Responses Of Ecosystem Water-use Efficiency Research Articles

Response of ecosystem water-use efficiency to global vegetation greening

Global vegetation has been greening during the past two decades. However, relatively less is known concerning the change of carbon–water cycle coupling in response to vegetation greening. A key indicator that can characterize this coupling is water-use efficiency (WUE) of ecosystems. Therefore, we explored the impact of global vegetation greening on ecosystem WUE using Theil-Sen trend analysis, Mann-Kendall (M−K) test, stepwise regression and Lindeman Merenda Gold model (LMG) model based on the remote sensing data from 1982 to 2018. Our results showed that global vegetation greening had a positive effect on the gross primary productivity (GPP), evapotranspiration (ET) and WUE during the past thirty years. The WUE in 70 % of these vegetation greening areas, including India, China, Russia, Japan, North Korea, Europe and North America, was increased, and the WUE was increased by 0.002 g C kg H2O yr−1, which was twice as much as that of the global vegetation cover areas. Results of stepwise regression analysis showed that vegetation greening contributed 80 % of the changes in WUE, much higher than the contribution of the climate factors. GPP contributes approximately 81 % to the changes in WUE as a result of vegetation greening. In addition, we also found that, in the greening areas around the world, leaf area index (LAI) explained 80 % of the changes in WUE, and that vegetation has a positive effect WUE. Therefore, the WUE should be considered in the design of ecological restoration programs in the future, and vegetation of low water consumption would be favored to realize sustainable ecosystem services and regional development.

Spatial responses of ecosystem water-use efficiency to hydrothermal and vegetative gradients in alpine grassland ecosystem in drylands

The carbon–water cycle is critical to ecosystem function and sustainability in dryland region. Ecosystem water use efficiency (WUE) is an important indicator reflecting the interaction between carbon and water, and investigating its spatial responses to hydrothermal and vegetative gradients is vital to understand the underlying mechanism of carbon–water coupling. The source region of the Yellow River (SRYR) is a typical alpine grassland ecosystem in drylands characterized by cold and arid climate, which is ecologically fragile and sensitive to environmental factors. In this study, we investigated the spatial variations of ecosystem WUE in the SRYR during 2000–2014 and examined its underlying drivers. The mean annual WUE showed significantly increasing trends along hydrothermal and vegetative gradients from semi-arid zone (SA) to dry sub-humid zone (DSH), with the average value of 0.59 ± 0.35 gC kg−1 H2O. The spatial variations of WUE were mainly controlled by gross primary productivity (GPP) rather than evapotranspiration (ET). Compared to precipitation (P), temperature (Ta) better explained the spatial variations of WUE. And a temperature threshold of 0 °C was found, which indicated a shift of sensitivity of WUE to thermal conditions from permafrost to seasonally frozen ground due to the effects of low temperature on photosynthetic process in alpine grassland ecosystem. The responses of WUE to LAI gradients reflected the indirect effects of hydrothermal conditions on WUE by altering LAI. This study improves our understanding of the spatial responses of the regional carbon and water interactions to environmental conditions.

Discrepant responses between evapotranspiration- and transpiration-based ecosystem water use efficiency to interannual precipitation fluctuations

Climate change intensifies precipitation fluctuation and increases drought frequency around the globe. Water use efficiency (WUE) has proven to be a crucial metric to quantify the trade-off linking global carbon and water cycles in many aspects of terrestrial ecosystem function. Investigating the response of ecosystem WUE to multiannual precipitation fluctuations has major implications for our understanding of ecosystem carbon and water dynamics. However, the impacts of water availability variation on evapotranspiration- and transpiration-based ecosystem WUE and their mechanisms are poorly understood due to limited observations. We investigated ecosystem WUE in response to interannual precipitation fluctuations in order to reveal the regularity underlying WUE changes under different levels of water stress. We optimized the parameters of two remote sensing models (PT-JPL and PM) based on different biophysical processes using the differential-evolution Markov Chain (DE-MC) method. We investigated ecosystem WUEET (GPP/ET) and WUET (GPP/T) in response to interannual precipitation fluctuations at 73 sites. We found ecosystem WUET appears to decline during drought years and to increase in wet years contrasting with WUEET, which was mostly attributable to differing sensitivities of GPP, ET and T to multiannual precipitation fluctuations. The vegetation generally consumes more T to improve ecosystem GPP during dry years, meanwhile, no apparent change in WUEET during dry years because of the trade of between GPP/T and T/ET. The replenishment of soil moisture to ecosystem transpiration is higher than we thought during dry years. This was masked in analysis that considered the responses of GPP and T to annual precipitation changes separately, but was revealed by the changes in ecosystem WUET. This research advances our understanding of the consequences of water fluctuation on ecosystem carbon and water exchange.

