Abstract
Unique ecosystems distributed in alpine areas of the Qinghai–Tibetan Plateau play important roles in climate change mitigation, local food supply, and conservation of species diversity. To understand the water use efficiency (WUE) of this fragile and sensitive region, this study combined observed data from the eddy covariance system and the Shuttleworth–Wallace (S-W) model to measure the continuous mass exchange, including gross primary productivity (GPP), evapotranspiration (ET), and canopy transpiration (T) throughout 2 or 3 years (2016–2018) in three common alpine ecosystems (i.e., alpine steppe, alpine meadow, and alpine swamp). These ecosystems represent a water availability gradient and thus provide the opportunity to quantify environmental and biological controls on WUE at various spatiotemporal scales. We analyzed the ecosystem WUE (WUEe; defined as the ratio of GPP to ET) and canopy WUE (WUEc; defined as the ratio of GPP and canopy T). It was found that the yearly WUEe was 1.40, 1.63, and 2.16 g C kg–1 H2O, and the yearly WUEc was 8.93, 2.46, and 5.19 g C kg–1 H2O in the three typical ecosystems, respectively. The controlling factors of yearly WUE diverged between WUEe and WUEc. We found that plant functional group proportion (e.g., gramineous and Cyperaceae) highly explained the yearly WUEe variation across sites, and a good correlation was observed between community species diversity and WUEc. These findings suggest that community composition and trait change are critical in regulating WUEe and WUEc across different alpine ecosystems and that the regulation mechanisms may differ fundamentally between WUEe and WUEc.
Highlights
The Qinghai–Tibetan Plateau has received international attention because of its contribution to the global carbon budget and the sensitivity and fragility of the alpine ecosystems
The community of these sites represents the most prevalent vegetation types of the Qinghai–Tibetan Plateau and exhibits a clear water availability gradient (Figure 1). They include an alpine steppe located in Maduo County, dominated by Stipa purpurea, the soil of this community is frigid calcic soil; an alpine meadow located in Batang, Yushu County, dominated by Kobresia humilis, the soil of this community is felty soil; and an alpine swamp located in Longbao Nature Reserve, Yushu County, dominated by Kobresia littledalei, the soil of this community is bog soil
The soil water content (SWC) at the alpine swamp site remained relatively high throughout the growing seasons, whereas SWC at the alpine steppe site was more variable
Summary
The Qinghai–Tibetan Plateau has received international attention because of its contribution to the global carbon budget and the sensitivity and fragility of the alpine ecosystems. The Three Rivers Headwaters Region, located in the central of Qinghai–Tibetan Plateau, is the most critical area for reserving water resources and maintaining alpine biodiversity (Liu et al, 2008; Wang et al, 2019), and is an important pasture for the local free-grazing livestock production (Zhang L.X. et al, 2014; Zhang et al, 2019). This area has already experienced a considerable climate change, including air temperature increase (0.16◦C decade−1) and precipitation change (Wang et al, 2008). A more accurate and deeper understanding of the dynamics of WUE at various observation scales is conducive to the verification and improvement of the accuracy of assessment of regional resource use (Hu et al, 2008; Niu et al, 2008) and the prediction of the climate effect on the limited forage supply and water resource change
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