Net ecosystem exchange over a non-cleared wind-throw-disturbed upland spruce forest—Measurements and simulations

• EC-fluxes of CO2, energy and water over a wind-throw-disturbed spruce forest.
• 2–6 years after disturbance GEP shows positive trend while Reco is highly variable.
• Annual net C-balance indicates steady trend toward switch from C-source to C-sink.

Net ecosystem exchange (NEE) was measured in a wind-throw-disturbed upland spruce forest in the Bavarian Forest National Park (Germany) continuously over four years from 2009 to 2013 by the eddy-covariance method. Estimated annual NEE (positive values stand for a net carbon source) of the non-cleared wind-throw resulted in 347 ± 104, 255 ± 77, 221 ± 66, 240 ± 52, and 167 ± 50 gC m−2. However, two to six years after the storm event (windstorm Kyrill, January 2007) GEP was already strong, increasing from 393 (2009) to 649 gC m−2 yr−1 (2013). Ecosystem respiration showed a high inter-annual variability during the measurement period, ranging from 656 to 816 gC m−2. Carbon dioxide (CO2) fluxes during snow-covered periods averaged about 0.8 μmol m−2 s−1 with only little variation.The contributions of spruces and grasses to the overall carbon exchange, and the differentiation into autotrophic and heterotrophic respiration were estimated by the biogeochemical model LandscapeDNDC (formerly MoBiLE). Comparisons with observations indicate that the model represents gross primary productivity very well, but underestimates ecosystem respiration during early spring and late autumn, and thus tends to diverge from measurements over multi-year simulation periods.These results show that (1) low productivity mountain forest sites may switch from a carbon source to a carbon sink within relatively few years after disturbance, and (2) model uncertainties are most prominently related to soil respiration, decomposition of coarse woody debris, and succession of ground cover species.