Long-Term Trends and Interannual Coupling of Spring Green-Up Onset and Canopy Maturation in Northern China’s Forests

The start of season (SOS) is widely used to assess forest phenological responses to climate change, but how well it represents subsequent canopy greening remains unclear. Using GIMMS NDVI3g and climate data for 1983–2014, we examined greenness-defined SOS and the start of canopy maturity (SOM) across deciduous needleleaf (DNF), mixed (MF), and deciduous broadleaf (DBF) forests in northern China, and assessed the additional explanatory information provided by seasonal climate beyond SOS. After detrending, SOS and SOM were positively correlated in all three forest types (r = 0.57–0.80, all p < 0.01). However, long-term trend estimates showed regional differences: SOS and SOM tended to advance in DNF and DBF, whereas SOS tended to advance and SOM to be delayed in MF. SOS alone accounted for 34.7–58.3% of the interannual variation in SOM; seasonal climate explained additional SOM variation in DBF after accounting for SOS. Thus, although spring green-up onset and later canopy greening are significantly correlated at the interannual scale, they do not necessarily follow parallel long-term trajectories. Incorporating both spring onset and subsequent canopy greening, together with their seasonal climate associations, into phenological models may improve projections of forest spring development under climate change.