Tropical and subtropical drylands dominate the interannual variability in global net terrestrial carbon flux irrespective of different global ecosystem classifications or geographic scales

Abstract

Global terrestrial ecosystems exhibit substantial interannual variability (IAV) in net carbon (C) flux. Determining the biogeographic origin of this variability is essential for the understanding and forecasting of global C cycling and carbon-climate feedbacks. Currently, most studies identify either global drylands or moist tropical forests as the dominant source of IAV. Considering this, we investigated whether the use of three different global ecosystem classifications of drylands and moist tropical forests, as well as two alternative geographical scales, could alter which ecosystem is the dominant contributor to terrestrial net C flux IAV. Using the simulation results of 18 dynamic global vegetation models from the TRENDY v11 model intercomparison, we calculated the absolute and area-weighted contributions of net C flux IAV for: individual 0.5° grid cells, global ecosystem classifications, and ecoregions (intermediate scale between grid cells and global ecosystems). For all three of the global ecosystem classification schemes, we found the drylands IAV contributions of 41%, 32%, and 37% were significantly greater than the associated IAV contributions of 20%, 19%, and 24% from the moist tropical forests (p < 0.001). However, the moist tropical forests had a higher IAV contribution per unit area across all three classification schemes (∼3% versus ∼1%–2% (p < 0.001)). At the ecoregion scale, this switch between drylands and moist tropical forests was absent; as seven of the ten highest absolute and nine of the ten highest area-weighted contributing ecoregions were drylands. Specifically, we found tropical and subtropical grasslands, savannas, and shrublands to be particularly substantial contributors to global terrestrial net C flux IAV, with the Cerrado’s absolute IAV contribution of 3.52% exceeding all but one of the other 763 global ecoregions IAV contributions (all p < 0.05). Our findings demonstrate that drylands persist as the dominant contributor to global terrestrial net C flux IAV, irrespective of different global ecosystem classifications or geographic scales.

Recommended citation: Bogucki, L., Feldman, A., Moore, D.J.P., Amaral, C., Wang, L., Green, J., Babst, F., Ojima, D., Pervin, R., Reed, S., Smith, W.K., Zhang, W. and MacBean, N. (2026) Tropical and subtropical drylands dominate the interannual variability in global net terrestrial carbon flux irrespective of different global ecosystem classifications or geographic scales. Environmental Research: Climate, 5, 035049.
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