Seasonal and annual stem respiration of Scots pine trees under boreal conditions

Stem respiration of trees is a major, but poorly assessed component of the carbon balance of forests, and important for geo-chemistry. Measurements are required under naturally changing seasonal conditions in different years. Therefore, intra- and inter-annual carbon fluxes of stems in forests were measured continuously from April to November in three consecutive years.

Stem respiratory CO2 fluxes of 50-year-old Scots pine (Pinus sylvestris) trees were continuously measured with a CO2 analyser, and, concomitantly, stem circumference, stem and air temperature and other environmental factors and photosynthesis, were also measured automatically.

There were diurnal, seasonal and inter-annual changes in stem respiration, which peaked at 1600 h during the day and was highest in July. The temperature coefficient of stem respiration (Q10) was greater during the growing season than when growth was slow or had stopped, and more sensitive to temperature in the growing season. The annual Q10 remained relatively constant at about 2 over the three years, while respiration at a reference temperature of 15 degrees C (R15) was higher in the growing than in the non-growing season (1.09 compared with 0.78 micromol m(-2) stem surface s(-1)), but was similar between the years. Maintenance respiration was 76 %, 82 % and 80 % of the total respiration of 17.46, 17.26 and 19.35 mol m2 stem surface in 2001, 2002 and 2003, respectively. The annual total stem respiration of the stand per unit ground area was 75.97 gC m(-2) in 2001 and 74.28 gC m(-2) in 2002.

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.