Greater temperature sensitivity of vegetation greenup onset date in areas with weaker temperature seasonality across the Northern Hemisphere

The temperature sensitivity (S_T) of the vegetation greenup onset date (VGD) is critical for the assessment and prediction of phenological response to climate warming. Spatial variations in S_T, though well documented based on satellite observations in recent studies, have remained largely under-explained. We determined VGD from satellite observation of the normalized difference vegetation index over the period 2000–2018 and showed that S_T is substantially smaller in areas with stronger temperature seasonality (i.e. the multiyear mean of standard deviations of monthly mean temperature over a 12-month period). Spatially, S_T averaged over each 2 °C bin of temperature seasonality increased from about −10.0 days K⁻¹ in areas with temperature seasonality of about 3.5 °C to about −3.3 to −2.3 days K⁻¹ in areas with temperature seasonality between 15.0 and 22.3 °C. Statistical analyses suggest that this pattern was not related with spatial variations in the length of chilling exposure, spring frost risk, inter-annual variability of spring temperature, or photoperiod. Rather, it may occur because areas with stronger temperature seasonality have stronger pre-VGD constraints because of a lower temperature and a faster temperature increase from the onset of greenup to summer, which could result in faster leaf development rates and thus higher speeds of completion of leaf development. Our results also show that the greater S_T associated with higher mean annual temperature and multiyear mean VGD across the northern middle and high latitudes observed in previous studies is probably caused by temperature seasonality. This study deepens our understanding of the spatial variation in the temperature sensitivity of vegetation spring leafing phenology in the Northern Hemisphere.