Parallel acceleration of vegetation growth rate and senescence rate across the Northern Hemisphere from 1982 to 2015

Growth and senescence rates are critical ecological indicators of seasonality shifts of vegetation, with both sensitive to climate change. Here we investigated daily mean vegetation growth and senescence rates, and the major climate forcing across the Northern Hemisphere (>30 degrees N) using satellite-derived normalized difference vegetation index (NDVI) and flux-based gross primary productivity (GPP) from 1982 through 2015. Both growth and senescence rates are higher at high latitudes than those at low latitudes, with spatially-averaged values increased by 1.0 x 10(-4) and 0.7 x 10(-4) NDVI-unitsday(-1) per degree latitude. These increases were greater in Eurasia than in North America. A parallel acceleration of growth (0.8 x10(-4) NDVI-unitsday(-1)decade(-1)) and senescence (0.6 x10(-4) NDVI-unitsday(-1)decade(-1)) rates was found for the 34-year study period. The warming-induced increases in vegetation peak growth (peak NDVI) contributed strongly to this parallel acceleration, while unequal advances or delays of three key phenological indicators (the start (SOS), peak (POS), and end (EOS) of the growing season) exerted influential effects on the rates. However, no single climatic factor during any period appeared responsible for the variations in growth and senescence rates. In areas with growth and senescence rates that are determined by peak growth, temperature and precipitation during the growth period accelerated both rates through elevating peak growth. On the other hand, in areas with growth rate determined by SOS, rising temperature before SOS decelerated the growth rate by advancing SOS. In areas with senescence rate determined by EOS, both temperature and radiation during the senescence period contributed to changes in senescence rate by influencing EOS. In sum, a central focus should be placed on the linkages among climate, phenology, and growth and senescence rates for quantifying vegetation seasonality and associated ecosystem function under the changing climate.