Effects of site preparation and stump harvest on carbon dynamics in forest soils
Sammanfattning: Efforts are being made to substitute fossil fuels with renewable alternatives to address increasing concerns regarding global climate change driven by anthropogenic emissions of CO2 and other greenhouse gases. Stumps of harvested trees could make a significant contribution to these efforts in Scandinavia. Silvicultural measures may all impact on greenhouse gas fluxes. Site preparation, being one of these, is conducted on a major share of the annual clear-cut area in Sweden to promote seedling development. More information on the effects of stump harvesting and site preparation methods, on soil and ecosystem C stocks is needed as all of these treatments disturb the soil, creating for instance areas of exposed mineral soil, and/or a double humus layer and/or a humus layer mixed with mineral soil. The overall aim of this thesis is to assess changes in C stocks (in soils and ecosystems overall) following several common mechanical site preparation methods and stump harvest. The results and conclusions yielded by the thesis are based on in-situ measurements of the following data: soil-surface CO2 flux (Rs), weight loss of needle litter and coarse roots in litterbags, soil temperature and moisture, and assessments of C stocks in soil, ground vegetation and trees as well as percentage area disturbed by the different methods. The results showed that site preparation and stump harvests have no or reducing effects on soil-surface CO2 flux (Rs), and that any initial increase in Rs is limited to the first few weeks. Removal of the humus layer, thereby exposing the mineral soil, led to lower Rs than in control plots. However, a double humus layer did not result in higher than control Rs values. Thus, Rs seems to be related to the presence of a humus layer rather than its thickness (if present). Mixing the humus layer and mineral soil did not lead to higher Rs than control fluxes, and in one experiment it even led to lower Rs. The location of incubated litter (in bags) strongly influenced the decomposition rate, as buried litter decomposed more rapidly than litter placed on the soil-surface. However, the higher decomposition rates associated with buried litter were not accompanied by increases in Rs from the entire soil profile. After three decades, the most pronounced effects of site preparation on the entire ecosystem C stocks were increases due to enhancement of the growth of planted trees (Pinus sylvestris, Picea abies and Pinus contorta), relative to control. General conclusions are that site preparation and stump harvests do not lead to reductions of the soil or ecosystem C stocks, and in the long-term promoting strong growth of forest trees is the most important measure for securing C stocks of entire forest ecosystems, including those of the soil.
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