The variation in dynamics of translocation between above- and below-ground biomass of Spartina alterniflora, the dominant blue carbon source in North American saltmarshes, was studied across latitude using Phenology-based Growth dynamic model (PG model). The study shows that the main sources of the carbon translocation to the below-ground tissues varies with latitude. The model analysis suggests both photosynthates and the remobilization of assimilates during growing and senescing periods serve as the main sources of the carbon translocation from above-to below-ground tissues in a higher latitude. However, in the lower latitude regions with a warmer environment, the main source to build up the below-ground biomass was the immediate photosynthesis that occurred during growing seasons. The total photosynthates translocation from above-to below-ground tissues during growing seasons increase as the latitude decreases, whereas the assimilates translocation from the senescing shoots to below-ground during fall seasons increases as latitude increases. Assimilates are allocated from below-to above-ground tissues during the dormancy period in higher latitude. The model enables us to predict both above- and below-ground biomass and quantify the carbon translocation, which helps us understand the main sources of allocation to the below-ground tissues, a critical component of potential blue carbon sequestration, at different phenological events.