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Temperature is a dominant factor driving arctic and boreal ecosystemphenology, including leaf budburst and gross primary production (GPP) onset in Alaskan spring. Previous studies hypothesized that both accumulated growing degree day (GDD) and cold temperature (chilling) exposure are important to leaf budburst.We test this hypothesis by combining both satellite and aircraft vegetation measurements with theCommunity LandModel Version 4.5 (CLM), in which the end of plant dormancy depends on thermal conditions (i.e. GDD).We study the sensitivity of GPP onset of different Alaskan deciduous vegetation types to aGDDmodel with chilling requirement (GCmodel) included. The default CLM simulations have a 1–12 d earlier day of yearGPP onset over Alaska vegetated regions compared to satellite constrained estimates from the Polar Vegetation Photosynthesis andRespirationModel. Integrating aGCmodel intoCLMshifts the phase and amplitude ofGPP. During 2007–2016,meanGPPonset is postponed by 5±7, 4±8, and 1±6 d over Alaskan northern tundra, shrub, and forest, respectively. TheGCmodel has the greatest impact duringwarm springs, which is critical for predicting phenology response to future warming.Overall, springGPP high bias is reduced by 10%. Thus, including chilling requirement in thermal forcingmodels improves northern high-latitude phenology, but leads to other impacts during the growing seasonwhich require further investigation.

Exposure to cold temperature affects the spring phenology of Alaskan deciduous vegetation types