Nonstationary linkage between summer warmth index and NDVI at high latitudes in Eurasia

Abstract The global increase in vegetation photosynthetic activity under the warming climate, commonly referred to as ‘greening’, has been a hot topic, especially in the high latitudes. However, in this study, it was found that, within the region of 60°–70°N and 110°–150° E, the interannual relationship between summer normalized difference vegetation index (NDVI) and summer warmth index (SWI) changes from being statistically positive in P1 (1982–2010) to statistically negative in P2 (2011–2021). Also, the interannual relationship between summer NDVI and summer soil moisture changes from being negative in P1 to positive in P2. The reason and possible mechanisms were investigated. On the one hand, the atmospheric vapor pressure deficit (VPD) has increased significantly in P2 corresponding to the increasing SWI, and the interannual relationship between the VPD and NDVI transforms into a significantly negative one. This is because, when the atmospheric VPD increases, leaf and canopy photosynthetic rates decline owing to stomatal closure, to protect vegetation from losing too much water. Therefore, the interannual relationship between the NDVI and VPD, and in turn the SWI, transforms into a significantly negative one in P2. On the other hand, it was found that the surface evapotranspiration is energy-limited in P1, but then with the decreasing soil moisture content it becomes soil-moisture-limited in P2. As one of the most important components of surface evapotranspiration, vegetation evapotranspiration is also limited by soil moisture. Therefore, the interannual relationship between soil moisture and NDVI becomes significantly positive in P2.