The response of an entire catchment to increased CO2 and temperature was studied by experimental ecosystem manipulation during a 4-year period. The project was part of the CLIMEX project (Climate Change Experiment) conducted in a mountainous pine–birch forest (Pinus sylvestris, Betula pubescens) at an elevation of 300 m above sea level at Risdalsheia, southernmost Norway. The trees were up to 160 years old with a maximum height of 9 m. The site is typical for large areas of upland boreal forest in Scandinavia. The project involved five catchments, two of which were divided into two subcatchments covered with a big greenhouse or a roof construction. The total set-up at the experimental site employed multiple treatments and controls. Data from four catchments are presented here: (1) control, (2) greenhouse control, (3) CO2 enriched from the ambient level of ca. 360 to 560 ppmv and air temperature increased 3–5 °C above ambient, and (4) soil temperature increased 3–5 °C with heating cables. The results showed that increased CO2 and/or temperature did not significantly influence tree growth—measured as tree ring increment. This finding is opposed the increased primary production found for the forest floor vegetation in the present experiment and it contradicts the results from many short-term studies, done with seedlings or young plants. It is concluded that effects of increased CO2 and temperature observed in experiments with seedlings, saplings and forest floor plants may not reflect the effects on mature trees and therefore cannot be directly extrapolated to whole ecosystem effects. In the end this may cause C sequestration in forests to be less than anticipated in general. An increased needle weight and shoot length in all roof- and greenhouse-covered catchments in the present study indicated that the reduced light conditions and shelter effect under the roof and greenhouses have blurred the possible small treatment effects.