Abstract
Water-use efficiency (WUE), defined as the ratio of net primary productivity (NPP) to evapotranspiration (ET), is an important indicator to represent the trade-off pattern between vegetation productivity and water consumption. Its dynamics under climate change are important to ecohydrology and ecosystem management, especially in the drylands. In this study, we modified and used a late version of Boreal Ecosystem Productivity Simulator (BEPS), to quantify the WUE in the typical dryland ecosystems, Temperate Eurasian Steppe (TES). The Aridity Index (AI) was used to specify the terrestrial water availability condition. The regional results showed that during the period of 1999–2008, the WUE has a clear decreasing trend in the spatial distribution from arid to humid areas. The highest annual average WUE was in dry and semi-humid sub-region (DSH) with 0.88 gC mm-1 and the lowest was in arid sub-region (AR) with 0.22 gC mm-1. A two-stage pattern of WUE was found in TES. That is, WUE would enhance with lower aridity stress, but decline under the humid environment. Over 65% of the region exhibited increasing WUE. This enhancement, however, could not indicate that the grasslands were getting better because the NPP even slightly decreased. It was mainly attributed to the reduction of ET over 70% of the region, which is closely related to the rainfall decrease. The results also suggested a similar negative spatial correlation between the WUE and the mean annual precipitation (MAP) at the driest and the most humid ends. This regional pattern reflected the different roles of water in regulating the terrestrial ecosystems under different aridity levels. This study could facilitate the understanding of the interactions between terrestrial carbon and water cycles, and thus contribute to a sustainable management of nature resources in the dryland ecosystems.
Highlights
According to the latest IPCC report [1], global surface temperature increased by 0.85 ̊C in last 100 years and will continue to rise by 0.3 ̊C to 4.8 ̊C relative to 1986–2005., Water shortage in arid/semi-arid regions is reported to become more serious due to various factors [2]
High temperature areas are in the desert neighboring areas such as the Kyzylkum Desert, the Karakum Desert and the Gobi Desert, which are mainly characterized as arid sub-region (AR)
Low temperature areas are in the northwestern part of Mongolia, the northeastern part of Kazakhstan, the mountainous Altai region connected to Siberia in Russia, and the Tianshan Mountains, which stretches over Kyrgyzstan, Tajikistan and China
Summary
According to the latest IPCC report [1], global surface temperature increased by 0.85 ̊C in last 100 years and will continue to rise by 0.3 ̊C to 4.8 ̊C relative to 1986–2005., Water shortage in arid/semi-arid regions is reported to become more serious due to various factors [2]. Temperate Eurasian Steppe (TES), the largest grassland belt in the world, stretches from Hungary to the northeast of China, lying between the boreal forest in Russia and the vast desert area in Central Asia. It spans around one-fifth of the longitude with an area of over 1.3×107 km2 [9]. Observed evidence is that regional annual precipitation decreased during the past 30 years [15]. Such trend increases the regional drought stress due to the higher water loss from the land surface. According to the recent analysis from Mohammat, Wang [16], the possibility of summer drought is enhancing since the 1990s
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