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Stem respiration is an important component in the annual carbon balance of a Scots pine stand, contributing 9 % to total carbon loss from the ecosystem and consuming about 8 % of the carbon of the ecosystem gross primary production. Stem (or air) temperature was the most important predictor of stem carbon flux. The magnitude of stem respiration is modified by photosynthesis and tree growth. Solar radiation indirectly affects stem respiration through its effect on photosynthesis.

The response of forest soil CO 2 efflux to the elevation of two climatic factors, the atmospheric concentration of CO 2 ( " CO 2 of 700 lmol mol À1 ) and air temperature (" T with average annual increase of 5 1C), and their combination (" CO 2 1 " T) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines (Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO 2 efflux in May-October increased with elevated CO 2 by 23-37%, with elevated temperature by 27-43%, and with the combined treatment by 35-59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO 2 was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO 2 alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO 2 efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO 2 emissions. The decrease in the temperature sensitivity of soil CO 2 efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO 2 and air temperature consistently increased forest soil CO 2 efflux over the 4-year period, their combined effect being additive, with no apparent interaction.

Abstract. Our objectives were to identify factors related to temporal variation of soil CO 2 efflux in a boreal pine forest and to evaluate simple predictive models of temporal variation of soil CO 2 efflux. Soil CO 2 efflux was measured with a portable chamber in a Finnish Scots pine forest for three years, with a fourth year for model evaluation. Plot averages for soil CO 2 efflux ranged from 0.04 to 0.90 g CO 2 m −2 h −1 during the snow-free period, i.e. May-October, and from 0.04 to 0.13 g CO 2 m −2 h −1 in winter. Soil temperature was a good predictor of soil CO 2 efflux. A quadratic model of lntransformed efflux explained 76-82 % of the variation over the snow-free period.The results revealed an effect of season: at a given temperature of the organic layer, soil CO 2 efflux was higher later in the snow-free period (in August and September) than in spring and early summer (in May and June). Regression coefficients for temperature (approximations of a Q 10 value) of month-specific models decreased with increasing average soil temperatures. Efflux in July, the month of peak photosynthesis, showed no clear response to temperature or moisture. Inclusion of a seasonality index, degree days, improved the accuracy of temperature response models to predict efflux for the fourth year of measurements, which was not used in building of regression models. During peak efflux from mid-July to late-August, efflux was underestimated with the models that included degree days as well as with the models that did not. The strong influence of the flux of photosynthates belowground and the importance of root respiration could explain the relative temperature insensitivity observed in July and together with seasonality of growth of root and root-associated mycorrhizal fungi could explain partial failure of models to predict magnitude of efflux in the peak season from mid-July to August.Correspondence to: S. M. Niinistö (sini.niinisto@iki.fi)The effect of moisture early in the season was confounded by simultaneous advancement of the growing season and increase in temperature. In a dry year, however, the effect of drought was evident as soil CO 2 efflux was some 30 % smaller in September than in the previous wet year. Soil temperature was a good overall predictor of soil CO 2 efflux, possibly partly because its apparent effect was strengthened by many environmental factors and ecosystem processes that varied in concert with its variation. However, the consistent underestimation by the predictive models for the peak season corroborates recent findings concerning the importance of seasonal changes in carbon inputs to processes producing CO 2 in soil.