Fig: Definition and influencing factors of WUE. After Ruggiero et al., 2017
Water supply has a significant impact on plant photosynthesis and water transport on the Tibetan Plateau (TP) (Huang et al., 2017), which could be reflected in the water use efficiency (WUE) of terrestrial ecosystems (Wang et al., 2020a, Sun et al., 2016). Defined as the ratio of ecosystem gross primary production (GPP) to transpiration (E) (Keenan et al., 2013, Yang et al., 2016), WUE refers to the trade-off between plant gain in carbon and cost in water. Numerous studies have shown that, lack of water suppresses both ecosystem productivity and transpiration at the same time (Lin et al., 2020, Sun et al., 2016, Wang et al., 2020b, Xue et al., 2015, Yang et al., 2016). However current studies reveal contrasting response of WUE to drought (Lin et al., 2020, Wang et al., 2020b, Yang et al., 2016). Therefore, by summarizing and comparing the following studies, we try to get a better insight into the relationship between WUE and water supply condition, which will undoubtfully benefit our understanding of how vegetation response to the future climate change in TP.
On the one hand, in drought, WUE may increase when assimilation rate decreases lesser than transpiration rate. In the study of Wang et al. (2020), using the eddy covariance technique, they found that WUE had a higher value during the low soil water content period, which indicated that water stress enhances WUE in the Nam Co alpine grassland ecosystem. However, their study focused on a relatively drier area, where water supply was not adequate and plant photosynthesis was greatly affected by water. And with lower soil water content, the suppression of transpiration was lower than that of assimilation, thus leading to the higher WUE (Wang et al., 2020b). And this phenomenon also appeared in other ecosystem, according to the research of Yu et al. (2008), WUE of forest ecosystem in eastern China appeared to increase with less precipitation (Yu et al., 2008).
On the other hand, when soil water supply in the soil layer was deficient, transpiration rate decreased less than assimilation rate, resulting a decreased WUE. Yang et al. (2015) calculated the Person’s correlation between WUE with wetness index (the ratio between precipitation and potential evapotranspiration) with two observational WUE datasets. Their result reveals that WUE decreases with drought in grassland, where WUE variability is mostly regulated by assimilation (Yang et al., 2016). In addition, through version 4.5 of the Community Land Model (CLM4.5), Lin et al. (2020) found that although precipitation significantly influenced assimilation and transpiration, its effect on WUE was slightly positive yet insignificant, which means less water supply will lead to a slightly lower WUE. The reason for this may be that the effect of precipitation on GPP offsets any precipitation related increase in transpiration.
In conclusion, in this blog, we focused on the influencing of water condition on WUE in TP. And we found that both the water consumption and photosynthesis capacity of vegetation determine the response of WUE to water stress. Therefore, researchers are supposed to carefully identify which one is the dominant factor.
Thanks my supervisor Wang Han for detailed comments that greatly improved the logic and quality of this review.
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