Background The water conservation function of a forest ecosystem is of great significance for sustainable development in a region. The transpiration of a plantation is an important component of forest water cycle and a key indicator reflecting the water conservation function of a forest. The measurement of sap flow can accurately estimate the process of a forest plantation.

Methods Sap flow density of plantation species, Cotinus coggygria, was continuously measured in situ during the growing season from April to September in 2021 in Mangshan National Forest Park in Beijing using thermal diffusion method. Environmental factors including air temperature (T_a), relative humidity (R_H), photosynthetically active radiation (PAR) and soil volumetric water content (VWC) were measured simultaneously. Besides, vapor pressure deficit (VPD) was calculated by substituting T_a and R_H into the formula. This study examined the variations of sap flow density and environmental factors on the diurnal and seasonal scale, as well as analyzed the correlation between them via regression analysis and path analysis.

Results The cumulative transpiration was 90 mm during the growing season. On the diurnal scale, sap flow density was mainly controlled by PAR, showing a positive correlation (R²=0.93), There was a time lag of 0.5 h between the peak values of sap flow density and PAR. On the seasonal scale, sap flow density was mainly regulated by VPD, and there was a threshold for this regulation at 1.5 kPa. When VPD is less than 1.5 kPa, the changes in sap flow density were directly related to VPD, showing a positive linear correlation between them (R²=0.94). When VPD is more than 1.5 kPa, the regulatory effect of VPD on sap flow density gradually weakened, while the regulatory effect of stomatal conductance (g_s) on sap flow density increased, and g_s had an exponential decline with the rise of VPD (R²=0.88). During the mid-growing season from June to August, sap flow density increased with the rise of T_a, VPD and PAR, but there was no significant relationship with VWC.

Conclusions It is concluded that C. coggygri had the characteristics of lower water consumption, and its sap flow density was mainly controlled by PAR on the diurnal scale and was regulated by VPD on the seasonal scale. This species can adapt to atmospheric drought by regulating stomatal conductance. This study provides scientific basis for the species selection in developing forest plantations for soil and water conservation in Beijing.