Climate and vegetation jointly govern continental-scale patterns of plant phenology and vegetation greenness

Phenological change is among the clearest biological fingerprints of climate change, shaping carbon cycling, ecosystem productivity, and trophic interactions. In this work, we synthesize more than 2,400 site-years of high-frequency phenocam imagery—from the PhenoCam Network, Australia’s Terrestrial Ecosystem Research Network, and Europe’s Integrated Carbon Observation System—to evaluate continental-scale patterns in phenology and vegetation greenness. We show that phenological and greenness metrics differ markedly across primary vegetation types and climate zones, with baseline phenology varying strongly across vegetation–climate combinations, revealing substantial ecological structuring of seasonal dynamics. In contrast, temporal trends are generally modest, heterogeneous, and seldom statistically distinguishable from zero; only a small number of vegetation–climate subgroups display detectable directional change. Generalized additive models fitted to multi-year mean site values indicate that climate and vegetation type together explain up to 58% of cross-site deviance, and that climate–vegetation interactions improve model performance by ~10% on average—most strongly for length-of-season metrics. Because Earth system models depend on realistic seasonal dynamics to constrain carbon–climate feedbacks, our results identify where model representation most needs improvement and which components of vegetation seasonality are most sensitive to climate forcing. Taken together, the findings suggest that spatial variation in plant phenology is strongly governed by vegetation–climate coupling, whereas coherent phenological shifts over time have yet to emerge at continental scales. We highlight the value of harmonized phenocam data for detecting early signals of ecological change and the need for continued international coordination toward a global phenocam dataset.