Biophysical controls on energy exchange and water use efficiency over a poplar plantation in Northern Hilly China

Abstract

During the last decades, the artificial afforestation is expanding in the northern China, understanding their ecological function has been of scientific interest. In this study, we explored how surface energy-balance components vary at seasonal and annual scales, and how biophysical factors control bulk surface parameters and energy exchange over a poplar plantation using the eddy-covariance measurements during 2016–2018. The plantation was characterized with a dry surface where available water played a dominant role in controlling energy partitioning. Annually, sensible heat flux (H) exceeded latent heat flux (λE) for most of the years, resulting in annual Bowen ratio (β) of 1.31–1.74. Seasonally within the year, variations of evaporative fraction (EF), Priestley-Taylor coefficient (α), surface conductance (gs) and decoupling coefficient (Ω) was significant different between drought and wet periods. The enhancement of λE by high vapor pressure deficit (VPD) was buffered by a concurrent suppression of gs which played a direct role in controlling energy partitioning. During dry periods, soil water availability restrained the atmospheric evaporative demand controls on water and energy exchange. High ET/precipitation (P) (>1) indicated high water consumption of the plantation, which increased local water deficit. Our results highlight the limitation of poplar water use, supply and energy partitioning in current water conditions and suggest more deliberation in species selection for afforestation in this area.