<b>Frosts </b><b>amplify</b><b> drought thresholds that delay spring green-up in northern ecosystem</b><b>s</b>

Under climate warming, increased frequency of droughts and greater risks of frost events both delay spring green-up dates (GUDs), reducing photosynthetic productivity. However, the quantitative drought thresholds that trigger delayed GUDs (DT-GUDs) and how frost events amplify these thresholds remain unclear. Using drought indices and multiple GUD layers from satellite/ground-based, ecosystem-phenology modelling, and future simulation datasets, we applied a copula-based Bayesian framework to quantify DT-GUDs. Results revealed DT-GUDs ranging from –3.28 ± 0.08 to –1.93 ± 1.35 (mean ± SD), with frost co-occurrence increasing these values by &gt;44%. This indicates that even mild droughts can delay GUDs under frosts. However, vegetation with higher physiological resilience and isohydricity can maintain relatively open stomata during drought stress, ensuring stable photosynthesis and physiological activities and reducing the risk of delayed GUDs under drought conditions. Our findings highlight nonlinear frost-drought interactions, revealing a critical pathway through which compound climate stresses drive spring phenological shifts in northern ecosystems.