Limited increases in savanna carbon stocks over decades of fire suppression

Savannas cover a fifth of the land surface and contribute a third of
terrestrial net primary production, accounting for three quarters of
global area burned and over half of global fire-driven carbon emissions.
Fire suppression and afforestation have been proposed as tools to increase
carbon sequestration in these ecosystems. A robust quantification of
whole-ecosystem carbon storage in savannas is lacking, however, especially
under altered fire regimes. Here, we provide the first direct estimates of
whole-ecosystem carbon response to over 60 years of fire exclusion in a
mesic African savanna. We found that fire suppression increased
whole-ecosystem carbon storage by only 35.4 ± 12% (mean ± standard error),
even though tree cover increased by 78.9 ± 29.3%, corresponding to total
gains of 23.0 ± 6.1 Mg C ha-1 at an average ~0.35 ± 0.09 Mg C ha-1 yr-1,
more than an order of magnitude lower than previously assumed. Frequently
burned savannas had substantial belowground carbon, especially in biomass
and deep soils. These belowground reservoirs are not fully considered in
afforestation or fire suppression schemes but may mean that the decadal
sequestration potential of savannas is negligible, especially weighed
against concomitant losses of biodiversity and function.