Net photosynthesis of current and one-year-old shoots in Scots pine (Pinus sylvestris L.) was measured during a two-year period (1977–1978). The measurements were carried out in Central Sweden using a system that allowed in situ measurements throughout the year. The photosynthetically active period, defined as months with a positive carbon dioxide balance, was nine months per year for one-year-old shoots and six months for current shoots. The daily rates of net photosynthesis were low in spring and late autumn and 95% of the annual photosynthetic production occurred from May to October. In spring net photosynthesis was limited by a low photosynthetic capacity and a restricted access to water caused by the frozen ground. It took more than three months in spring and early summer to restore full photosynthetic capacity in the one-year-old shoots, but the rate of recovery varied considerably between the two years. In late autumn the rates of photosynthesis were mainly limited by low photon flux densities.
The seasonal patterns of photosynthesis were similar between the two years, and even though there were pronounced differences for individual months, the total annual photosynthetic production of the two years only differed by 1.4% for the one-year-old shoots. The differences in photosynthetic production for current shoots were much higher as an effect of different rates of needle development and differences in photosynthetic capacity during the two years. An estimate of the carbon balance of the current shoots showed that the shoots had not photosynthesized their own weight in terms of carbon until late August (1978) or early September (1977), which means that their photosynthetic production was of minor importance for the formation of stem wood.