COMPARATIVE RESPONSE ON GAS-EXCHANGE OF PICEA SPP EXPOSED TO INCREASED ATMOSPHERIC CO2 IN OPEN-TOP CHAMBERS AT 2 TEST SITES

We took comparative measurements of gas exchange response curves of tw o species of spruce (Picea abies (L.) Karst and Picea sitchensis (Bong .) Carr.) exposed to high levels of atmospheric carbon dioxide (CO2) i n two test stations: Vielsalm (Belgium) and Glendevon (United Kingdom) . The photosynthetic response of these two species to variations in co ncentrations of intercellular CO2 and to variations in light intensity were measured in situ using an integrated transportable differential CO2 and water vapour exchange measuring system. The response curves we re adjusted by the Mitscherlich function. The statistical analysis of our measurements and adjustments reveal similarities in the reaction o f Picea abies and Picea sitchensis to a doubling of the present level of atmospheric CO2. Regarding the photosynthesis response curves to in tercellular CO2 variation, we noted a decrease in the maximum photosyn thesis rate and the carboxylation rate accompanied by an increased com pensation point. Regarding the photosynthesis response curves to the l ight variation, we found that dark respiration and photochemical effic iency remained unchanged, and the maximum photosynthesis rate was slig htly higher in an atmosphere enriched in CO2. These experimental conte xts would seem to indicate that the current and forecast levels of CO2 are not ecological factors limiting primary productivity, and that th e increase in atmospheric CO2 interacts with other environmental facto rs.