Greening in the middle and high latitudes in the Northern Hemisphere is determined by changes in the growth rate of natural vegetation during the growing season

Natural vegetation is an essential component of terrestrial ecosystems, and its growth has a significant impact on the vegetation greening process. However, the growth rate of natural vegetation throughout the growing season in the Northern Hemisphere's middle and high latitudes (NHMHR) (30 degrees N-90 degrees N) is unknown. Additionally, the mechanics underlying how climatic factors drive changes in vegetation growth rates are unclear. Using the GIMMS normalized difference vegetation index (NDVI) dataset from 1982 to 2015, we examined the growth rate of natural vegetation in the NHMHR (V-NDVI, NDVI difference for two consecutive months) and the influence of several climatic conditions (temperature, precipitation, solar radiation) on the development and senescence of natural vegetation. The results showed that (1) from 1982 to 2015, the obvious greening of natural vegetation in the NHMHR was caused mainly by significant increases in V-NDVI in June and October (1.34 x 10(-4)/yr and 8.21 x 10(-4)/yr, p < 0.01). (2) The slowing of vegetation senescence in October, as well as an increase in defoliation in the final month of the growing season (November), resulted in a significant decline in V-NDVI in November (-1.42 x 10(-3)/yr, p < 0.01). (3) Preseason temperature and precipitation had strikingly varied effects on the V-NDVI during vegetation growth in different areas. Preseason temperature controlled the fluctuation in the V-NDVI from 50 degrees N to 70 degrees N, whereas preseason precipitation had a greater impact in southeastern North America and western Asia. The significant increase in temperature in October was the main factor that slowed the rate of vegetation senescence. Therefore, when simulating the influence of vegetation on climate change, we should consider climate control on the V-NDVI in different seasons. Our results are valuable for measuring how climatic conditions affect vegetation growth and senescence and for systematically clarifying the mechanism of vegetation greening in the future.