Environmental controls on water use efficiency in a hilly tea plantation in southeast China
Introduction
Carbon and water fluxes are tightly coupled through the stomata in terrestrial ecosystems and are considerably important properties of ecosystem processes and functions (Liu et al., 2017, Singh et al., 2014, Tan et al., 2015). WUE, the ratio of carbon assimilation to water loss, has been used as a key integrated ecophysiological indicator in hydrological and ecology studies, as it reflects the coupled relationship between carbon and water cycles from leaf to ecosystem levels (Singh et al., 2014, Tong et al., 2014, Yu et al., 2008). At the ecosystem level, WUE is commonly defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET), and these two parameters can be estimated simultaneously by the eddy covariance (EC) method (Baldocchi, 2003, Law et al., 2002). The ecosystem WUE is effective because it indicates the water use strategy and adaptability to climate change of plants in different environments (Lin et al., 2018, Xie et al., 2016), and can be used to assess the effect of water resources on terrestrial ecosystem carbon sinks (Tong et al., 2014). Moreover, because WUE is an important functional indicator, it has become the key parameter of some models for simulating ecosystem production (Tang et al., 2006, Zhou et al., 2015). Understanding the impact of environmental and management factors on WUE is critically important for assessing the response of ecosystems to global climatic change and optimizing water and carbon management strategies to suit local climatic conditions.
Theoretically, any controlling factors of both GPP and ET can affect WUE. Previous studies have shown that at the ecosystem level, WUE is mainly controlled by both environmental factors and physiological factors, such as air temperature (Ta), net radiation (Rn), vapor pressure deficit (VPD), soil water content (SWC), precipitation, stomatal conductance (gc) and leaf area index (LAI) (Li et al., 2015, Liu et al., 2017, Tong et al., 2014, Wang et al., 2018b). Unlike the regulatory mechanism at the leaf scale, the ecosystem WUE combines photosynthesis, respiration, evaporation and transpiration processes and has a much more complicated regulatory mechanism (Li et al., 2015). Moreover, management practices such as mulching and fertilization affect the soil water and nutrient supply and hence the GPP, ET, and WUE of croplands and plantations (Ali et al., 2018, Liu et al., 2016). However, the magnitude and direction of the response of GPP, ET and WUE to these environmental factors and management practices might be different or even opposite. (Li et al., 2018) found that radiation exerted a stronger positive effect on ET than GPP, resulting in a negative response of WUE across biomes. The different responses deeply reflect the complexity of GPP, ET and WUE variability and their underlying mechanisms.
Tea is an intensively managed evergreen broadleaved shrub widely planted in hilly areas in tropical and subtropical regions as an important cash crop. In China, tea plantations covered approximately 3.11 × 106 ha in 2019 (National Bureau of Statistics of China, 2020), and are still rapidly expanding especially in the hilly region of southeast China. However, the impact of tea plantations on carbon assimilation and water loss has been poorly studied (Li et al., 2011, Lv et al., 2013, Zhang et al., 2017). The management practices of tea plantations, especially pruning, influence carbon and water dynamics. GPP was suppressed by pruning, while respiration was enhanced, resulting in a carbon source during the pruning season (Pang et al., 2019); ET was impacted by a low leaf area index through its effect on canopy surface conductance and decreased 36.73 ~ 48.32% after pruning practice (Lv et al., 2013, Geng et al., 2020). However, few studies have clarified the seasonal divergence of GPP, ET and WUE of tea plantations and their response to environmental factors and management practices. An improved understanding of seasonally different behaviors of ecosystem WUE and its key controlling variables is important for optimal tea plantation management and assessing the impact of expanding tea plantations on regional hydrology and ecosystem functioning.
In this study, the CO2 and water vapor flux between tea plantations and the atmosphere was measured by the EC technique over a five-year period (2014–2018) in the subtropical hilly area of southeast China. The specific objectives were as follows: (1) to analyze the seasonal and interannual variations in GPP, ET and WUE in a subtropical tea plantation and (2) to reveal the response patterns of tea plantation WUE to environmental variables and management practices.
Section snippets
Site description
A tea plantation was studied at Tianmuhu station (31°16′14″ N, 119°27′15″ E), a field station of the Key Laboratory of Watershed Geographic Sciences, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, located in Jiangsu Province, southeast China. This station is located upstream of the Taihu basin and is influenced by a subtropical monsoon climate. The annual mean air temperature and precipitation were 15.8 ℃ and 1155.8 mm, respectively. 76% of the annual precipitation
Seasonal variation in environmental factors
The seasonal patterns of daily air temperature (Ta), soil temperature (Ts), vapor pressure deficit (VPD), net radiation (Rn), soil water content (SWC) and precipitation (P) during 2014–2018 are shown in Fig. 1. The air temperature and precipitation followed the typical seasonal trends of this area, which presents warm-humid summers and cold-dry winters. Ta and Ts showed a pronounced seasonal pattern, with the maximum value occurring in July or August and the minimum value occurring in December.
Comparison of WUE with other sites
WUE represents the trade-off between the amount of carbon gain and the amount of water loss controlled through stomatal behavior during the process of photosynthesis (Keenan et al., 2013). The EC method provides direct and continuous measurements of carbon and water fluxes simultaneously for evaluating the WUE of different vegetation types at the ecosystem level (Hu et al., 2008, Niu et al., 2011, Xiao et al., 2013, Yu et al., 2008). The WUE values of reported subtropical and tropical forests
Conclusions
Here, the seasonal variations in the ecosystem GPP, ET and WUE of a subtropical hilly tea plantation in southeast China are presented. The dominant drivers of these variables were also evaluated. The results showed that the mean annual GPP, ET and WUE ranged from 1426.61 ~ 1896.05 g C m−2, 607.05 ~ 805.83 mm and 2.22 ~ 2.68 g C kg−1 H2O, respectively. The relatively high WUE was primarily attributed to the application of a large amount of fertilizer and other management practices leading to a
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank the financial support provided by the National Natural Science Foundation of China (41877513, 41901222), the National Key R&D Program of China (2018YFD1100102), the Ecological Civilization Project Toward a Beautiful China (XDA23020403), and the Engineering Laboratory of Lake Environmental Governance and Ecological Restoration Project, Chinese Academy of Sciences (KFJ-PTXM-021). Great thanks to the editors and anonymous reviewers for providing valuable comments
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