Emerging evidence for delaying effect of winter warming on green-up onset in alpine grasslands on the Tibetan Plateau

Experimental and observational evidence indicates that spring warming advances the green-up onset of woody plants, while winter warming delays it. However, evidence for herbaceous plants is limited, leaving a gap in our understanding of how their green-up onset respond to winter warming, which complicates predictions of phenological shifts under long-term climate change. Particularly in the alpine grasslands of the Tibetan Plateau (TP), there are strong debates over whether green-up onset is influenced by winter temperatures. We conducted a three-year in situ manipulative winter warming experiment in an alpine grassland in the central TP (4550 m above sea level) and found that the experimental winter warming delayed green-up onset for three out of four species (three dominant and one common in the TP alpine grasslands) and for the community, by reducing chill accumulation and increasing growing degree-days. This effect was more pronounced in species with earlier green-up onset, indicating a stronger reliance on chilling cues to avoid frost risk. However, the convergent cross mapping method did not detect a causal effect of winter temperature on green-up onset from long-term ground and satellite observations across the TP. Our findings indicate the impacts of winter warming on the green-up onset of alpine herbaceous plants on the TP with implications for improving phenology models.