A model coupling the effects of soil moisture and potential evaporation on the tree transpiration of a semi‐arid larch plantation

Abstract

Quantifying the tree transpiration (T) response to the coupling effects of changing soil moisture and weather conditions provides insights into water use by forests, especially in dryland regions. This study was conducted in a semi‐arid pure larch (Larix principis‐rupprechtii) plantation in northwest China. The sap flow density of trees, weather conditions in open field, and soil moisture of main root zone (0–60 cm) were measured synchronously and continuously throughout a growing season in 2010. The response of daily T to the relative extractable soil water (REW) and potential evapotranspiration (PET) was analyzed. The T varied in the range of 0.08–2.18 mm/day during the study period. With increasing REW, it initially rose quickly and almost linearly; but after the REW reached a threshold, it leveled off and gradually approached its maximum (Tmax). However, Tmax positively related with PET. Based on the T‐REW and Tmax‐PET relationships, an integrated model (T = (−0.05PET2 + 0.65PET) × (1 − exp (−5.39REW))) was developed to describe the complex T‐REW‐PET relationship and it fit the measured data well. For the whole study period, due to the limit of PET and REW, the actual T was reduced by 32% (88.6 mm) and 16% (45.0 mm) respectively, compared with potential T. Although this model cannot be widely applied until it is validated with measured data from more sites, it does provide a way to accurately estimate the daily T using REW and PET data and to quantify the coupling effects of changing REW and PET on the T.

Modeling the Response of Daily Evapotranspiration and its Components of a Larch Plantation to the Variation of Weather, Soil Moisture, and Canopy Leaf Area Index

“…In this study, the relationship between T and ET ref was established as a binomial equation using the upper boundary line, indicating that the T initially increased and gradually approached its maximum at the threshold of ET ref of 4.0 mm/day. This response pattern is in line with that of Li et al (), and the ET ref threshold is consistent with that of Bréda et al () obtained in a mature Quercus petraea stand located in Nancy, France. However, several studies have also reported that daily T increased with rising ET ref without an apparent threshold (Granier et al, , ).…”

“…Previous studies have shown that the REW threshold varied with climatic conditions, being higher under dry conditions than wet conditions (Lagergren & Lindroth, ; She et al, ). This might explain the higher REW threshold (0.45) of the larch stand growing in a semiarid area (Li et al, ) than that of this study in a semihumid area (0.4).…”

“…The stand T has been shown in previous studies to be affected by various meteorological factors (e.g., vapor pressure deficit, air temperature, and solar radiation intensity), but this can be simplified as the effect of ET ref (Wu et al, ), which can reflect the capacity of the atmosphere to absorb the transpired water (Li et al, ). ET ref affects the transpiration rate mainly by controlling the leaf‐air vapor pressure difference and the stomatal behavior/movement (Petzold et al, ).…”

“…In typical forests without extreme climate conditions, the T is usually jointly limited by multiple factors (e.g., ET ref and REW). Therefore, the coupling effects of influencing factors for stand T must be considered and quantified, such as the study in a semiarid larch stand (Li et al, ) which found a reduction of 32% and 16% of actual T by the limit of ET ref and REW, respectively. Besides ET ref and REW, the forest canopy LAI is also a key factor affecting the stand T, as indicated in this study.…”

“…The existing forest ET studies at fine temporal scales (e.g., daily) have shown that forest ET is strongly controlled by the instantaneous variation of weather, soil moisture, and vegetation (Fernandes et al, ; Ge et al, ; Granier et al, ). Increasing attempts were given to quantify the response of forest ET components to the change of individual influencing factors, such as the integrated daily T model coupling the T responses to ET ref and soil moisture (Li et al, ) and the canopy interception model coupling the components of rainwater adsorption and evaporation during rainfall events under the influences of P and LAI (Xu et al, ). However, synthetic modeling studies that quantify and couple the different responses of all ET components to all the main driving factors remain rare.…”