Trees are an important component of urban green infrastructure that can provide a wide range of ecosystem services, including mitigating heat island effects, alleviating urban waterlogging, improving air quality, reducing noise pollution, increasing carbon sequestration, and enhancing the aesthetics of the landscape. However, the expanding urban land pavement significantly alters the micro environment for trees, thereby the growth, carbon metabolism, water use, and eventually ecosystem services of trees are affected. Global warming is expected to aggravate the urban heat island effect and exert more serious environmental stress on urban trees. Studying the combined effects of pavement and other abiotic stresses (such as drought) on tree eco physiology can help understand and foresee the response of urban trees under increasing environmental stress in the future, and thus promote the healthy growth of urban trees better and continuously provide their ecological benefits. However, related studies are scanty at present.
In this study, a potted field simulation experiment was carried out for two years. Two physiologically contrasting urban greening tree species, ash (Fraxinus chinensis) and ginkgo (Ginkgo biloba), which are extensively cultivated in Beijing, were planted and four treatments (control, pavement, drought, and pavement+drought) for each species were arranged. The physiological and biochemical characteristics, photosynthesis, leaf transpiration, water consumption, growth rate, and biomass accumulation and allocation of ash and ginkgo under pavement and drought were comprehensively studied. Furthermore, some suggestions on the species selection and management for greening trees on the pavement were put forward. Results showed that:
(1) The air temperature and surface temperature in pavement were, respectively, 0.11 °C and 5.35 °C higher than those in non paved land, because pavement has higher heat absorption and storage capacity, higher long wave radiation and sensible heat ratio. Pavement significantly reduced air humidity by 1.5% because they block soil evaporation and lack of plant transpiration. The difference in temperature and humidity between the two kinds of land was more obvious at nighttime. Pavement significantly reduced soil moisture, because of lower surface water permeability and weak water hold capacity of compacted soil underneath. Pavement aggravated the increase of soil temperature and the shortage of soil moisture brought by drought. Therefore, the heat stress and drought stress on plants will be strengthened in pavement when drought occurs.
(2) The single leaf area, length, and width decreased under insufficient water supply regardless of pavement for ash but decreased only when combined with pavement for ginkgo. When water supply is insufficient pavement decreased relative water content and nitrogen content and increased C/N ratio, proline and malondialdehyde contents in leaf for ginkgo but not ash, while the chlorophylls contents were inhibited for both of the two species. There was no variation of the ratio of leaf length to width, specific leaf weight, and soluble sugar content among treatments. Water supply was the key factor influencing the variation of leaf morphological and biochemical parameters, whereas most of the parameters were affected significantly only under the combination of pavement and water deficiency.
(3) When water supply was sufficient, the net photosynthetic rate and maximum photosynthetic capacity of ginkgo were significantly inhibited by pavement. Drought decreased net photosynthetic rate by 31.6% and 25.7% for ash and ginkgo, respectively, and maximum photosynthetic capacity of them was also inhibited. The net photosynthetic rate of ash and ginkgo decreased more by 44.9% and 35.9%, respectively, under the combination of drought and pavement. Stomatal regulation (characterized by gs) exerted key effects on gas exchanges under drought and its combination with pavement, while the non stomatal regulation for photosynthesis was also very important. By analyzing the effects of pavement and drought on photosynthetic processes, the role of non stomatal regulation was evident and was shown by parameters such as photochemistry efficiency and electron transfer in PSII (characterized by Fv/Fm, ΦPSII, and ETR and Jmax), Rubisco content and activity (Vcmax), and triose phosphates (TPU). These parameters were significantly reduced under drought treatment and reduced more under its combination with pavement, which resulted in a significant reduction of photosynthetic rates.
(4) Soil evaporation and evapotranspiration could be enhanced by pavement irrespective of water supply. Under sufficient water supply pavement decreased leaf transpiration which was regulated by stomatal conductance, while increased stem sap flow and whole tree transpiration because of higher total leaf area for ginkgo but not ash. The reduction of stem sap flow and whole tree transpiration under insufficient water supply was mitigated and exacerbated by pavement for ash and ginkgo, respectively.
(5) When water supply was sufficient, pavement did not significantly affect the total biomass, but significantly enhanced the total leaf area and leaf biomass of ginkgo. Drought and pavement+drought treatments significantly reduced the plant height, basal diameter, total leaf area, organ biomass and total biomass of ash but not ginkgo. Water deficiency is the key factor to inhibit the growth and biomass of ginkgo. Under the combined effects of pavement and drought, the root mass fraction and the root shoot ratio of ash was significantly increased, indicating the decrement of belowground biomass was lighter than that of the aboveground biomass.
Overall, the leaf morphological and biochemical traits, photosynthesis and whole tree transpiration of ash showed more tolerance to pavement and drought than ginkgo. Thus, ash is more suitable to be planted in sites with stronger environmental stress, such as pavement. Since supplying sufficient water can maintain the physiological function, photosynthesis and healthy growth of trees in pavement, it is necessary to irrigate the newly planted trees properly, especially in the continuous dry and hot weather, without aggravating the contradiction between trees and other water use.