Leaf- and ecosystem-scale water use efficiency and their controlling factors of a kiwifruit orchard in the humid region of Southwest China

Abstract

Understanding the characteristics of water use efficiency (WUE) and its controlling factors in the agricultural ecosystem can help us better understand the coupled process between water use and carbon assimilation spatially. The characteristics of the leaf- and ecosystem-level water, carbon and WUE variations as well as their drivers in a kiwifruit orchard were systematically analyzed during the growing seasons of 2018–2020. The results showed that leaf transpiration rate (Tr), net photosynthetic rate (Pn) and instantaneous WUE (WUEi) in exposed leaves were 2.25–2.77, 8.32–9.44 and 2.53–3.77 times higher than those of shaded leaves. Tr and Pn were significantly affected by photosynthetic active radiation (PAR) and stomatal conductance (gs). Air temperature (Ta), leaf water vapor deficit (VPDl) and stomatal conductance (gs) affected WUEi through leaf water consumption, while PAR affected WUEi through leaf photosynthetic process. Averaged evapotranspiration (ET), gross primary productivity (GPP) and ecosystem WUE (eWUE) were 551.30 ± 75.92 kg H2O m-2, 1475.37.59 ± 201.25 g C m-2 and 2.68 ± 0.04 g C kg−1 H2O, respectively. Global total radiation (Rg), Ta and VPD were the dominant climatic factors affecting ET and GPP, while Rg and wind speed (U2) were significantly correlated with eWUE (p < 0.01). WUEu, defined as the ratio of Pn·VPDl0.5 and Tr, could be considered as an effective indicator quantifying the coupled relationship between Tr and Pn·VPDl0.5 at the leaf scale. The coupled relationship between ET and GPP was strengthened after incorporating the effect of VPD on GPP due to reduced time lags among GPP, ET and VPD at the ecosystem scale. Rg, Ta, U2 and VPD contributed to greater effects through ET than that of GPP on eWUE, while the differences between them were reduced after incorporating VPD on GPP, which may be the source of the dependence of iWUE (GPP·VPD/ET) and uWUE (GPP·VPD/ET) on environmental conditions. This study enriches the scarce literature on what drives multi-scale water and carbon in a humid orchard and contributes to improving the understanding of the coupled process of water and carbon incorporating the effect of VPD at leaf and ecosystem scales.