The Seasonal Divergence in the Weakening Relationship between Interannual Temperature Changes and Northern Boreal Vegetation Activity.

The response of boreal vegetation to global warming has shown a weakening trend over the last three decades. However, in previous studies, models of vegetation activity responses to temperature change have often only considered changes in the mean daily temperature (Tmean), with the diurnal temperature range (DTR) being neglected. The goal of this study was to evaluate the temporal trends of the relationships between two temperature factors (Tmean and DTR) and the vegetation activity across the boreal regions on both annual and seasonal timescales, by simultaneously employing satellite and climate datasets. We found that the interannual partial correlation between the growing season (GS) NDVI and Tmean (RNDVI-Tmean) has shown a significant decreasing trend over the last 34 years. At the seasonal scale, the RNDVI-Tmean showed a significant upward trend in the spring, while in the summer and autumn, the RNDVI-Tmean exhibited a significant downward trend. The temporal trend characteristics of the partial correlation between the NDVI and DTR (RNDVI-DTR), at both the GS and seasonal scales, were fully consistent with the RNDVI-Tmean. The area with a significant decrease in the GS RNDVI-Tmean and RNDVI-DTR accounted for approximately 44.4% and 41.2% of the boreal region with the 17-year moving window, respectively. In stark contrast, the area exhibiting a significant increasing trend in the GS RNDVI-Tmean and RNDVI-DTR accounted for only approximately 22.3% and 25.8% of the boreal region with the 17-year moving window, respectively. With respect to the seasonal patterns of the RNDVI-Tmean and RNDVI-DTR, the area with a significant upward trend in the spring was greater than that with a significant downward trend. Nevertheless, more areas had a significant downward trend in the RNDVI-Tmean and RNDVI-DTR in summer and autumn than a significant upward trend. Overall, our research reveals a weakening trend in the impact of temperature on the vegetation activity in the boreal regions and contributes to a deeper understanding of the vegetation response to global warming.