Combining sap flow measurements and modelling to assess water needs in an oasis farmland shelterbelt of Populus simonii Carr in Northwest China

Abstract

a b s t r a c tFarmland shelterbelts provide an ecological protection screen for an oasis but exhibit high mortality in the face of water shortage. It is necessary to understand farmland shelterbelt tree transpiration under different levels of water stress and stand ages for proper management. Sap flux measurement techniques and models are among the most useful method to detect water stress and to evaluate plant water consumption. The usefulness of both methods decreases, however, when applied to species, such as Populus simonii Carr, that have an outstanding tolerance to drought and a remarkable capacity to take up water from drying soils. Our hypothesis is that analysis using simultaneous measurements of sap flow and models in the same trees is useful for assessing the irrigation needs in farmland shelterbelts. To test our hypothesis, we analysed the relationships between canopy transpiration, canopy conductance, relative extractable water and atmospheric factors in a farmland shelterbelt and evaluated the effectiveness of the model. Measurements were made during one growing season. The time courses of sap flow measured and modelled on days of contrasting weather and soil water conditions were analysed to evaluate the usefulness of the method to assess the crop water needs. We calculated the daily tree water consumption from sap flow measurements and the parameterized modified Jarvis-Stewart model, and we evaluated the model's usefulness to assess the final water needs under water stress and stand ages for farmland shelterbelt irrigation. The transpiration decreased as the soil drought increased, and it increased as the atmospheric drought increased. The time course of the water needs showed that the occurrence of water stress in the farmland shelterbelt trees had a large impact on their water consumption, which increased as the water stress decreased, following the equation y = 1/[1 + e −60.67 × (REWx−0.402) ]. The simultaneous use of modelling and tree structural data increased the reliability of assessing water needs from youth to maturity. A similar analysis with the water consumption values, from which stand age values were derived, showed that water needs increased with the tree age following the equation y = 847 − 844/[1 + (x/87.9) 1.9 ]. We conclude that compared to the use of sap flow records alone, the simultaneous use of sap flow records and model values provides more detailed information to assess water needs in a farmland shelterbelt, which has an important significance for farmland shelterbelt protection.

Direct Penman–Monteith parameterization for estimating stomatal conductance and modeling sap flow

“…In coniferous and other stands, with strong coupling between the canopy and the atmosphere, gc (see Table 1 for the list of abbreviations) has a major effect on transpiration (Jarvis and McNaughton 1986;Meinzer et al 1997;Phillips and Oren 1998;Hernandez-Santana et al 2016). If evapotranspiration can be measured, actual stomatal conductance can then be easily calculated by using an inverted Penman-Monteith equation (Martin et al 1997;Cienciala et al 1997;Granier et al 2007;Whitley et al 2009;Braun et al 2010;Wang et al 2014;Fu et al 2016). This equation combined with sap flow measurements provides a powerful tool for estimating canopy conductance for both wholetree and forest stand levels with results comparable to leaf level measurements with subsequent upscaling (Ewers et al 2007b).…”

“…This approach is still widely used because it is simple and its modular structure makes it easy to incorporate in the larger models, and because its responses to the environmental factors can be separated and visualized (Egea et al 2011; Buckley and Mott 2013). For its simplicity this kinds of models are also used to derive stomatal conductances from the sap flow measurements (Lu et al 1995;Cienciala et al 1997;Whitehead 1998;Oren et al 1999;Ewers and Oren 2000;Ewers et al 2007a;García-Santos et al 2009;Wang et al 2014;Fu et al 2016). In such a cases, the simple models are often preferred over the more recent Ball-Berry model and its modifications, which are otherwise widely used in canopy and global circulation models (Verhoef and Egea 2014).…”

“…For the sake of simplicity and to make our work comparable to similar studies (i.e. Oren et al 1999;Ewers and Oren 2000;Ewers et al 2007a;García-Santos et al 2009;Wang et al 2014;Fu et al 2016) we decided to use those kinds of empirical models that do not include the photosynthesis.…”

Cover Crop Effects on Corn Plant Sap Flow Rates and Soil Water Dynamics

“…However, no direct studies have quantified CC effects on corn plant sap flow rates. Plant sap flow measurement is a useful method to detect direct plant water consumption (Fu et al, 2016), and the experimental approach used by this study can quantify the effects of CC on water availability for transpiration by the cash crop.…”

“…Plant sap flow measurement techniques are among the most useful methods to characterize plant-water relations, water stress, plant transpiration, and plant water consumption (Alarcón et al, 2000;Backes and Blanke, 2007;Rousseaux et al, 2009;Juhász et al, 2012;Fu et al, 2016). This method uses a constant power heat balance gauge for accurate stem flow measurements, where the xylem mass flow rate is calculated from a balance of heat in and out of a stem segment (Dugas, 1990).…”