How Do Climatic and Non-Climatic Factors Contribute to the Dynamics of Vegetation Autumn Phenology in the Yellow River Basin, China?

Autumn phenology is considered to be an important indicator of ecosystem carbon cycle processes, yet, its driving factors are not well-understood. Using the Normalized Difference Vegetation Index (NDVI) data from 1982 to 2015, this study investigated the dynamics of the end date of vegetation growing season (EGS) in the Yellow River basin (YRB) and quantified the impacts of both climatic and non-climatic drivers on the EGS. Our results show that the EGS was delayed by 5.6 days in the earlier period (1982-1999), and it was further delayed by another 3.4 days in the later period (2000-2015). Based on partial derivatives method, during the entire period (1982-2015), the average contributions of daytime temperature (T-ma(x)), nighttime temperature (T-min), precipitation and solar radiation to EGS trends across the YRB were -0.64, 1.69, 0.52, and 0.30 days/10 yr, respectively. T-min was the climatic factor that provided the highest explanation of the EGS trends. However, for the two subperiods, the dominant factor controlling EGS trends has changed from T-min to precipitation. Nonclimatic factors were found to have a strong positive impact on the EGS after 2000 as a result of increased PM2.5 concentration in the previous winter. This study provides the first quantitative insights into spatiotemporal differences in autumn phenology within a large fluvial basin context.