流域尺度全球景观生态风险时空演变及其对NPP变化的影响

人类活动强度的迅速增加，不断改变全球下垫面，造成区域景观生态风险持续变化。该研究以全球主要流域和亚流域为研究对象，使用1982—2015年连续34 a的土地利用栅格数据，构建了景观生态风险指数，分析了流域尺度全球景观生态风险时空演变特征，并进一步量化了不同驱动因素对景观生态风险的影响，同时分析了流域尺度全球植被净初级生产力（net primary productivity，NPP）的时空变化特征，揭示了流域尺度全球景观生态风险对NPP的影响。结果表明：1）温带和寒温带、干旱气候带以及北极附近的极地带区域的流域多年平均景观生态风险较高。在1982—2015年间，17.44%和23.45%的流域景观生态风险分别呈现显著增加和显著减少趋势（P<0.05）。2）干旱气候带、亚热带和热带地区的景观生态风险变异系数的空间变化幅度较大，36.82%的流域景观生态风险存在显著突变点，这些突变点主要发生在1988—2006年（P<0.05）。3）在多年尺度，各种因素对流域景观生态风险的影响从大到小依次为降水量、气温、NDVI、人口密度、海拔、坡度和GDP。此外，各种因素对景观生态风险的变化存在增强的交互作用。4）超过50%的流域年NPP变化具有显著增加趋势，而仅有12.60%的流域年NPP呈显著减少趋势（P<0.05），超过20%的流域年景观生态风险与年NPP间存在显著相关关系（P<0.05）。研究成果可为气候变化背景下，实现全球碳中和，增加陆地生态系统碳汇，适应气候变化提供坚实的科学依据。

Spatiotemporal changes of global landscape ecological risk and its impact on NPP changes at the watershed scale

The rapid increase in the intensity of human activities has continuously changed the global underlying surface, resulting in ecological risks to the regional landscape. In this study, an ecological risk system was constructed for the watershed-scale landscape using a variety of landscape pattern indices. The global major basins and sub-basins were taken as the research objects. The land use raster data was collected for 34 consecutive years from 1982 to 2015. The spatiotemporal ecological risk of the global landscape was analyzed at the watershed scale. The various driving factors of landscape ecological risks were then quantified using geo-detectors. At the same time, the spatiotemporal variation was determined for the global net primary productivity (NPP) of vegetation at the watershed scale, in order to reveal the impact of global landscape ecological risks. The results showed that: 1) The range of spatial variation was 0-0.29 for the multi-year average ecological risk of the global landscape at the watershed scale. Specifically, the high-value areas were distributed mainly in the temperate and cold temperate zones, arid climate zone, and polar zone near the Arctic. Meanwhile, 17.44 % and 23.45 % of the watershed landscape ecological risks showed a significant increase and decrease trend from 1982 to 2015, respectively (P<0.05). 2) There was a large spatial variation in the coefficient of variation of landscape ecological risk in the arid climate, subtropical, and tropical zones. Furthermore, 36.82 % watershed of significant change-points were found in the landscape ecological risk from 1988 to 2006. The spatial distribution of the watersheds with the significant changes in the landscape ecological risk was basically consistent with that of watersheds with significant change-points (P<0.05). The global underlying surface changed greatly in the 1990s, indicating the great variation in the landscape ecological risk. 3) There was the greatest impact of precipitation on the landscape ecological risk of major- and sub-watersheds at the multi-year scale. The influencing factors on the landscape ecological risk of watersheds were ranked the descending order of the precipitation, temperature, NDVI, population density, altitude, slope, and gross domestic product. There were also complex interactions among the various influencing factors on the landscape ecological risk. 4) The annual NPP of 52.32 % and 12.60 % of the watersheds increased and decreased significantly from 1982 to 2015, respectively. The landscape ecological risk was conducive to the NPP in the 9.11 % watershed with a significant positive correlation between annual landscape ecological risk and annual NPP (P<0.05). By contrast, the annual landscape ecological risk was significantly negatively correlated with the annual NPP in the 11.43 % watershed (P<0.05), where the reduction of landscape ecological risk was conducive to the NPP. The finding can also provide a solid scientific basis for global carbon neutrality and carbon sinks in the terrestrial ecosystems under climate change.