Excessive reliance on afforestation in China's ari...

Revegetation has significantly contributed to improvements in ecosystem services, such as carbon sequestration and soil retention. Yet, vegetation expansion in water-limited regions may generate conflict of water demand between nature and humans. Present studies are still lacking when it comes to identifying the permissible vegetation capacity, i.e. net primary productivity (NPP) threshold, based on the local water resources limits, and further proposing adjustment and optimization strategies to keep water use balanced in anthropogenic-biological systems. Under such a circumstance, this study assesses the difference between the actual NPP and NPP thresholds at regional and ecosystem scales in China. The results show that 8 out of 31 provinces have their provincial actual NPP above the regional NPP thresholds, mainly concentrated in northern China between 400 and 800 mm iso-precipitation line, i.e. North China Plain (Beijing-Tianjin-Hebei), the middle reaches of the Yellow River Basin (Shaanxi and Henan provinces), and the Northeast China (Heilongjiang, Jilin and Liaoning). Forest ecosystems dominate the difference between the actual total woodland and grassland ecosystems NPP and the permissible NPP thresholds in these regions, ranging from 67% (Beijing) to 99% (Tianjin). If the current vegetation intensity in these regions remains unchanged, the areas of woodland and grassland ecosystems should be optimized 0–48% and 0–100% of their present areas to balance the water demand between the ecosystems and humans, without considering the potential consequence of climate change and soil erosion. Although 23 provinces have their regional actual NPP below their permissible NPP thresholds, 6 out of 23 provinces still have their woodland and grassland ecosystems NPP above the corresponding NPP thresholds, mainly focusing on the Northwestern China north over the 400 mm iso-precipitation line, including Inner Mongolia, Qinghai, Hainan, Shanxi, Gansu and Xinjiang. Forest ecosystems also dominate the negative NPP differences in these regions, ranging from 91% (Inner Mongolia) to 46% (Gansu). These reveal the hidden and potential pressure in the 6 provinces to balance limited water resources in the local anthropogenic-biological system. This study provides a method to assess the water-resources permissible NPP threshold and further proposes the specific adjustment and optimization plans for the areas with actual NPP above the corresponding NPP thresholds, which can provide guidance for ecological restoration program implementations in a more sustainable way.

重建植被极大地促进了生态系统服务的改善，如碳固存和土壤保持。然而，在水资源有限的地区，植被的扩张可能会产生自然与人类之间的用水需求冲突。目前的研究仍然缺乏在当地水资源限制的基础上确定允许的植被容量，即净初级生产力(NPP)阈值，并进一步提出调整和优化策略以保持人类-生物系统中的用水平衡。在这种情况下，本研究评估了中国区域和生态系统尺度上的实际NPP和NPP阈值之间的差异。结果表明，31个省份中有8个省份的实际NPP高于区域NPP阈值，主要集中在华北平原(京津冀)、黄河中游(陕西、河南)和东北地区(黑龙江、吉林、辽宁)。在这些地区，森林生态系统在林地和草地生态系统实际总NPP和允许NPP阈值之间的差异中占主导地位，范围从67%(北京)到99%(天津)。如果这些地区目前的植被密度保持不变，林地和草地生态系统的面积应该优化为目前面积的0-48%和0-100 %,以平衡生态系统和人类之间的需水量，而不考虑气候变化和土壤侵蚀的潜在后果。尽管23个省份的区域实际NPP低于其允许NPP阈值，但其中6个省份的林地和草地生态系统NPP仍高于相应的NPP阈值，主要集中在400 mm等雨量线以上的中国西北地区，包括内蒙古、青海、海南、山西、甘肃和新疆。森林生态系统也在这些地区的负NPP差异中占主导地位，范围从91%(内蒙古)到46%(甘肃)。这揭示了这6个省在平衡当地人类-生物系统中有限的水资源方面隐藏的和潜在的压力。本研究提供了一种评估水资源允许净第一性生产力阈值的方法，并对实际净第一性生产力高于相应的净第一性生产力阈值的地区提出了具体的调整和优化方案，为生态修复工程的可持续实施提供了指导。