Ecosystem water use efficiency (WUE) acts as an integrated functional indicator for understanding land‐atmosphere interactions. The temporal patterns in the daily variations of WUE and their underlying drivers during different seasons in alpine meadow ecosystems, which are particularly vulnerable to changing climate, still remain poorly understood in spite of increasing efforts. In this study, we investigated the potential divergence in the response of WUE to climatic and biological drivers during different seasons at two alpine meadow ecosystems in the northeastern Tibetan Plateau using continuous eddy‐covariance measurements of carbon and water fluxes between 2013 and 2015. We found that variations in CO2 concentration exert significantly positive effects on variations in WUE in spring, but not in summer and autumn. Notably, vapor pressure deficit (VPD) overrode other factors playing a dominant role in regulating daily variations in WUE during all seasons in these alpine meadow ecosystems. Variations in VPD explained 29.5 to 52.3% of the variance in WUE between different seasons. We further showed that carbon gain and water loss processes responded divergently to different drivers; higher VPD significantly increased ecosystem evapotranspiration; whereas, variations in soil moisture and leaf area index significantly and positively affected gross primary productivity. Our findings highlighted the increasing importance of atmospheric drought in shaping land‐atmosphere interactions in alpine meadow ecosystems, particularly in a warming climate.
It is generally believed that evapotranspiration at night is too miniscule to be considered. Thus, few studies focus on the nocturnal evapotranspiration (ETN) in alpine region. In this study, based on the half-hour eddy and meteorological data of the growing season (from May to September) in 2019, we quantified the ETN of alpine desert (AD), alpine meadow (AM), alpine meadow steppe (AMS), and alpine steppe (AS) in the Qinghai Lake Basin and clarified the different response of evapotranspiration to climate variables in daytime and nighttime with the variation of elevation. The results show that: (1) ETN accounts for 9.88~15.08% of total daily evapotranspiration and is relatively higher in AMS (15.08%) and AD (12.13%); (2) in the daytime, net radiation (Rn), temperature difference (TD), vapor pressure difference (VPD), and soil moisture have remarkable influence on evapotranspiration, and Rn and VPD are more important at high altitudes, while TD is the main factor at low altitudes; (3) in the nighttime, VPD and wind speed (WS) control ETN at high altitudes, and TD and WS drive ETN at low altitudes. Our results are of great significance in understanding ETN in the alpine regions and provide reference for further improving in the evapotranspiration estimation model.
The responses of photosynthesis to change of temperature and CO2 concentration are divergent among various alpine plant species at high altitude regions; however, very few direct in situ measurements have been conducted to compare photosynthetic capacity among different functional plants along altitude gradients in the Qinghai‐Tibet Plateau (QTP). This study measured the net photosynthetic assimilation rate (An), maximum carboxylation rate (Vcmax), maximum electron transport rate (Jmax), stomatal conductance (gs) and mesophyll conductance (gm) for CO2 of three plant functional types (PFTs, sedge, grass and shrub) and functional traits at different altitudes in the Qinghai Lake watershed during growing season. Meanwhile we simulated An of the PFTs under potential future scenario. Results indicate that grass maintain a relatively stable An by decreasing Vcmax, Jmax, ratio of Jmax to Vcmax (J/V) and gs, while slightly increasing gm with increasing altitudes. In contrast, the An of sedge and shrubs increased with rising Vcmax, Jmax, gs and gm values, resulting in a large increment in the An at low altitudes. Grass was less sensitive to temperature by reducing the supply of CO2, while sedge and shrub increased. Ca was a more dominant factor than Ta in affecting the An of grass. The order of rising An in PFTs was shrub > sedge > grass, and the An of alpine meadow was found to increase more under the Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios. Our results indicate that grass may be more resistant to future warming and suggested that considering PFTs is of great significance for improving the simulating of the response of vegetation physiological and ecological processes to future climate change in alpine regions.
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