Climate change impact on vegetation growth in the humid-arid transition zone in northern China during 1982-2013

As an ecologically sensitive region, the humid-to-arid climate transition zone in the eastern and central north China (ECNC) has experienced varied vegetation change in recent decades, but the exact impacts of climate change and human activities remain uncertain. Based on the Normalized Difference Vegetation Index (NDVI), we established the trend of vegetation change in the past three decades (1982-2013) in the ECNC, and examined the impact of climate and non-climate factors on vegetation growth within the different eco-regions. For the study period, the climate in ECNC was getting significantly warmer and slightly drier, and the overall NDVI increased significantly, but with great spatial variations. Our results suggest that temperature increase has promoted vegetation growth in places that are colder and/or wetter, and with higher vegetation coverage. Precipitation increase has promoted vegetation growth in drier places with sparse vegetation and inhibited growth in wet places with high vegetation coverage. As different eco-regions typically have different vegetation coverage and occupy different climate zones, their response to climate change also varies. For the study period, increasing temperature and decreasing precipitation promoted vegetation growth in the forest of northern ECNC. Increasing temperature and precipitation led to NDVI increase in the grassland in the south, whereas grassland in the north and west showed no significant change despite temperature increase and precipitation decrease. Cropland responded mostly positively to temperature increase, although correlations between NDVI and climate factors were generally weaker. Using multiple regression models, we found that 60% of the NDVI increase was attributed to climate factors whereas the remaining 40% was likely caused by human activities. Although farming practices and crop rotations might have caused significant decrease in NDVI in small areas, human impact largely led to significant NDVI increase in the grass-crop transition zones, most likely due to ecological restoration programs.