Recent spring phenology shifts in western Central Europe based on multiscale observations

Aim: Although numerous studies have reported advanced Northern Hemisphere spring phenology since the 1980s, recent studies based on remote sensing have reported a reversal or deceleration of this trend. This study aimed (1) to fully understand recent spring phenology shifts using both in situ observations and satellite-based normalized difference vegetation index (NDVI) datasets, and (2) to test whether the NDVI methods capture the trends observed in situ. Location: Western Central Europe. Methods: Temporal spring phenology trends (leaf unfolding dates) were examined using 1,001,678 in situ observations of 31 plant species at 3984 stations, as well as NDVI-based start-of-season dates, obtained using five different methods, across the pixels that included the phenology stations. Results: In situ and NDVI observations both indicated that spring phenology significantly advanced during the period 1982-2011 at an average rate of -0.45 daysyr^-1. This trend was not uniform across the period and significantly weakened over the period 2000-2011. Furthermore, opposite trends were found between in situ and NDVI observations over the period 2000-2011. Averaged over all species, the in situ observations indicated a slower but still advancing trend for leaf unfolding, whereas the NDVI series showed a delayed spring phenology. Main conclusions: The recent trend reversal in the advancement of spring phenology in western Central Europe is likely to be related to the response of early spring species to the cooling trend in late winter. In contrast, late spring species continued to exhibit advanced leaf unfolding, which is consistent with the warming trend during spring months. Because remote sensing does not distinguish between species, the signal of growing-season onset may only reflect the phenological dynamics of these earliest species in the pixel, even though most species still exhibit advancing trends.