Land pavement depresses photosynthesis in urban trees especially under drought stress

Investigations into the photosynthetic response of urban trees on paved land under drought stress would help to improve the management of trees under rapid urbanization and climate change. An experiment was designed to grow two common greening tree saplings, ash (Fraxinus chinensis Roxb.) and ginkgo (Ginkgo biloba L.), in environments of both land pavement and drought. The results showed that (1) land pavement increased surface and air temperatures and decreased air humidity as well as net photosynthetic rate (P_n) and photosynthetic capacity (A_max) of ginkgo significantly; (2) drought significantly decreased P_n, A_max and maximum net photosynthetic rate (P_nmax) as well as other photosynthetic parameters of both ash and ginkgo; (3) the negative effects of the combination of land pavement and drought on photosynthetic parameters were more significant than the effects of drought treatment for both ash and ginkgo. This implies that urban trees, especially those growing on land pavements, will confront harsher environments and a greater decline of photosynthesis under the severe and more frequent droughts predicted in the future. Overall, ash showed more tolerance to land pavement and drought than ginkgo, indicating that the selection of tolerant tree species is important for urban planting.