Impact studies on Nordic forests: effects of elevated CO 2 and fertilization on gas exchange

The effects of rising atmospheric carbon dioxide concentration, [CO 2 ], 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 (α), stomatal conductance (g s ), and stomatal limitation of photosynthesis (L s ) after 2–4 years of CO 2 exposure. Fertilization alone did not significantly change A sat , α, g s , or L s for any of the species, but α and A sat were linearly related to foliage nitrogen content when compared across all treatments. No significant CO 2 effects were detected for α, g s , or A sat when compared at the same intercellular [CO 2 ], i.e., no downregulation of A sat was apparent. "Long-term" CO 2 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 CO 2 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 CO 2 and temperature in cool climates.