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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. Key Points: Green‐up is controlled by energy‐related drivers, but senescence is controlled by a combination of moisture‐ and energy‐related driversSpring green‐up is more sensitive to warming in warmer areasAutumn senescence is more sensitive to moisture deficit in dry areas [ABSTRACT FROM AUTHOR]

The timing of leaf emergence in temperate and boreal forests is changing, which has profound implications for a wide array of ecosystem processes and services. Spring phenology models, which have been widely used to predict the timing of leaf emergence, generally assume that a combination of photoperiod and thermal forcing control when leaves emerge. However, the exact nature and magnitude of how photoperiod and temperature individually and jointly control leaf emergence is the subject of ongoing debate. Here we use a continuous development model in combination with time series of land surface phenology measurements from MODIS to quantify the relative importance of photoperiod and thermal forcing in controlling the timing of canopy greenup in eastern temperate and boreal forests of North America. The model accurately predicts biogeographic and interannual variation in the timing of greenup across the study region (median RMSE = 4.6 days, median bias = 0.30 days). Results reveal strong biogeographic variation in the period prior to greenup when temperature and photoperiod influence greenup that covaries with the importance of photoperiod versus thermal controls. Photoperiod control on leaf emergence is dominant in warmer climates, but exerts only modest influence on the timing of leaf emergence in colder climates. Results from models estimated using ground-based observations of cloned lilac are consistent with those from remote sensing, which supports the realism of remote sensing-based models. Overall, results from this study suggest that apparent changes in the sensitivity of trees to temperature are modest and reflect a trade-off between decreased sensitivity to temperature and increased photoperiod control, and identify a transition in the relative importance of temperature versus photoperiod near the 10 °C isotherm in mean annual temperature. This suggests that the timing of leaf emergence will continue to move earlier as the climate warms, and that the magnitude of change will be more pronounced in colder regions with mean annual temperatures below 10 °C. • A Bayesian model is developed to study environmental controls on spring greenup. • Photoperiod and temperature exert major controls on the timing of spring greenup. • Photoperiod control is dominant in warmer climates with shorter pre-season periods. • Greenup in colder ecosystems will continue to move earlier as the climate warms. [ABSTRACT FROM AUTHOR]