Slowdown of spring green-up advancements in boreal forests

There is a consensus that the spring phenology of deciduous forests is advancing in response to global warming. Since the late 1990s, however, this tendency of spring phenology advancement has been weakened in over 60% of boreal forests, particularly in Siberia (−0.58 day yr−1 for 1982-1997 vs. −0.17 day yr−1 for 1982-2013) and northwestern North America (NWNA; −0.42 day yr−1 for 1982-1997 vs. 0.07 day yr−1 for 1982-2013). This study investigated the major factor in the weakening trends in the advancement of the start of the growing season (SOS) based on the satellite-observed normalized difference vegetation index (NDVI) in two regions by quantifying the effects of four climatic fields-winter duration (WD; the number of freezing days), pre-season temperature (PT; accumulated temperature from late winter to early spring), green-up temperature (GT; accumulated temperature around the green-up date), and pre-season precipitation (PR; accumulated precipitation before the green-up date)-on changes in the spring green-up trend. The GT explained the majority of the slowdown in the SOS trends in recent decades. In Siberia, the GT increases contributed to the advancement of the SOS during the 1980s and 1990s; however, the GT increase reduced to less than half of these periods resulting in a slowdown of the SOS advancing trend since the early 2000s. In NWNA, GT increases and WD shortening drove the SOS advancement until the late 1990s; however, both effects have been diminished to near zero to result in no further SOS advancements. This study demonstrates that the recent slowdown of the SOS advancing trends over Siberia and NWNA was largely attributed to the weakening of the warming trends, rather than the sensitivity changes of spring phenology to climate variables. This study suggests that the natural climate variability strongly affects the decadal variations in the boreal forest spring phenology.