Snowmelt timing has a stronger effect on northern spring green-up in non-forest than forest ecosystems

In high-latitude regions, snow dynamics influence plant phenology and ecological processes, ultimately feeding back to the climate system. However, it remains unclear how the timing of snow melt regulates the start of the growing season (SOS) across northern forest and non-forest ecosystems, and how these effects depend on moisture and temperature regimes. Here, we combine satellite-derived plant phenology with reanalysis snow and environmental datasets spanning 2001–2024 across the Northern Hemisphere north of 50°N to better understand these mechanisms. We partitioned the pre-growing season into three cascading phases: (P1) pre-melt snow accumulation, (P2) snowmelt, and (P3) post-melt vegetation activation. Linear mixed-effects models show that air temperature during P3 explains the largest share of SOS variation (β≈ -0.61, p < 0.001), while the end of snowmelt represents a key physical threshold for SOS, particularly in non-forest ecosystems (β ≈ 0.1, p < 0.001). Overall, environmental conditions during P3 exert the strongest control on spring green-up, being more than twice as important than that of earlier stages (P1 and P2). Using piecewise structural equation modeling, we identify different ecological strategies between ecosystems: Non-forest ecosystems exhibit greater sensitivity to snow melt timing, with an effect size about three times greater than in forests. These findings indicate that forests are comparatively less sensitive to snowmelt timing, whereas non-forest ecosystems remain highly vulnerable to shifts in snowmelt timing and the associated changes in temperature and moisture conditions.