Response of Water Use Efficiency to Global Environmental Change Based on Output From Terrestrial Biosphere Models

Abstract

Water use efficiency (WUE), defined as the ratio of gross primary productivity and evapotranspiration at the ecosystem scale, is a critical variable linking the carbon and water cycles. Incorporating a dependency on vapor pressure deficit, apparent underlying WUE (uWUE) provides a better indicator of how terrestrial ecosystems respond to environmental changes than other WUE formulations. Here we used 20th century simulations from four terrestrial biosphere models to develop a novel variance decomposition method. With this method, we attributed variations in apparent uWUE to both the trend and interannual variation of environmental drivers. The secular increase in atmospheric CO2 explained a clear majority of total variation (66 ± 32%: mean ± one standard deviation), followed by positive trends in nitrogen deposition and climate, as well as a negative trend in land use change. In contrast, interannual variation was mostly driven by interannual climate variability. To analyze the mechanism of the CO2 effect, we partitioned the apparent uWUE into the transpiration ratio (transpiration over evapotranspiration) and potential uWUE. The relative increase in potential uWUE parallels that of CO2, but this direct CO2 effect was offset by 20 ± 4% by changes in ecosystem structure, that is, leaf area index for different vegetation types. However, the decrease in transpiration due to stomatal closure with rising CO2 was reduced by 84% by an increase in leaf area index, resulting in small changes in the transpiration ratio. CO2 concentration thus plays a dominant role in driving apparent uWUE variations over time, but its role differs for the two constituent components: potential uWUE and transpiration.

Response of Ecosystem Water Use Efficiency to Drought over China during 1982–2015: Spatiotemporal Variability and Resilience

“…The discrepancy in those studies could be partly attributed to the different environmental conditions, plant species, planting areas and cultivation methods. Moreover, the significant increasing trend (8.5 × 10 −3 g C kg −1 H 2 O per year) of annual WUE found in this study which may be due to the increased atmospheric CO 2 concentration [17,55,56]. However, the WUE trend was much higher than Sun et al (2018) (7.32 × 10 −4 g C kg −1 H 2 O per year) [30] which may induced by their different data sources.…”

“…Many previous studies have investigated the spatiotemporal changes of WUE. For example, Zhou et al (2017) showed that underlying WUE (GPP × VPD 0.5 /ET) increased slowly during 1901-1975 but increased rapidly from 1976 to 2010 over the world based on terrestrial biosphere models [17]. Tang et al (2014) found that the WUE had a similar latitudinal trend on earth, increasing from the subtropics to about 51 • N and then decreased in higher latitudes [18].…”

Forest‐Type‐Dependent Water Use Efficiency Trends Across the Northern Hemisphere

“…Increased WUE in forests under eCO 2 has been reported in many previous studies, including those using field experiments (De Kauwe et al, ; Leakey et al, ), tree ring measurements (Dekker et al, ; D. C. Frank et al, ; Saurer et al, ), isotopic compositions of atmospheric CO 2 (Keeling et al, ), eddy covariance (EC) flux tower measurements (Keenan et al, ; Mastrotheodoros et al, ) and model simulations (Cheng et al, ; Huang et al, ; Zhou et al, ). However, these studies have not consistently agreed with the statement that increased WUE under eCO 2 should correspond to increased carbon assimilation and/or decreased water losses in forests.…”

Gas exchange and water‐use efficiency in plant canopies

“…; Zhou et al . ). Land‐cover change can alter ecosystem water‐use efficiency, for example, by changing the balance between C 3 and C 4 vegetation, by replacing annual with perennial species, or by changing canopy structure and leaf area index in such a way that it impacts the partitioning between evaporation and transpiration at the ecosystem level (Nosetto et al .…”

“…Recent estimates suggest that the overall contribution of land‐cover change since 1900 has been to reduce water‐use efficiency of terrestrial ecosystems, on average, compared to what it would be in the absence of land‐cover change (Zhou et al . ).…”