Effect of pavement and water deficit on biomass allocation and whole-tree transpiration in two contrasting urban tree species

Aiming to explore the species-specific responses of biomass allocation and whole-tree transpiration in urban trees to pavement and drought, a field manipulation experiment grew saplings of two common greening trees, ash (Fraxinus chinensis Roxb.) and ginkgo (Ginkgo biloba L.). Pavement slightly increased leaf area and leaf biomass in ginkgo under sufficient water supply. When water deficit occurs, the height, basal diameter, and biomass in ash reduced and more biomass was allocated to roots, pavement aggravated the reduction of stem biomass in ash. Pavement slightly increased soil evaporation and total evapotranspiration, irrespective of whether the water supply was sufficient or not. However, whole-tree transpiration and stem sap flow increased under pavement when the water supply was sufficient for ginkgo but not so for ash. The down-regulation of water use generated by an insufficient water supply was aggravated and retarded by pavement for ginkgo and ash, respectively. Ash grows faster and can maintain its transpiration better under pavement and drought stress, making it the preferred tree for planting on paved land in urban areas.