Impact studies on Nordic forests:: effects of elevated CO2 and fertilization on gas exchange

The effects of rising atmospheric carbon dioxide concentration, [CO2], and fertilization on gas exchange of four field-grown tree species were examined using the branch bag technique (Picea abies (L.) Karst., Pinus sylvestris L., Fagus sylvatica L.) or whole tree chambers (Populus trichocarpa Torr. & Gray). Results are presented on changes in light-saturated rates of net photosynthesis (A(sat)), carboxylation efficiency (alpha), stomatal conductance (g(s)), and stomatal limitation of photosynthesis (L-s) after 2-4 years of CO2 exposure. Fertilization alone did not significantly change A(sat), alpha, g(s), or L-s for any of the species, but alpha and A(sat) were linearly related to foliage nitrogen content when compared across all treatments. No significant CO2 effects were detected for alpha, g(s), or A(sat) when compared at the same intercellular [CO2], i.e., no downregulation of A(sat) was apparent. "Long-term" CO2 enrichment increased A(sat) significantly by 49, 53, 86, and 114% in Populus trichocarpa, Picea abies, F. sylvatica, and Pinus sylvestris, respectively. In all the species the relative CO2 effect on A(sat) increased linearly with temperature. Thus, application of a simple linear relationship could improve predictions of future tree growth responses to increasing CO2 and temperature in cool climates.