To accurately quantify tree transpiration and determine related hydrologic and physiological processes, it is important to have reliable information on environmental and physiological controls on sap flow. The objective of this study is to explore the environmental and physiological controls on sap flow in a headwater catchment in the subhumid mountainous region of North China, which has pronounced environmental specificities in hydro-climatic conditions, bedrock properties, and edaphic features. Sap flow in Aspen (Populus davidiana) (one of the dominant tree species in the region) and relevant environmental and physiological factors were measured from 2013 to 2014. The results indicate the following: (1) The dominant controlling factor of sap flow switched from a meteorological to a physiological factor when leaf area index (LAI) dropped to a low value (approximately <1 m2 m−2) around early October; (2) LAI exhibited a threshold control on possible maximum sap flow with a LAI threshold value of 3.5 m2 m−2, while environmental factors led to fluctuations in sap flow within the upper bound that was determined by the physiological factor LAI; (3) Photosynthetically active radiation (PAR) was the key environmental factor controlling sap flow as a whole, while at the monthly timescale the controls of environmental factors on sap flow had significant seasonal variability; (4) The diurnal relationships between sap flow and environmental factors revealed evident hysteresis loops, which were markedly influenced by the radiation factor. With the combination of the environmental factor PAR and the physiological factor LAI, an empirical regression equation (SFD = 0.7059PAR * LAI + 4.5068, R2 = 0.8862, n = 207) was established for sap flow estimation in the study area. These results shed light on the hydrologic and physiological processes involved in tree transpiration, and contribute to the refinement of tree transpiration models in regions under similar environmental conditions.