Photosynthesis and transpiration of 20-year-old Scots pine [Pinus sylvestris, fertilization, irrigation, mesophyll conductance, respiration, stomatal conductance]

A system of air-cooled assimilation chambers was developed for continuous measurement of gas exchange in a 20-year-old stand of Scots pine. The period of net photosynthetic activity was approximately eight months a year, starting in April as soon as the soil was no longer frozen. Photosynthesis stopped in December largely because of low irradiance and air temperature. Calculation of the mesophyll conductance for carbon dioxide showed that it took more than two months of the growing season to repair winter damage to the photosynthetic apparatus. Needle age had a much more pronounced effect on photosynthetic efficiency and performance than had the position of needles within the crown. The variation between trees was small, and smaller for photosynthesis than for transpiration. Stem and branch respiration measured in situ exhibited large seasonal variation. The pronouncedly increased rate of respiration during the growth period was probably caused by "growth respiration" plus an adaptation to temperature of the maintenance respiration. In light, stem and branch respiration decreased as a result of the refixation of carbon dioxide in the chlorophyll-containing tissue immediately beneath the thin periderm.