Seasonal asymmetric changes between photosynthesis and canopy structure phenology across the northern hemisphere

Vegetation phenology is a critical indicator of terrestrial ecosystem responses to climate change. However, our understanding of the asynchrony between photosynthetic and canopy structural phenology remains limited. Using satellite-derived datasets of gross primary productivity (GPP) and leaf area index (LAI) from 1982 to 2018, we systematically quantify this phenological asynchrony across the Northern Hemisphere (>30°N). At the primary 50% relative-amplitude threshold, GPP-derived transition dates preceded the corresponding LAI-derived transition dates by 7.02 d in spring and 17.51 d in autumn on average, although spring offsets were more spatially variable. During the study period, ΔSOS had a trend of −0.584 d decade−1, indicating a widening spring lead of the GPP-derived transition. By contrast, ΔEOS had a trend of + 0.643 d decade−1, indicating a narrowing autumn offset. Boosted regression tree models identified spring temperature as the climate variable most strongly associated with spring asynchrony, whereas autumn asynchrony showed comparable associations with autumn temperature and solar radiation. Our findings provide phenological evidence for a seasonally asymmetric and shifting structure-function relationship in northern terrestrial ecosystems under climate change. Accounting for the asynchrony between photosynthetic and canopy structural phenology is thus essential for advancing satellite-based vegetation monitoring and carbon cycle modelling.