Effects of soil water content on forest ecosystem water use efficiency through changes in transpiration/evapotranspiration ratio
Introduction
Ecosystem water use efficiency (WUE) is defined as the ratio of gross primary production (GPP) over evapotranspiration (ET) of a given ecosystem, and is a key parameter of the carbon-water coupling for an ecosystem. In recent years, WUE has been employed to investigate ecosystem responses to changing environmental conditions, especially under the context of climate change (Niu et al., 2011). It is therefore important to identify the factors that affect ecosystem WUE and quantify their impacts. From a plant physiological point of view (at the leaf level), VPD is a key factor that affects WUE and efforts have been devoted to study the effect of VPD on WUE (Dekker et al., 2016; Gentine et al., 2019; Lin et al., 2015; Zhou et al., 2015). At the leaf-level, the photosynthetic uptake of CO2, A, is related to stomatal conductance (gs), and the concentration of CO2 inside (ci) and outside (ca) the leaf cells (Beer et al., 2009):
Transpiration (T), is proportional to the difference between water vapor pressure in the leaf cells (ei) and the ambient air (ea):
The water vapor pressure in the leaf cells is usually assumed to be saturated at the leaf temperature. Assuming an isothermal condition between the leaves and the ambient air, the term (ei-ea) can be approximated by atmospheric VPD. The leaf-level WUE (A/T) is therefore related to VPD:
The term (1-ci/ca) is proportional to VPD0.5 based on the optimality theory (Katul et al., 2009; Lloyd and Farquhar, 1994), even though the exponent of VPD varies greatly from 0.2 to 0.8 (Stoy et al., 2019). Assuming an exponent of 0.5, WUEleaf is proportional to VPD−0.5.
The relationship between WUE and VPD at the ecosystem scale has been evaluated using eddy covariance data and results suggest that ecosystem WUE is best related to VPD0.5 at sub-daily and daily timescales (Zhou et al., 2014, 2015). An underlying water use efficiency, uWUE, can therefore be defined as
The other key parameter related to water availability for plants and ecosystems is soil water content (SWC), which is not explicitly included in Eq. (3) but still affects WUE through its effect on ci/ca (Perez‐Priego et al., 2018). Plants extract water from soil and stomatal conductance responds to soil water availability (Gentine et al., 2019), leading to changes in GPP and ET. Ecosystem WUE has been reported to increase or decrease in dry years or during droughts (Grünzweig et al., 2003; Krishnan et al., 2006; Reichstein et al., 2007, 2002), where SWC may play a role. Yang et al. (2016) conducted a global examination of the relationship between WUE and drought, and found a large contrasting response of WUE to drought in different ecosystems at the monthly timescale. Because both VPD and SWC may be contributing to changes of WUE during droughts, the effect of SWC is not straightforward to disentangle. Yang et al. (2010) stratified half-hourly WUE data into different VPD ranges to eliminate the correlation between VPD and WUE, and observed opposite responses of WUE to SWC at low and high VPD values, using data from an Ozark Forest during a severe drought. Their results suggest that ecosystem WUE increases with SWC when VPD is high, but decreases with SWC when VPD is low. At the plant scale, contrasting effects of SWC on WUE have also been reported. Moriana et al. (2002) reported that WUE was not influenced by SWC except in cases of severe soil water stresses at sub-daily timescales, while Villalobos et al. (2012) found that WUE is higher under soil water stress at hourly timescale. It is therefore not clear even whether WUE increases or decreases with SWC at either ecosystem or plant scales.
In this contribution we evaluate, using flux data from 36 eddy-covariance sites across four forest types, the effect of soil water content on WUE changes in forest ecosystems at the hourly timescale. At longer timescales, VPD and SWC are highly coupled (Zhou et al., 2019a), making it challenging to evaluate the individual effects of VPD and SWC on WUE (Gentine et al., 2019). However, at the hourly timescale SWC and VPD are largely decoupled, and it is possible to separate the effects of VPD and SWC. We first evaluate whether WUE increases or decreases with SWC at constant VPD. We find that ecosystem WUE is strongly dependent on SWC, and the dependence changes with the VPD conditions. We then discuss whether the observed change in WUE with SWC is due to changes in the plant water use efficiency (GPP/T) or changes in the transpiration/evapotranspiration ratio (T/ET). At the ecosystem scale water loss is not only due to transpiration (T), but also evaporation (E), and only the total amount of ET is measured. We quantify the contributions of GPP/T changes and T/ET changes to the effect of SWC on ecosystem WUE through ET partitioning following the method presented in Li et al. (2019).
Section snippets
FLUXNET dataset and preprocessing
We extracted half-hourly and hourly eddy-covariance flux data from the FLUXNET2015 Tier 1 dataset which were acquired from the FLUXNET data portal (http://fluxnet.flux-data.org). The FLUXNET2015 dataset was quality checked and gap filled. All the selected flux sites have data records for at least three years. The dataset provides meteorological data including radiation, precipitation, air humidity, air temperature, etc. Available soil water content (SWC) and photosynthetic photon flux density
The coupling between VPD and SWC
In Fig. 2 the correlation coefficients between VPD and SWC of the 36 forest sites are plotted for hourly, daily, monthly and seasonal timescales. The correlation between VPD and SWC increases with increasing timescales. Our results are consistent with previous observations where VPD and SWC are more tightly coupled at longer timescales. At hourly and daily timescales VPD and SWC are weakly coupled with correlation coefficients of -0.24±0.17 and -0.26±0.20. The correlations bewteen VPD and SWC
Relationships between WUE and SWC
The main finding of this study is that there exist significant negative correlations between ecosystem WUE and SWC and the negative correlation is more apparent at higher VPD. It is demonstrated that during peak growing seasons ET increases faster than GPP with increasing SWC, leading to a decrease in WUE with the increase of SWC. Previous studies have demonstrated the important role of soil water availability in controlling water use efficiency at both ecosystem scale and leaf scale, but the
Conclusions
Based on hourly meteorological and ecosystem flux data from 36 forest eddy-covariance sites in the FLUXNET2015 Tier 1 dataset, we investigated the effects of SWC on forest WUE at different VPD levels. Results show that variations in ecosystem WUE, GPP and ET are correlated with soil moisture. Two statistical model results show that the effects of SWC on ecosystem WUE are larger under high VPD levels. At low VPD, WUE is almost independent on SWC, because both GPP and ET increase slightly with
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.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (No. 91647212, No.51809007) and Major Basic Research Development Program of the Science and Technology Agent, Qinghai Province (2019-SF-146). We thank editor Kimberly A, Novick and two anonymous reviewers for their insightful comments and suggestions that helped us improve the manuscript.
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