| 摘要: |
| 黄土高原实施退耕还林还草工程20余年，区域生态环境发生巨大变化，过度植被恢复产生的负效应也逐步显现，有关黄土高原高强度植被恢复的可持续性问题引起讨论。当前，黄土高原植被恢复已进入一个节点性时期，但植被重建的终极目标并不十分明确。未来50—100 a，黄土高原退耕还林还草工程的环境效应与可持续性如何，目前缺乏前瞻性的科学认识。鉴于这一问题，本文对黄土高原近20 a大规模植被恢复和重建产生的生态、水文、侵蚀和气候效应进行了回顾，并对在甘肃庆阳南小河沟长期退耕还林和还草小流域持续开展近10 a的观测研究成果进行了总结分析，探讨长期自然和人为植被恢复背景下小流域的生态水文效应，最后从小流域视角对黄土高原退耕还林还草的可持续性进行了探讨。针对当前黄土高原退耕还林还草工程取得的成绩和面临的挑战，提出了退耕还林还草的两阶段论。第一阶段为退耕还林还草实施至今20 a，主要目标是增绿、控蚀、减泥沙，目前这一目标已基本实现；第二阶段为未来20—30 a，目标是稳绿、增水、促发展，当前工作的重点是需要针对第二阶段的目标开展规划和研究工作，提高黄土高原生态质量的持久性和区域人地关系的协调。 |
| 关键词: 植被恢复 生态效应 水文效应 侵蚀效应 气候效应 可持续性 |
| DOI：10.7515/JEE221003 |
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| 基金项目:中国科学院战略性先导科技专项（B类）（XDB40000000）；国家自然科学基金项目（41790444） |
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| Ecohydrological effects and sustainability of the Grain for Green Project on the Chinese Loess Plateau |
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JIN Zhao
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1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
2. National Observation and Research Station of Earth Critical Zone on the Loess Plateau in Shaanxi, Xi’an 710061, China
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| Abstract: |
| Background, aim, and scope The Grain for Green Project (GGP) has been initiated on the Loess Plateau for over 20 years, which has caused enormous changes in the regional eco-environment. Negative impacts of excessive vegetation restoration have emerged, such as soil water deficit and regional water resources shortage. The environmental effects of GGP in the next few decades have not been fully assessed. Therefore, the sustainability of large-scale vegetation restoration on the Loess Plateau is uncertain. This paper aims to articulate possible environmental trade-offs of GGP if it is continuously implemented in the next 50 to 100 years. The results of this paper can provide knowledge reference for scientific decision-making and proper implementation of high-intensive vegetation rehabilitation on the Loess Plateau. Materials and methods To tackle the questions as mentioned above, this paper reviewed the ecological, hydrological, pedological, and climatic effects of large-scale vegetation restoration on the Loess Plateau during the past 20 years. Then, we synthesized the results obtained from the paired catchments (tree-planting catchment vs. grass-revegetating catchment) in the Nanxiaohegou watershed of Qingyang City, Gansu Province. Environmental feedback as well as the positive and negative effects of long-term vegetation restoration were comprehensively evaluated. Finally, sustainability and consequence of converting farmland to forests in headwater catchments on the Loess Plateau were discussed. Results (1) Over the past 20 years, vegetation coverage on the Loess Plateau doubled, soil water deficit intensified, surface runoff and river discharge decreased, river sediment load reduced, and vegetation restoration showed positive feedback to regional rainfall increase. (2) After long-term returning farmland to forest, vegetation coverage of the tree-planting catchments was significantly higher than that of the catchments under grass revegetation. Two vegetation restoration measures resulted in varied ecohydrological processes of the paired catchments. Compared with the natural grassland catchment, the water yield capacity of the afforested catchment decreased by 90% after 70 years of plantation restoration, and the mechanisms of runoff generation also changed. The tree-planting catchment produced limited sediment output, and the water resource was highly consumed. At the scale of small watersheds, topographic factors and afforestation contributed equally to land surface energy balance. Discussion Sustainability of the conversion of farmland to forests on the Loess Plateau depends on the balance between maintaining reasonable runoff and sediment output, i.e., how to maintain the sediment transport of the Yellow River sustainably. The level of sediment transport of the Yellow River determines the intensity and duration of GGP. According to the preliminary results of the paired catchments, 70 years of afforestation resulted in almost no runoff and sediment output in the headwater catchment. From the point of view of system science, this situation has no problem to maintain the balance and stability of small catchments, but it is challenging to keep the material balance of large systems, such as the balance of water and sediment of the Yellow River. Existing research showed that the Yellow River Delta would face severe shrinkage and transgression due to the dramatic decrease in sediment transport of the Yellow River. Therefore, soil and water conservation on the Loess Plateau needs to be coordinated from small watershed management to whole watershed management to maintain the sustainability of environmental protection on the Loess Plateau and the overall health and high-quality development of the Yellow River Basin. Conclusions After 20 years of intensive vegetation restoration, ecology, hydrology, soil erosion, and climate of the Loess Plateau underwent dramatic changes, and the regional eco-environment change showed trade-off consequences. The most significant trade-off is trading water for green and sediment reduction. Our 10-year research through paired catchments observation indicated that the streamflow and sediment load will continuously decrease if maintaining the method of high-density tree plantation for 50 to 100 years in the future. Given the achievements and challenges of the GGP on the Loess Plateau, the problem should be viewed from the perspective of two stages. The first stage is 2000—2020, and the goals are increasing vegetation coverage, controlling soil erosion, and reducing sediment. The second stage is for the next few decades, and the goals are maintaining vegetation coverage, reducing vegetation density, increasing river water, controlling erosion in critical areas, and improving economic and social development. Recommendations and perspectives Currently, the goals of the first stage of GGP on the Loess Plateau have been achieved, and the focus of work now is to carry out planning and research for the goals of the second stage, promote the coordination of the human-environment relationship on the Loess Plateau, and serve the ecological protection and high-quality development of the Yellow River Basin. |
| Key words: vegetation restoration ecological effect hydrological effect soil erosional effect climatic effect sustainability |
