The effect of forest management on trace gas exchange at the pedosphere-atmosphere interface in beech (Fagus sylvatica L.) forests stocking on calcareous soils

The effect of forest management (thinning) on in situ carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) trace gas exchange between soil and atmosphere was studied in three consecutive years at three beech forest sites, which differ in aspect [southwest (SW), northeast (NE), northwest (NW)]. At all sites adjacent thinning plots ("T") and untreated control plots ("C") were established. Measurements at the SW and NE sites covered the years 4-6 after thinning while at the NW site measurements covered the year before and the first 2 years after thinning. Mean N2O fluxes were < 3 mu g N2O-N m(-2) h(-1) at all plots except for the newly thinned NWT plot. CH4 uptake was rather low, too. Very low CH4 oxidation rates during dry periods are explained by physiological drought stress for CH4 oxidizers. Heterotrophic litter decomposition constitutes the largest part of total soil respiration. On the whole, no significant positive or negative effects of the silvicultural treatment on the magnitude of CO2-, CH4- and N2O-trace gas exchange could be observed at the SW site 4-6 years after thinning. Also at the NE site, no effects of thinning on CO2 and N2O fluxes could be demonstrated. However, at this site a significant moisture-induced lower CH4 uptake could be shown. At the NW site forest management led to a dramatic increase in N2O emissions in the first two summers after thinning and to distinct effects on CO2 emissions and CH4 uptake in the first year after the felling. The unambiguous effects of thinning at the NW site are mainly related to higher C input by dead residues leading to enhanced mineralization activity, to a shift in the competition for nutrients favoring microorganisms as compared to trees and to changes in the soil water availability at the thinned plot. Considering the data obtained from the NE and SW site we expect that with the development of an understorey vegetation at the NW site the observed effects on the magnitude of trace gas exchange due to thinning will continue to decline in the following years. Our results implicate that it is indispensable to take account of the effects of forest management in order to accurately calculate trace gas emission inventories for the investigated forest ecosystem in case thinning took place immediately before.