气候变化和人类活动对中国北方旱区植被变绿的定量贡献

气候变化和大规模的生态恢复使中国北方旱区植被发生了显著变化，量化气候变化和人类活动对植被动态的相对贡献，对于旱区生态系统管理和应对未来气候变化具有重要意义。目前，中国北方旱区植被变化影响因素的时间动态（2000年大规模生态恢复工程实施前后）和空间异质性（沿干旱梯度）仍需进一步的定量研究。基于多源数据，采用趋势分析、偏相关分析和随机森林模型等方法，分析了1981-2018年中国北方旱区气候和植被的时空变化规律，量化了2000年前后气候变化和人类活动对植被动态的相对贡献并分析其在干旱梯度上的空间差异性。结果表明：（1）1981-2018年期间，中国北方旱区的叶面积指数（LAI）平均增加速率为（0.0037±0.0443） a^-1，且增加速率沿干旱梯度增大。2000年前仅10.46%（P<0.05）的地区显著变绿，而2000年后达到36.84%，且植被变绿主要归因于非树木植被。（2）2000年后降水对植被变绿的正效应在不同干旱梯度均增加，而在半干旱区和亚湿润干旱区，温度对植被变绿由正向促进转为负向抑制，而辐射在干旱区由负效应转向正效应。（3）2000年前后，气候变化均主导着植被的动态，贡献率分别为96.07%和73.72%，人类活动的贡献在2000年后进一步增强（从3.93%增加到26.28%），且沿着干旱梯度而增加，其中人类活动对植被变绿的贡献在半干旱地区增加最显著（+0.0289 m² m^-2 a^-1，P<0.05）。研究结果可为未来气候变化下中国北方旱区的植被恢复和可持续发展提供科学依据。

Quantifying the contributions of climate change and human activities to vegetation greening in the drylands of northern China

Climate change and large-scale ecological restoration have led to significant changes of vegetation in the drylands of northern China, and quantifying the relative contributions of climate change and anthropogenic activities to vegetation dynamics is essential to ecosystem management and coping with future climate change in dryland ecology. The temporal dynamics (before and after the implementation of large-scale ecological restoration projects in 2000) and spatial heterogeneity (along the aridity gradient) of the impact factors of vegetation change in the drylands of northern China still need further quantitative studies. Based on multi-source data, our study analyzed the spatial and temporal patterns of climate and vegetation changes in the drylands of northern China from 1981 to 2018 by using trend analysis, partial correlation analysis and random forest model. We then quantified the relative contributions of climate change and human activities to vegetation dynamics before and after 2000 and their spatial heterogeneity along the aridity gradient. The results indicated that:(1) the average leaf area index (LAI) increase rate in the drylands of northern China was (0.0037±0.0443) a^-1 during 1981-2018, and increased along the aridity gradient. Temporally, only 10.46% of the area was experienced a significant vegetation greening trend before 2000, while 36.84% (P<0.05) after 2000 in the drylands, and vegetation greening was mainly attributed to non-tree vegetation. (2) The positive effect of precipitation on vegetation greening increased in all aridity gradients after 2000, while temperature shifted from positive promotion to negative inhibition in semi-arid and dry sub-humid areas, and radiation shifted from negative to positive effect in arid areas. (3) Climate change dominated vegetation dynamics both before 2000 and after 2000, with contributions of 96.07% and 73.72%, respectively. The contribution of human activities further increased after 2000 (from 3.93% to 26.28%) and strengthened along the aridity gradient, the most significant increase of human-induced LAI trend was found in the semi-arid area (+0.02887 m² m^-2 a^-1, P<0.05). These results can provide scientific references for the restoration and sustainable development of the drylands of northern China in response to future climate change.