Thesis supervisor: Senior Research Fellow Krista Lõhmus and Senior Research Fellow Ivika Ostonen-Märtin,University of Tartu
Opponent: professor Jukka Pumpanen, PhD, Universiy of Helsinki
Soil CO2 efflux i.e. soil respiration can form a remarkable part of total forest ecosystem respiration and as northern forests at higher latitudes play an important role in global carbon cycle it is highly actual to quantify their soil CO2 effluxes. There are three novel aspects in this thesis. First, in the light of global climate change, the knowledge about the effect of elevated air humidity on soil CO2 effluxes will help to predict and understand the con¬sequences of a changing pattern of humidity on the forest carbon cycle. Second, the first results are obtained about soil respiration partitioning into the heterotrophic and autotrophic components in Estonian silver birch and Norway spruce forests. Third, for the first time in Estonia the effect of stump harvesting on soil respiration is analysed. The general aim of the thesis was to ascertain the effect of several factors: abiotic (elevated air humidity, soil temperature and moisture); biotic (stand age and development stage, fine root biomass and turnover, above-ground litter input, soil microbial biomass and activity) and forest management (stump harvesting) on total soil respiration and its autotrophic and heterotrophic components in silver birch and Norway spruce stands of different ages. The findings showed that soil temperature was the main climatic factor explaining the seasonal variation of soil respiration and soil moisture had a weak effect. Increased air humidity reduced soil respiration, however, it increased the fine root biomass and production of the understorey and the basal respiration of microbes. Stand age affected soil CO2 effluxes in both silver birch and Norway spruce stands, which were mostly explained by changes in biotic factors such as fine root dynamics than by changes in soil temperature and moisture. The temperature sensitivity of soil respiration components showed opposite response for deciduous and for coniferous tree species. First results of Estonian case study imply that stump harvesting does not affect the soil respiration on fertile sites. This thesis demonstrates the complexity of below-ground respiration processes and the importance to consider the effect of several natural and anthropogenic factors into the predictions of the carbon cycle of forest ecosystem in changing climate.