Spatial synchrony in the start and end of the thermal growing season has different trends in the mid-high latitudes of the Northern Hemisphere

Changes in spatial synchrony in the growing season have notable effects on species distribution, cross-trophic ecological interactions and ecosystem stability. These changes, driven by non-uniform climate change were observed on the regional scale. It is still unclear how spatial synchrony of the growing season on the climate gradient of the mid-high latitudes of the Northern Hemisphere and ecoregions, has changed over the past decades. Therefore, we calculated the start, end, and length of the thermal growing season (SOS, EOS, and LOS, respectively), which are indicators of the theoretical plant growth season, based on the daily-mean temperature of the Princeton Global Forcing dataset from 1948 to 2016. Spatial variations in the SOS, EOS and LOS along spatial climate gradients were analyzed using the multivariate-linear regression model. The changes of spatial synchrony in the SOS, EOS and LOS were analyzed using the segmented model. The results showed that in all ecoregions, spatially, areas with higher temperature tended to have an earlier SOS, later EOS and longer LOS. However, not all the areas with higher precipitation tended to have a later SOS, later EOS, and shorter LOS. The spatial synchrony in the SOS decreased across the entire study area, whereas the EOS showed the opposite trend. Among the seven ecoregions, spatial synchrony in the SOS in temperate broadleaf/mixed forests and temperate conifer forests changed the most noticeably, decreasing in both regions. Conversely, spatial synchrony in the EOS in the taiga, temperate grasslands/savannas/shrublands and tundra changed the most noticeably, increasing in each region. These may have important effects on the structure and function of ecosystems, especially on the changes in cross-trophic ecological interactions. Moreover, future climate change may change the spatial synchrony in the SOS and EOS further; however, the actual impact of such ongoing change is largely unknown.