No Down-Regulation of Leaf Photosynthesis in Mature Forest Trees after Three Years of Exposure to Elevated CO₂

Abstract

The photosynthetic responses of six species of mature forest trees to long-term exposure to elevated CO₂ (ca. 530 ppm) were determined at the Swiss Canopy Crane (SCC) site near Basel, Switzerland. In the third year of growth in elevated CO₂, using web-FACE technology, net photosynthesis (A_s) in fully sunlit, upper canopy foliage was stimulated by ca. 40 % compared to ambient controls. This enhancement did not differ from the instantaneous increase in A_s found in ambient-grown leaves that were temporarily measured at elevated CO₂. A complete lack of down-regulation of photosynthesis was found in all species and in both the early and the late growing season. Neither was leaf nitrogen content significantly affected by long-term exposure to elevated CO₂. Our results document a persistent enhancement in leaf level photosynthesis in response to elevated CO₂ in mature forest trees over a period of three years. Circumstantial evidence suggests that the additional assimilates feed into large sinks other than stem and shoot growth.

Key words

Assimilation - down-regulation - global change - Swiss canopy crane - web-FACE.

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Editor: M. C. Ball