Abstract
Tree transpiration (T) is a major water budget component and varies widely due to the integrated effects of many environmental and vegetation factors. This study aimed to separate, quantify, and then integrate the effects of the main individual factors, to improve water use estimation and manage the hydrological impacts of forests. A field study was conducted at 3 plots of larch (Larix principis-rupprechtii) plantation in the semi-humid area of the Liupan Mountains, northwest China. The main influencing factors were the atmospheric evaporative demand expressed by potential evapotranspiration (PET), the soil water availability expressed by volumetric soil moisture (VSM) within the 0–100 cm layer, and the canopy transpiration capacity expressed by forest canopy leaf area index (LAI). The daily stand T was estimated through the up-scaling of sap-flow data from sampled trees. It displayed a high degree of scattering in response to PET, VSM and LAI, with an average of 0.76 mm·day−1 and range of 0.01–1.71 mm·day−1 in the growing season of 2014. Using upper boundary lines of measured data, the response tendency of T to each factor and corresponding function type were determined. The T increases firstly rapidly with rising PET, VSM and LAI, then gradually and tends to be stable when the threshold of PET (3.80 mm·day−1), VSM (0.28 m3·m−3) and LAI (3.7) is reached. The T response follows a quadratic equation for PET and saturated exponential function for VSM and LAI. These individual factor functions were coupled to form a general daily T model which was then fitted using measured data as: T = (0.793PET − 0.078PET2)·(1 − exp(−0.272LAI))·(1 − exp(−9.965VSM)). It can well explain the daily T variation of all 3 plots (R2 = 0.86–0.91), and thus can be used to predict the response of daily T of larch stands to changes in both environmental and canopy conditions.
Highlights
In the dryland regions of China, vegetation restoration has been implemented for several decades through large-scale afforestation programs such as the “Three-North Forest Shelterbelt” Program since 1978, the “Natural Forest Protection” Program since 1998, and the “Grain for Green” Program since 1999, with the main purpose of protecting soil against serious erosion and sandstorms[1,2,3,4]
Li et al.[33] studied the T of a larch plantation in the middle of the growing season to analyze the effect of potential evapotranspiration (PET) and relative extractable soil water (REW), but without consideration of leaf area index (LAI)
The results show that the T reduction due to the limit of actual PET amounts to 80.78 mm (35.67%) in the study period for all plots; the corresponding T reduction is 26.30 (11.61%), 23.81 (10.51%) and 25.01 mm (11.04%) due to the insufficient volumetric soil moisture (VSM), and 25.71 (11.35%), 20.20 (8.92%) and 23.71 mm (10.49%) due to the insufficient LAI in P1, P2 and P3, respectively
Summary
In the dryland regions of China, vegetation restoration has been implemented for several decades through large-scale afforestation programs such as the “Three-North Forest Shelterbelt” Program since 1978, the “Natural Forest Protection” Program since 1998, and the “Grain for Green” Program since 1999, with the main purpose of protecting soil against serious erosion and sandstorms[1,2,3,4]. As a result of ignoring the vegetation carrying capacity of limited water resources, the large-scale afforestation has induced several unexpected problems, such as soil desiccation4–6, “little-old-trees”[7] and water yield reduction[8,9,10]. This threatens the forest stability, forest ecosystem services, and the regional water supply security and sustainable development. The lack of field studies, which consider the coupled effects of all the main influencing factors, limits the understanding and accurate prediction of forest T under changing environmental conditions, and the integrated forest-water management in a multifunctional way
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