The opponent was Professor Ülo Mander from the University of Tartu, and kustos Professor Atte Korhola from the University of Helsinki. The dissertation is published as a part of series Dissertationes Schola Doctoralis Scientiae Circumiectalis, Alimentariae, Biologicae and can be found in helda.
Read also the short article on the Elisa's research in University of Helsinki news (in finnish).
Methane (CH4) is a strong greenhouse gas, and its ecosystematmosphere exchange depends on the consumption and production rates. The boreal zone includes nearly one third of the world’s forests, and boreal forest soil is the largest carbon stock among different ecosystem types. Upland soils are a globally important sink of CH4 due to microbes oxidizing atmospheric CH4. During the last decades, the understanding of the CH4 dynamics of forests has been reshaped and increased substantially, as the trees have been shown to contribute to the CH4 exchange. The newly-found aerobic CH4 emissions from plants have also revealed the existence of previously unknown processes. Meanwhile, the ecosystem-scale studies on CH4 exchange have shown that forests may occasionally be net sources of CH4.
In this thesis, the objective was to quantify the CH4 exchange in a boreal pine forest, regarding the contributions of soil, ground vegetation and trees. The effects of soil water conditions and the CH4-consuming and -producing microbes were also studied. The research included the most abundant boreal tree species: Scots pine, downy birch and Norway spruce. The effect of ground vegetation on the forest floor CH4 flux was studied by classifying the vegetation into four groups, and by measuring the CH4 fluxes of three common shrubs (bilberry, lingonberry, and heather) in the laboratory. The forest floor CH4 flux was upscaled to the whole research site from topography-modelled soil moisture.
The results demonstrated that the CH4 flux of the forest floor is strongly dependent on the soil moisture. All the studied tree species emitted CH4 from the stems and the branches, and the stem-emissions were significantly higher from trees growing at wet soil compared to drier soil. The ground vegetation species and soil moisture are strongly connected, and based on the results, both affect the CH4 flux. In the laboratory, heather shoots resulted in mean CH4 emissions, while bilberry and lingonberry shoots indicated uptake. Thus, the studied shrub species seem to have different CH4 dynamics. In addition, the shrubs increased the amount of CH4-consuming microbes and thus CH4 uptake in the soil. While the forest floor at the site was on average a sink of CH4 throughout the growing season, the upscaled forest floor CH4 flux revealed high spatial variation and CH4-emission patches at the area. The size and CH4 flux of these patches was related to temporal variation in the soil moisture.