城市与区域生态国家重点实验室知识库

      城市树木作为城市绿色设施的重要组成部分，具有缓解热岛效应、减轻城市内涝、净化空气、降低噪音污染、提高碳吸存和美化景观环境等重要生态服务功能。 然而， 不断扩大的 城市 硬化地表 可 显著改变树木生长的环境条件 ，进而影响树木生长、碳代谢和水分利用 ，最终限制其生态服务功能的发挥。全球变暖 预计将加剧 城市热岛效应， 使城市树木遭受 更加严重的环境 胁迫 。研究硬化地表与其他胁迫因子（如干旱等）的复合作用对树木生理生态的影响，可为 预见 未来环境胁迫加重的趋势下城市树木的响应提供参考，进而 更好地 保证城市树木健康 生长并持续发挥 其 生态 功能 ，但是目前相关研究较少 。
      本研究采用盆栽田间模拟的试验方法，选取北京市广泛种植 且生长速率和生理特征存在明显区别的 2 种绿化树白蜡（ Fraxinus chinensis ）和银杏 Ginkgobiloba 分别设置 4 种处理（对照、硬化、干旱和硬化 干旱） 试验周期为2年 。 综合探讨 硬化地表和干旱胁迫 下， 白蜡和银杏 叶片生理生化特性、光合作用、叶片蒸腾和植株耗水 、 生长速率 及 生物量积累和分配 的响应。 在此基础上，提出关于 硬化地表上绿化树种选择和健康管理等 的 参考建议。主要研究结果如下：
      （1）硬化地表较 自然地表 使空气温度 和 地面温度 分别 显著升高 0.11 °C和5.35 °C，原因是硬化材料具有较高的吸热和储热能力、较高的长波反辐射和显热比例。硬化地表使空气湿度显著降低 由于 硬化地表阻隔土壤蒸发且缺少如自然地表上草本植物的蒸腾。 2种地表的温湿度差别在夜间表现得更加明显。硬化地表显著降低土壤水分，由于地表水分入渗 较少且土壤压实不利于水分涵养。硬化地表可加重干旱发生时土壤温度的升高 及 土壤水分的不足，因此，硬化地表和干旱复合作用时植物 受到 的热胁迫和干旱胁迫将更加严重。
      （2）无论是否位于硬化地表上，干旱使白蜡单叶面积、长和宽均显著降低而银杏的这些指标仅在硬化 +干旱处理下显著下降 。 当供水不足时，硬化地表显著降低银杏叶片 相对含水量和 氮含量、白蜡和银杏叶片叶绿素含量，提高银杏碳氮比、脯氨酸和丙二醛含量。 不同处理未对 2种供试树种的叶片长宽比 、比叶重和可溶性糖含量 产生显著影响。 水分供应是控制叶片形态和生化特征参数的关键因素， 但 同时大部分参数的显著变化主要发生在硬化地表和干旱 的 共同作用下。
      （3）水分供应充足时硬化地表对银杏净光合速率和最大光合能力 均 产生抑制作用。干旱导致白蜡和银杏净光合速率 分别显著降低 31.6%和 25.7%且 最大光合能力下降， 同时 硬化地表和干旱复合作用时 净光合速率 下降 更大分别为 44.9%和 35.9%。气孔调节在干旱及硬化和干旱复合作用对树木叶片气体交换的影响上起关键作用，但同时非气孔调节作用亦不可忽视。非气孔调节作用 可 通过叶片光合过程 受到 的影响 进行 阐释 ，相关的参数 包括 PSII光合系统的 光化学效率和电子传递速率（ Fv/Fm、 ΦPSII、 ETR和 Jmax）、 Rubisco含量和活性（ Vcmax）及磷酸丙糖利用速率（ TPU）；这些参数在干旱条件下降低且在硬化地表和干旱复合作用时更大程度的降低，进而导致光合作用的显著下降。
      （4）硬化地表无论在供水是否充分的情况下均能提高土壤蒸发量和总蒸散量。在供水充足条件下硬化地表使银杏叶片蒸腾速率降低、 受气孔调节影响， 树干液流和 植株蒸腾量增加 、可能由于更高的总叶面积 ，但未对白蜡造成显著影响。硬化地表加剧了银杏 树干液流 和植株蒸腾在供水不足时的下降，白蜡则相反。
      （5 ）供水充足时，硬化地表虽未显著影响树木总生物量，但明显促进了银杏叶片生长和 叶 生物量的积累。干旱和硬化 干旱 处理下 均使白蜡株高、基径、总叶面积及各器官和总生物量显著降低，但并未对银杏造成显著影响。水分缺乏是抑制白蜡生长和生物量积累的关键因素。硬化地表和干旱复合作用时，白蜡 根重占比和 根冠比显著提高，根系生长 的受抑制程度 较地上部分生长更轻。
      综合而言，白蜡 叶片形态和生化特性 、 光合作用 和植株蒸腾 较银杏表现出 对硬化地表和干旱 更强的 耐受 性 更适合种植于硬化地表等环境胁迫 较 强的地点。由于充分的水分供应能保持硬化地表上树木 生理功能、光合作用和健康生长 ，因此， 尤其在持续干热的天气， 必须对 新栽 树木进行适当的灌溉 但同时须避免加剧树木与其他用水之间的矛盾。

      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.