Carbon balance of different aged Scots pine forests in Southern Finland

We estimated annual net ecosystem exchange (NEE) of a chronosequence of four Scots pine stands in southern Finland during years 2000-2002 using eddy covariance (EC). Net ecosystem productivity (NEP) was estimated using growth measurements and modelled mass losses of woody debris. The stands were 4, 12, 40 and 75 years old. The 4-year-old clearcut was a source of carbon throughout the year combining a low gross primary productivity (GPP) with a total ecosystem respiration (TER) similar to the forest stands. The annual NEE of the clearcut, measured by EC, was 386 g C m^-2. Tree growth was negligible and the estimated NEP was -262 g C m^-2 a^-1. The annual GPPs at the other sites were close to each other (9281072 g C m^-2 a^-1), but TER differed markedly, being greatest at the 12-year-old site (905 g C m^-2 a^-1) and smallest in the 75-year-old stand (616 g C m^-2 a^-1). Measurements of soil CO2 efflux showed that different rates of soil respiration largely explained the differences in TER. The NEE and NEP of the 12-year-old stand were close to zero. The forested stands were sinks of carbon. They had similar annual patterns of carbon exchange and half-hourly eddy fluxes were highly correlated, indicating similar responses to the environment. The NEE in the 40-year-old stand varied between -179 and -192 g C m^-2 a^-1, while NEP was between 214 and 242 g C m^-2 a^-1. The annual NEE of the 75-year-old stand was 323 g C m^-2 and NEP was 252 g C m^-2. This indicates that there was no reduction in carbon sink strength with stand age.