Abstract
Global climate changes have increased the imbalance of water resources, especially in northern China, which comprises typical arid and semiarid regions. Large-scale afforestation has been implemented over the past three decades in northern China. The ecosystem water use efficiency (WUE) connects the carbon cycle and water cycle of the terrestrial ecosystems and is defined as the ratio of the gross primary productivity (GPP) to the evapotranspiration. However, there are still an insufficient number of studies on the impact of the afforestation on the WUE. In this study, we applied the random forest (RF) model to explore the impacts of climate and nonclimate factors on the WUE in northern China. The results showed that in areas with high precipitation, the forests had the highest WUE, while in the arid areas, the croplands had the highest WUE. Of the total area, 44.34% showed a significant increase, and 5.89% showed a significant decrease in the WUE from 1982–2015 in northern China. The main driving factors for the changes in the WUE were climate factors, including the precipitation, temperature and solar radiation, which contributed to approximately 84% of the WUE trends, while human activities, such as afforestation, contributed to approximately 16% of the WUE trends. Overall, although the climate had a larger impact on the WUE dynamics than the human activities, our results suggested that the impacts of the afforestation programs on forest carbon and water cycles should be considered in the context of climate change.
Highlights
Global climate changes have increased the imbalance of water resources [1], especially in arid regions
To verify the effectiveness of the water use efficiency (WUE) estimated in the Three North Regions (TNR) from the Global Land Surface Satellite (GLASS) products, the WUE calculated by the GLASS gross primary productivity (GPP) and ET was compared with the WUE calculated by the Moderate Resolution Imaging Spectroradiometer (MODIS) GPP and ET, which are widely used remote sensing products [10,11,52]
We explored the impacts of climate factors and human activities, especially land cover change, on the WUE dynamics in the TNR during the period of 1982 to 2015
Summary
Global climate changes have increased the imbalance of water resources [1], especially in arid regions. Newly planted vegetation needs more water to grow [3] while storing carbon and providing biofuel to contribute to climate change mitigation [4], which has a significant impact on the ecological sustainability in arid and semiarid regions. Based on the GLASS GPP and ET data, this study explored the spatiotemporal changes in the WUE and the relationship between the climate factors and the ecosystem WUE. The ability of the plants to survive under limited water conditions is indispensable for the ecosystems of this region [8] Ecological restoration projects, such as afforestation programs, have contributed to the accelerated greening trends in the TNR [31], but they have fewer effects on the biomass change in Northeast China [32] and on the hydrological cycle over the entire TNR during 1989–2009 [33]. The forests include forests with canopy cover greater than 30%, woods with canopy cover between 10–30%, and shrubs and other afforested land [42]
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