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Projected changes in temperature and precipitation are expected to influence spring and autumn vegetation phenology and hence the length of the growing season in many ecosystems. However, the sensitivity of green-up and senescence to climate remains uncertain. We analyzed 488 site years of canopy greenness measurements from deciduous forest broadleaf forests across North America. We found that the sensitivity of green-up to temperature anomalies increases with increasing mean annual temperature, suggesting lower temperature sensitivity as we move to higher latitudes. Furthermore, autumn senescence is most sensitive to moisture deficits at dry sites, with decreasing sensitivity as mean annual precipitation increases. Future projections suggest North American deciduous forests will experience higher sensitivity to temperature in the next 50 years, with larger changes expected in northern regions than in southern regions. Our study highlights how interactions between long-term and short-term changes in the climate system influence green-up and senescence.

Plain Summary language Vegetation activity is sensitive to climate change, but relatively few studies have quantified this across large spatial extents. We used a large dataset of leaf color derived from digital images to look at how the timing of green-up and fall senescence vary in relation to environmental factors across North America. We found that green-up is more sensitive to temperature at warmer sites, but fall is more sensitive to moisture availability. If the climate continues to change as currently projected, we predict that the timing of spring will exhibit more dramatic swings in onset, but the timing of autumn will likely become less sensitive to moisture availability in the next 50 years.