Ecosystem carbon partitioning varies without detectable differences in total stocks across fire-frequency gradients in Madagascar’s grassy ecosystems

<title>Abstract</title> <p>Grassy ecosystems are among Earth’s most fire-prone landscapes, yet the consequences of their fire regimes for ecosystem carbon (C) storage remain context dependent and incompletely understood. We assessed associations of fire frequency and seasonality with ecosystem C stocks, allocation and composition, and whether soil fertility accounted for these relationships. Across 60 plots spanning medium- and high-altitude grasslands and tapia savannas in Madagascar’s Central Highlands, we combined fire-history data with measurements of C in woody and ground vegetation, litter, fine roots and topsoil (0–5 cm), soil fertility, and pyrogenic C (PyC). Fire-related patterns were associated with frequency rather than seasonality and were strongest in grasslands. Relative to unburned areas, annually burned areas had 99.9% lower odds of woody C occurrence and contained 41.9% less ground-vegetation C, 55.5% less litter C and 27.7% less fine-root C. Conversely, topsoil C was 15.7% greater and belowground C 12.4% greater, while total measured ecosystem C did not differ detectably. Soil fertility explained variation but was not the dominant explanation for fire–C relationships. Aboveground indicators may therefore misrepresent fire-related C outcomes. Carbon-oriented management and monitoring should consider fire frequency, ecosystem context, and above- and belowground C pools rather than changes in burning seasonality alone.</p>