Environmental control of carbon dioxide gas exchange in needles of a mature Pinus cembra tree at the alpine timberline during the growing season

Seasonal courses of foliage CO2 gas exchange were measured in the upper sun and the lower shade crown of an adult Pinus cembra tree at the alpine timberline during the growing season 2002. Although photosynthetic photon flux density was significantly higher in the upper than in the lower crown daily mean net CO2 uptake did not differ significantly within the tree and was at an average only 8 % higher in the upper than in the lower crown. Under the prevailing climatic conditions in the timberline ecotone lack of light was the dominating factor limiting carbon gain during the growing season. Data presented in this study combined with data obtained for the winter time (WIESER 1997) allowed the calculation of an annual carbon gain of the foliage of about 2.76 g CO2 per g needle dry weight.