Labile carbon (C), which is principally comprised of non-structural carbohydrates, is an essential intermediary between C assimilation and structural growth in deciduous forests. We developed a new approach that combined meteorological and biometric C cycling data for a mixed deciduous forest in Michigan, USA, to provide novel estimates of whole-ecosystem labile C production and reallocation to structural net primary production (NPP). We substantiated inferred seasonal patterns of labile C production and reallocation to structural NPP with measurements of Populus grandidentata and Quercus rubra wood non-structural carbohydrate concentration and mass over two years. Our analysis showed that 55% of annual net canopy C assimilate (Ac) was first allocated to labile C production rather than to immediate structural NPP. Labile C produced during the latter half of summer later supported dormant-season structural growth and respiration, with 34% of structural NPP in a given year requiring labile C stored during previous years. Seasonal changes in wood non-structural carbohydrate concentration and mass generally corroborated inferred temporal patterns of whole-ecosystem labile C production and reallocation to structural NPP. Our findings confirm that disparities can arise between same-year meteorological and biometric net ecosystem production when meteorologically measured C assimilation and biometrically measured growth are asynchronous because of temporary photosynthate allocation to labile C storage. We conclude that a broader understanding of labile C production and reallocation at the ecosystem scale is important to interpreting lagged canopy C cycling and structural growth processes.