia speciosa (Ls); under one-week non-irrigated treatment. Results showed that Piand Smmaintained their water use rate while Lsexperienced reduction in water use under soil drying condition. Further analyses on stomatal conductance and leaf area suggested that leaf drop, which may be induced by soil drought, resulted in increases of stomatal conductance. The counteracting effect of increased stomatal conductance and decreased leaf area seems to explain the unaffected water use in Piand Smunder drought conditions. However, a smaller compensatory effect was observed in Ls, resulting in significant reduction of water use in the non-irrigated saplings. Thus, these results indicate that Piand Smmay be preferred for street tree planting since they would be less affected by drought stress than Ls. Nevertheless, further investigations, including detailed measurements of tree hydraulic variables and longer experimental duration, are advised to confirm these findings for extrapolation to a larger scale.

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