Seasonal dynamics of energy distribution in urban forest ecosystem of Beijing

  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 (R_n) 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 (g_s) and decoupling coefficient (Ω) were all positively correlated with soil water content (VWC) and normalized difference vegetation index (NDVI). The enhancement of α, g_s and Ω by vapor pressure deficit (VPD) was buffered when the VPD was too high. β had a negative correlation with VWC, VPD and NDVI. VWC and VPD control latent heat flux and energy partitioning by affecting canopy conductance.

  Conclusion  This study indicates that drought conditions can significantly reduce the canopy conductance of the ecosystem, thereby reducing latent heat flux and affecting energy partitioning. In addition, in order to increase the proportion of latent heat flux and maximize the cooling function and value of urban green space, irrigation during the canopy development period and drought period during growing season is a practical management measure.