Effects of stand patchiness due to windthrow and bark beetle abatement measures on soil CO2 efflux and net ecosystem productivity of a managed temperate mountain forest

In central European mountain forests, bark beetle abatement measures implemented after wind and bark beetle disturbances represent a well-defined disturbance regime. We used a biometric approach extended by soil CO2 efflux measurements to study the effects of disturbance-induced stand patchiness on the net ecosystem productivity (NEP) of a mature Norway spruce (Picea abies L. Karst.) forest at the Integrated Monitoring (IM) Long Term Ecological Research (LTER) site Zobelboden, Austria. In the disturbed stand area, roughly 28 % of the spruce trees were salvaged after an initial disturbance and during small-scale bark beetle abatement actions in 2007 and 2008. Aboveground C stocks declined by approximately 33 %, resulting in a 31 and 29 % reduction of biomass NPP and litterfall during the study year 2011. Soil CO2 efflux in the disturbed area (7.5 +/- A 0.5 t C ha(-1) y(-1)) was significantly lower than in the undisturbed area (9.3 +/- A 0.6 t C ha(-1) y(-1)). The difference was mainly attributed to the fact that the autotrophic contributions in the disturbed area were lower (22 %) than in the undisturbed area (31 %). Heterotrophic soil respiration was similar in both stand areas (5.9 t C ha(-1) y(-1) in the disturbed and 6.5 t C ha(-1) y(-1) in the undisturbed area). Four years after the initial disturbance, the NEP estimated for the undisturbed area was 0.68 and 0.09 t C ha(-1) y(-1) for the disturbed stand area. Although disturbance and abatement measures substantially lowered the NEP, the C uptake of the remaining trees, along with their dampening effect on soil temperature, ensured that the disturbed forest still served as a weak C sink.