Impact of forest disturbances on decomposition and soil nutrient release

Mountain ecosystems are especially vulnerable to climate change, since these areas will experience stronger temperature fluctuations than the global climate. As a consequence, the intensity and frequency of natural forest disturbance in Central Europe is set to increase, raising serious questions over the future provision of key ecosystem services. Stand-replacing forest disturbance (e.g., windthrow, insect infestations, and harvest) can cause abrupt and long-lasting changes in ecosystem structure and function over huge spatial areas, but empirical data on disturbance impacts on nutrient cycling is lacking, particularly in the context of the Limestone Alps. In these mountains, shallow soils with only small pools of exchangeable nutrients can frequently be met on pure limestone or dolomite and nutrient cycling is restricted to the organic soil layer and the vegetation. Thus, the interruption of the nutrient uptake by the arboreal vegetation after disturbance may cause accelerated nutrient losses with seepage water, associated with detrimental effects on nutrition and productivity status of the regenerating forest. A potential disturbance-driven nutrient loss may add to nitrate levels in drinking water, already critical from the viewpoint of human health. We propose to study disturbance impacts on decomposition, mineralization and soil nutrient release at forested manipulation sites in Upper Austria, which will be artificially disturbed after a short pre-treatment period. Replacement disturbances will be mimicked by clearcut with and without deadwood left on site, and by girdling. We hypothesize that i) forest disturbances affect nutrient release and retention and may cause severe depletions of nutrient pools, ii) rates of decomposition and mineralization (processes, releasing nutrients) are different between leaf litter, root litter, hol organic- and mineral soil, and iii) a net soil carbon source or sink can be attributed to individual biogeochemical processes and soil depths as function of the specific forest disturbance regime. After measuring nutrient losses in the field via input-output balances (solute fluxes via throughfall and soil solution) over 2.5 years (and relating them to nutrient soil and biomass pools), we will explain how disturbance affects nutrient cycling. Field incubations of leaf- and root litter in mesh bags, as well as holorganic and mineral soil in polyethylene bags, will be used to identify possible sources of nutrient release. Finally, we try to calculate a relatively short term soil carbon balance. Our goals are to i) develop guidelines for post-disturbance management of nutrient release and retention after ii) elucidating the impact of forest disturbances on soil biogeochemical processes, and iii) estimating net soil C sequestration under varying disturbance regimes. Investigating sites with and without deadwood left on site, the project will reveal how post-disturbance management can influence ecosystem nutrient loss.

The intensity and frequency of natural forest disturbance is set to increase, raising serious questions over the future provision of key ecosystem services. Forest disturbance (e.g., windthrow, insect infestations) can cause abrupt changes of ecosystem functions, but empirical data are lacking, particularly in the context of the Limestone Alps. In these mountains, shallow soils with only small pools of exchangeable nutrients can frequently be met on pure limestone or dolomite and nutrient cycling is restricted to the organic soil layer and the vegetation. Thus, the interruption of the nutrient uptake by vegetation after disturbance may cause accelerated nutrient losses with seepage water, associated with detrimental effects on nutrition and productivity status of the regenerating forest. Hence, we mimicked disturbances of a beech forest by clear cut, with and without deadwood left on site, and by girdling of trees to study impacts on decomposition, mineralization and soil nutrient release at forested manipulation sites in Upper Austria. Depending on type of harvest, removed nutrients were expressed in percent of the remaining soil nutrient stocks. Values greater than 50 % are critical, since nutrient supply would not be sufficient for the next forest regeneration without external inputs. This limit value was exceeded for potassium, because weathering supply is negligible on pure limestone bedrock. Hence, harvest must be restricted to (thick) stem wood only and foliage and branches have to remain on the cut stand. Debarking the stem on the clear cut area improves the nutrient balance markedly. Deposition of nitrogen in the open (11 to 14 kg/ha/year) and sulfur (3 to 4 kg/ha/year) was low. Soil solution was sampled bi-weekly and chemically analyzed. All artificial treatments caused increased soil solution nitrate concentrations. Mean concentrations of 30 mg/l were recorded during the first vegetation period after the harvest, already critical for human health. Woody debris left on site (branches and twigs with foliage) caused higher nitrate peaks than cleared clear cut sites. Girdling, simulating, e.g., bark beetle attack, caused longer lasting soil leaching effects. We coupled litter decomposition in microcosms (open rings) with soil efflux of carbon dioxide, nitrous oxide and methane. Roughly, two thirds of litter carbon (C) were emitted to the atmosphere, while one third of this litter C was subject of physical- or biotic transport or leaching as dissolved organic C into the soil. Decomposition was studied using the so-called litterbag method as well, this means, plant material was exposed in mesh bags, and mass loss over time was monitored. Our litter bag studies do not support the general opinion that clear cuts increase decomposition. Fine root and fresh leaves decayed quickly at similar rates, followed by foliage litter and, finally, coarse roots.