The Forest Primary Production Research Group was born in the Department of S- viculture, University of Helsinki in the early 1970s. Intensive field measurements of photosynthesis and growth of forest vegetation and use of dynamic models in the interpretation of the results were characteristic of the research in the group. Electric instrumentation was based on analogue techniques and the analysis of the obtained measurements was based on self-written programs. Joint research projects with the Research Group of Environmental Physics at the Department of Physics, lead by Taisto Raunemaa (1939–2006) started in the late 1970s. The two research groups shared the same quantitative methodology, which made the co-operation fruitful. Since 1980 until the collapse of the Soviet Union the Academy of Finland and the Soviet Academy of Sciences had a co-operation program which included our team. The research groups in Tartu, Estonia, lead by Juhan Ross (1925–2002) and in Petrozawodsk, lead by Leo Kaipiainen (1932–2004) were involved on the Soviet side. We had annual field measuring campaigns in Finland and in Soviet Union and research seminars. The main emphasis was on developing forest growth models. The research of Chernobyl fallout started a new era in the co-operation between forest ecologists and physicists in Helsinki. The importance of material—uxes was realized and introduced explicitly in the theoretical thinking and measurements.