Comparing carbon flux and high-resolution spring phenological measurements in a northern mixed forest

► Dominant tree phenology causes overall positive carbon assimilation trends. ► Climate is the cause of day-to-day variations in carbon assimilation. ► Tree phenological variations can be adequately represented with a sample size of 30. ► Visual phenological observations can be related to under-canopy light measurements. ► Sampling every 4 days minimizes data uncertainty and field work expenses.