Intensive ground vegetation growth mitigates the carbon loss after forest disturbance

Slow or failed tree regeneration after forest disturbance is increasingly observed in the central European Alps, potentially amplifying the carbon (C) loss from disturbance. We aimed at quantifying C dynamics of a poorly regenerating disturbance site with a special focus on the role of non-woody ground vegetation.

Soil CO₂ efflux, fine root biomass, ground vegetation biomass, tree increment and litter input were assessed in (i) an undisturbed section of a ~ 110 years old Norway spruce stand, (ii) in a disturbed section which was clear-cut six years ago (no tree regeneration), and (iii) in a disturbed section which was clear-cut three years ago (no tree regeneration).

Total soil CO₂ efflux was similar across all stand sections (8.5 ± 0.2 to 8.9 ± 0.3 t C ha^-1 yr.^-1). The undisturbed forest served as atmospheric C sink (2.1 t C ha^-1 yr.^-1), whereas both clearings were C sources to the atmosphere. The source strength three years after disturbance (-5.5 t C ha^-1 yr.^-1) was almost twice as high as six years after disturbance (-2.9 t C ha^-1 yr.^-1), with declining heterotrophic soil respiration and the high productivity of dense graminoid ground vegetation mitigating C loss.

C loss after disturbance decreases with time and ground vegetation growth. Dense non-woody ground vegetation cover can hamper tree regeneration but simultaneously decrease the ecosystem C loss. The role of ground vegetation should be more explicitly taken into account in forest C budgets assessing disturbance effects.