Assessment of climate change and human activities on vegetation development in Northeast China

Vegetation in Northeast China (NEC) has faced dual challenges posed by climate change and human activities. However, the factors dominating vegetation development and their contribution remain unclear and cannot be precisely discussed. In this study, we conducted a comprehensive evaluation of the response of vegetation in different land cover types, climate regions, and time scales to water availability from 1990 to 2018 based on the relationship between normalized difference vegetation index (NDVI) and Standardized Precipitation-Evapotranspiration Index (SPEI). The effects of human activities and climate change on vegetation development were quantitatively evaluated using the residual analysis method. We showed that the area percentage with a positive correlation between NDVI and SPEI increases with the extension of time scales. NDVI of grass, sparse vegetation, rain-fed crop, and built-up land as well as sub-humid and semi-arid areas (drylands) correlated positively with SPEI, and the correlations increased with the extension of time scales. The negatively correlated area was concentrated in humid areas or areas covered by forests and shrubs. The maximum water surplus period for irrigated crops and forests, shrubs, wetlands, humid areas were 1-month and 6-months, respectively. Vegetation water surplus in humid areas weakens with warming, and vegetation water constraints in drylands enhance. Moreover, potential evapotranspiration had an overall negative effect on vegetation and precipitation is a controlling factor for vegetation development in semi-arid areas. Within the period of study, 53% of the vegetated area in NEC showed a trend of improvement, which is mainly attributed to human activities (93%), especially through the implementation of ecological restoration projects in NEC. The relative role of human activities and climate change in vegetation degradation areas were 56% and 44%, respectively. Our findings highlight that the government should more explicitly consider the spatiotemporal heterogeneity of the influence of human activities and water availability on vegetation under changing climate, and improve the resilience of regional water resources. The relative proportions and roles map of climate change and human activities in vegetation change areas provide a basis for government to formulate local-based management policies.