Gas exchange in young European larches (Larix decidua Mill.) on the north- and east-facing slopes of an avalanche gully (2185 m a.s.l.) was measured with climatised gas exchange cuvettes during the summer of 1983. Air and soil temperatures, photon flux density, and leaf-air water vapour pressure differences were recorded concurrently.
The differences in maximum net photosynthesis rates on the two sites were surprisingly great. In the tree on the less favourable N-facing slope, CO₂ assimilation reached at the most only two thirds of that in the tree on the E-facing slope. An even greater difference was apparent in the net balance of CO₂ uptake: average CO₂ uptake during the measuring period was 57.2 mmol CO₂ m^-2 d^-1 in the tree on the E-slope but only 28.3 mmol CO₂ m^-2 d^-1, or about half as great, in the tree on the N-slope.
Of the main factors affecting CO₂ uptake, soil temperature seems to be particularly important. Net balance of CO₂ uptake was also greatly affected by the length of the production period, which was 20% longer on the E-slope, and by slow recovery of net photosynthesis in the N-slope larch after a marked temperature drop at the beginning of August. Presumably also differences in soil (nutrient supply) contribute to the observed great photosynthetic differences. A considerable influence on CO₂ uptake is ascribed to the differing physiological states of the trees on the two contrasting habitats.