Physiological and environmental control on ecosystem water use efficiency in response to drought across the northern hemisphere

Drought, a natural hydrometeorological phenomenon, has been more frequent and more widespread due to climate change. Water availability strongly regulates the coupling (or trade-off) between carbon uptake via photosynthesis and water loss through transpiration, known as water-use efficiency (WUE). Understanding the effects of drought on WUE across different vegetation types and along the wet to dry gradient is paramount to achieving better understanding of ecosystem functioning in response to climate change. We explored the physiological and environmental control on ecosystem WUE in response to drought using observations for 44 eddy covariance flux sites in the Northern Hemisphere. We quantified the response of WUE to drought and the relative contributions of gross primary production (GPP) and evapotranspiration (ET) to the variations of WUE. We also examined the control of physiological and environmental factors on monthly WUE under different moisture conditions. Cropland had a peak WUE value under moderate drought conditions, while grassland, deciduous broadleaf forest (DBF), evergreen broadleaf forest (EBF), and evergreen needleleaf forest (ENF) had peak WUE under slight drought conditions. WUE was mainly driven by GPP for cropland, grassland, DBF, and ENF but was mainly driven by ET for EBF. Vapor pressure deficit (VPD) and canopy conductance (Gc) were the most important factors regulating WUE. Moreover, WUE had negative responses to air temperature, precipitation, and VPD but had a positive response to Gc and ecosystem respiration. Our findings highlight the different effects of biotic and abiotic factors on WUE among different vegetation types and the important roles of VPD and Gc in controlling ecosystem WUE in response to drought.

Divergent responses of ecosystem water-use efficiency to extreme seasonal droughts in Southwest China

The recurrent drought extremes have resulted in deleterious impacts on ecological security. Despite that many attempts have been made to explore ecosystem responses to different severities of droughts, a deep understanding of how ecosystem water-use efficiency (WUE) responds to extreme seasonal droughts is critical for predicting the trends under future climate change, especially in the ecologically-fragile karst ecosystem across Southwest China. This study systematically examined the spatio-temporal variations of ecosystem WUE over the karst and non-karst areas, as well as their divergent responses to different seasonal droughts. Our findings revealed the apparent increase in drought frequency, duration, and severity in Southwest China during the past four decades. Meanwhile, spring and summer drought events were the prevailing drought types. Compared with the non-karst area, multi-year mean WUE in the karst area was relatively lower, whereas the area exhibiting significant increase in WUE (p < 0.01) accounted for 39.3% and 22.3%, respectively. However, the effects of drought on ecosystem WUE varied in different seasons with more severe consequence in the karst ecosystem. During the early stage of autumn-spring drought in 2009/2010, ecosystem WUE was apparently larger than the baseline condition with the difference turning to be negative anomalies during the peak period, whereas the effect of summer drought in 2011 led to negative anomalies nearly throughout the duration. Further analysis revealed that the anomalies in evapotranspiration acted a prominent role in altering WUE at the onset of both droughts, while ecosystem WUE was mainly determined by the sensitivity of gross primary production during the later stage. All analyses are beneficial for expecting the coupling relationship between global carbon and water cycles to future climate change, particularly as droughts are projected to increase in terms of frequency and severity.

A global examination of the response of ecosystem water-use efficiency to drought based on MODIS data

Ecosystem water-use efficiency (WUE) plays an important role in carbon and water cycles. Currently, the response of WUE to drought disturbance remains controversial. Based on the global ecosystem gross primary productivity (GPP) product and the evapotranspiration product (ET), both of which were retrieved from the moderate resolution imaging spectroradiometer (MODIS), as well as the drought index, this study comprehensively examined the relationship between ecosystem WUE (WUE=GPP/ET) and drought at the global scale. The response of WUE to drought showed large differences in various regions and biomes. WUE for arid ecosystems typically showed a negative response to drought, whereas WUE for humid ecosystems showed both positive and negative response to drought. Legacy effects of drought on ecosystem WUE were observed. Furthermore, ecosystems showed a sensitive response to abrupt changes in hydrological climatic conditions. The transition from wet to dry years should increase ecosystem WUE, and the opposite change in WUE should occur when an ecosystem experiences a transition from dry to wet years. This indicates the resilience of ecosystems to drought disturbance. Knowledge from this study should provide an in-depth understanding of ecosystem strategies for coping with drought.