Abstract
In dryland regions, soil moisture is an important limiting factor for canopy transpiration (T). Thus, clarifying the impact of soil moisture on T is critical for comprehensive forest—water management and sustainable development. In this study, T, meteorological factors (reference evapotranspiration, ETref), soil moisture (relative soil water content, RSWC), and leaf area index (LAI) in a Larix principis-rupprechtii plantation of Liupan Mountains in the dryland region of Northwest China were simultaneously monitored during the growing seasons in 2017–2019. A modified Jarvis—Stewart model was established by introducing the impact of RSWC in different soil layers (0–20, 20–40, and 40–60 cm, respectively) to quantify the independent contribution of RSWC of different soil layers to T. Results showed that with rising ETref, T firstly increased and then decreased, and with rising RSWC and LAI, T firstly increased and then gradually stabilised, respectively. The modified Jarvis—Stewart model was able to give comparable estimates of T to those derived from sap flow measurements. The contribution of RSWC to T in different soil layers has obvious specificity, and the contribution rate of 20–40 cm (13.4%) and 0–20 cm soil layers (6.6%) where roots are mainly distributed is significantly higher than that of 40–60 cm soil layer (1.9%). As the soil moisture status changes from moist (RSWC0–60cm ≥ 0.4) to drought (RSWC0–60cm < 0.4), the role of the soil moisture in the 0–20 cm soil layer increased compared with other layers. The impacts of soil moisture that were coupled into the Jarvis—Stewart model can genuinely reflect the environmental influence and can be used to quantify the contributions of soil moisture to T. Thus, it has the potential to become a new tool to guide the protection and management of forest water resources.
Highlights
As the main component of evapotranspiration, canopy transpiration (T) accounts for 50–90% of the forest evapotranspiration [1,2], and plays a vital role in regional water supplies
The objectives of this study were: (1) to determine the response relationship of T to ETref, relative soil water content (RSWC) in different soil layers, and leaf area index (LAI); (2) to improve the Jarvis-Stewart model which can reflect the impact of soil moisture in different soil layers; and (3) to quantify the independent contributions of RSWC in different soil layer on T across different years and soil moisture conditions based on the modified Jarvis-Stewart model
RSWC increased with the increase of soil depth in the range of 0–60 cm soil layer, and was 0.30, 0.29, and 0.31 at the 0–20 cm soil depth, 0.34, 0.29, and 0.35 at 20–40 cm soil depth, and 0.39, 0.46, and 0.47 at 40–60 cm soil depth for 2017, 2018, and 2019, respectively
Summary
As the main component of evapotranspiration, canopy transpiration (T) accounts for 50–90% of the forest evapotranspiration [1,2], and plays a vital role in regional water supplies. The temperature in the Loess Plateau has increased by 1.91 ◦ C and since 1961 the annual precipitation has decreased by 29.1 mm due to warming [4]. The large-scale afforestation plan in the 1980s caused severe forest water conflicts while causing soil and water conservation [5,6]. Many artificial forests in the planned area grew abnormally and could not form canopy closures. This is because soil drought has deteriorated the ecological environment in some areas [7], causing water loss and senescence of plant leaves and accelerated shedding [8]. Accurately simulating canopy T and quantifying the effects of soil moisture in a complex environment could significant help coordinate forest-water conflicts and water resource management
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