<p>Most savannas and grasslands worldwide are sustained by frequent fires that topkill perennial plants that resprout from belowground organs, emphasizing the need to understand the belowground component of such communities in the context of fire regimes. We used a long-term fire frequency experiment in a native upland longleaf pine (<i>Pinus palustris</i>) savanna in Georgia, USA to study relationships among aboveground and belowground biomass among plant life-forms and fire frequencies. We measured aboveground biomass and belowground biomass of plants categorized by size class and life-form (pine, other woody, forb, graminoid) in 0.1 m² subplots within plots treated with fire at 1–4 year intervals for two decades, and we compared subplots with and without wiregrass (<i>Aristida beyrichiana</i>). Fire frequency had little effect on the measured variables, suggesting that the belowground system responds slowly to changes in fire frequency. Woody + forb belowground biomass showed negative correlations with graminoid aboveground and belowground biomass and was lower where wiregrass was present, suggesting competitive interactions between graminoids and woody/forb species at small spatial scales. Aboveground and belowground biomass in the studied pine savanna were within the range of measurements in other humid savannas systems worldwide. Results from our study reinforce findings in other frequently burned humid savannas with high belowground to aboveground biomass ratios, local inverse relationships between grass and woody plant belowground biomass, and coexistence of perennial grasses, forbs, and woody plants through the equalizing effect of topkilling by fire, which contribute to the stability of these old-growth grassy ecosystems.</p>