Our aim is to reveal the sources and sinks of methane (CH4) in boreal forests, and quantify the contribution of trees to the CH4 exchange. We also aim at identifying the role of CH4 producing microbes in whole-ecosystem CH4 fluxes. The final objective is to identify process drivers in order to bring the process understanding of CH4 emissions from trees to the level where we can construct a sound soil-tree-atmosphere process model.