Recent developments in Earth System Models have granted researchers attempting to model global climate change significant new ability (Fisher et al., 2017). These models use carbon dioxide output and sequestration rates to calculate atmospheric CO2 levels and the Earth’s potential to trap heat (Dybzinski, 2019). However, the models are only as good as the assumptions they make; due to a lack of research into the topic, different terrestrial models commonly make contradictory assumptions about the roles of nitrogen availability and fine root mass in a tree’s rate of nitrogen uptake, leading to inaccuracy and inconsistency (Dybzinski et al., 2019). As most of North American tree growth is nitrogen-limited, the rate at which a particular species is able to absorb nitrogen is critical to its ability to grow and take up carbon--and to the models aiming to predict these rates. In this study, we attempted to quantify the true roles of each factor in tree nitrogen uptake rate and predicted that nitrogen availability would have far greater effect than fine root mass. In 18 single-species plots at the Morton Arboretum, we obtained the nitrate and ammonium availability per area, fine root mass of target species per area, and nitrogen uptake rate of the plot’s target species trees per area. We found that there is blankity blank relationship between fine root mass and uptake rate and absolutely blank relationship between nitrogen availability and uptake rate. We hope that these results will be incorporated into existing ESM models to allow for more accurate assessments of forests across the world and to inform efforts to understand global carbon sequestration.