北京市行道树绒毛梣的水力结构对城市不透水表面比例的响应

Response of hydraulic architecture in Fraxinus velutina street trees to the percentage of impervious pavement in Beijing

了解和量化城市树木耐旱性的变异性和潜在的驱动机制对于预测和管理全球气候变化下的城市生态系统稳定性至关重要。该研究以北京市常见行道树绒毛梣(Fraxinus velutina)为研究对象,选取6个不同不透水表面比例(用归一化建筑指数(NDBI)反映)的实验地点。利用遥感数据提取各地的NDBI和月地表平均温度(Ts)参数,并实地测定了空气水汽压亏缺(VPD)和黎明前水势(Ψ_(pd)),在生长季采用自然干燥法拟合各地点绒毛梣枝条木质部脆弱性曲线,并计算得到木质部栓塞脆弱性,测定各地点凌晨和中午枝条的自然栓塞化程度以获取不同地点木质部栓塞的恢复程度;同时测定了比导率(k_(s))和叶比导率(LSC)、气孔导度(G_(s))和最大光化学量子效率(F_(v)/F_(m))。研究了不同不透水表面比例下树木栓塞脆弱性(耐旱性)的适应性,以及耐旱性与其他水力性状之间的相关性。结果表明:1)不透水表面比例与50%导水率损失对应的水势值(Ψ_(50))显著负相关,不透水表面比例越高耐旱性越高;2)Ψ_(50)与Ψ_(pd)及VPD显著相关;3)k_(s)与Ψ_(50)间有显著权衡,但LSC与Ψ_(50)相关关系不明显;4)栓塞修复能力与Ψ_(pd)呈正相关关系;5)处于不同不透水表面比例地区的绒毛梣的F_(v)/F_(m)无显著差异;6)随不透水表面比例的增加,绒毛梣光合速率下降。该研究结果表明,不透水表面比例是影响绒毛梣耐旱性的关键城市环境指标之一,不透水表面比例提高导致绒毛梣耐旱性的提高,同时绒毛梣白天出现的栓塞可以在夜晚进行积极的修复,这两个特征均说明城市树木对于城市环境表现出水力结构的适应性,但这种适应性以光合速率下降为代价。该研究结果为快速城市化和全球气候变化场景下北京城市生态系统健康程度、弹性和稳定性评价提供了重要研究数据,并为决策者制定切实可行的北京城市树木管理策略提供了理论依据。

Aims Understanding and quantifying the variability of drought tolerance and the potential driving mechanism in urban trees are critical to the prediction and management of urban ecosystem stability under global climate change.The objectives of this study were:1)to identify the branch hydraulic traits of trees at urban sites with different percentages of impervious pavements in Beijing,and 2)to investigate if the drought tolerance of urban trees is adapted to urbanization.Methods The investigated species in the study was Fraxinus velutina.This species is widely applied to street planting in Beijing.We selected six sites along the north-south axis of the city with different percentages of impervious pavements as represented by normalized difference built-up index(NDBI).The NDBI and monthly surface temperature at each site were obtained by remote sensing.The bench dehydration technique was used to assess site-specific branch vulnerability to drought-induced xylem cavitation.Net photosynthesis rate,stomatal conductance,and maximal efficiency of PSII photochemistry(F_(v)/F_(m))were also measured with a photosynthesis instrument.Important findings The percentage of impervious pavements was positively correlated with the water potential corresponding to 50% loss of hydraulic conductivity(Ψ_(50)),while Ψ_(50) was found to correlate with pre-dawn xylem water potential(Ψ_(pd))and vapor pressure deficit.A significant trade-off relation was found between specific conductivity and Ψ_(50),but not between leaf specific conductivity and Ψ_(50).The embolism repair ability was significantly positively correlated with Ψ_(pd).The net photosynthetic rate decreased with the increase in percentage of impervious pavement,whereas the F_(v)/F_(m) did not show significant difference among sites.The results suggest that the percentage of impervious pavements is one of the key urban environmental indicators affecting the drought tolerance of urban trees.The hydraulic architecture of F.velutina showed adaptability to the urban environment in the city.The study not only provides important research data for evaluation of the health,resilience,and stability of the urban ecosystems under the scenarios of rapid urbanization and global climate change,but also a theoretical support for decision-makers to formulate practical and feasible management strategies for street planting in Beijing.