北京城市森林生态系统能量分配的季节动态研究

Seasonal dynamics of energy distribution in urban forest ecosystem of Beijing

【目的】森林是陆地生态系统的主要组成部分,对全球碳水循环和能量交换过程有重大影响,然而对城市森林生态系统能量交换过程的了解仍很有限,本文在北方城市森林开展生态系统水热通量的机理研究,对制定科学合理的人工林水分管理策略具有重要意义。【方法】本研究利用涡度协方差观测法,探讨了2018年北京奥林匹克森林公园各表面能量组分的日变化和季节变化,以及控制能量分配的主要生物物理因子。【结果】全年日均净辐射通量(Rn)为133 W/m2,日均感热通量(H)为23.6 W/m2,日均潜热通量(LE)为26.9 W/m2。波文比(β=H/LE)为0.88。在非生长季,感热通量大于潜热通量,在生长季开始,潜热通量逐渐超过感热通量,占据主导地位。Priestley-Taylor系数(α系数)、冠层导度(gs)和解耦系数(Ω)都与土壤含水量(VWC)和归一化植被指数(NDVI)呈正相关的关系,与饱和水汽压差(VPD)则呈现先上升后下降的关系。β与VWC、VPD和NDVI都呈现负相关的关系。VWC和VPD通过影响冠层导度,控制潜热通量及能量分配。【结论】本研究结果表明:干旱条件能显著降低生态系统冠层导度,从而降低潜热通量,影响能量分配。此外,为提高潜热通量比例,最大化城市绿地的降温功能及其价值,在冠层构建期和生长季干旱期进行一定程度的灌溉是一项合理的用水管理措施。

[Objective]As the main body of terrestrial ecosystems,forests play an important role in the global carbon water cycle and energy exchange process.However,the understanding of energy exchange process of urban forest ecosystems is still limited.[Method]In this study,eddy-covariance measurement was used to investigate the daily and seasonal variation of surface energy components of the Beijing Olympic Forest Park in 2018,and how the main biophysical factors control energy exchange.[Result]The daily mean net radiation flux(Rn)was 133 W/m2,the daily mean sensible heat flux(H)was 23.6 W/m2,and the daily average latent heat flux(LE)was 26.9 W/m2.The annual Bowen ratio(β,i.e.H/LE)was 0.88.In non-growing season,the sensible heat flux was greater than the latent heat flux.At the beginning of the growing season,the latent heat flux gradually exceeded the sensible heat flux and dominated in most of the growing season. Priestley-Taylor coefficient (α), surface conductance (gs) and decoupling coefficient (Ω)were all positively correlated with soil water content (VWC) and normalized difference vegetation index(NDVI). The enhancement of α, gs and Ω by vapor pressure deficit (VPD) was buffered when the VPD wastoo high. β had a negative correlation with VWC, VPD and NDVI. VWC and VPD control latent heat fluxand energy partitioning by affecting canopy conductance. [Conclusion] This study indicates that droughtconditions can significantly reduce the canopy conductance of the ecosystem, thereby reducing latent heatflux and affecting energy partitioning. In addition, in order to increase the proportion of latent heat flux andmaximize the cooling function and value of urban green space, irrigation during the canopy developmentperiod and drought period during growing season is a practical management measure.