The effect of stand age on CO2 efflux from wood ant (Formica rufa group) mounds in boreal forests

Recent studies suggest that wood ants (Formica rufa group) mounds are point sources of carbon dioxide (CO2), which increase the heterogeneity of soil carbon (C) emissions in forest ecosystems. However, little is known about the impact of anthropogenic activities, such as logging and subsequent forest succession, on these fluxes. In this study, we measured the CO2 efflux and temperature of wood ant mounds and the surrounding forest floor in managed Finnish boreal forests of different ages (5, 30, 60, and 100 years old) to assess how the effluxes vary with stand age. We conducted efflux measurements from the mounds and the surrounding forest floor throughout the ants' active season (May–September) and during the onset of hibernation (October). The annual CO2 efflux was then estimated using mound or forest floor temperatures, which were measured for one year. The average annual CO2 efflux from the ant mounds was 10.2 (±5.8 SD) kg m−2 year−1, increasing from 3.9 (±0.3 SD) kg m−2 year−1 in the 5 year-old stands to 14.3 (±3.0 SD) kg m−2 year−1 in the 100 year-old stands. Temperatures was significantly higher in the ant mounds than in the forest floor, and the average temperature difference between mounds and forest floor increased with stand age, being the lowest in the 5 year-old (4.1 (±3.1 SD) °C) and highest in the 100 year-old stands (10.3 (±5.2 SD) °C). There were no statistical differences in the mound CO2 efflux per volume among forest age classes, suggesting higher ant CO2 efflux in the older stands likely come from larger ant populations in the bigger mounts. The different mound temperature regimes among stand age classes indicates that the activity of wood ants changes with forest succession, particularly after clear-cutting, which alters CO2 efflux from the mounds. The impact of ant mounds on total CO2 efflux from the soil, estimated from mound area and volume, respectively, increased with forest age, from 0.05 (±0.05 SD) % to 0.31 (±0.18 SD) % and from 0.05 (±0.06 SD) % to 0.90% (±1.11 SD).

► We found that the mound CO2 effluxes increased with stand age.
► Forest clear-cutting might be a reason of the decrease in mound CO2 effluxes.
► The impact of ant mounds on the total soil CO2 efflux also increased with forest age.
► Stand age could affect the functional role of ants in forest ecosystems.