A meta‐analysis of leaf gas exchange and nitrogen in trees grown under elevated carbon dioxide

Abstract

The response of trees to rising atmospheric CO 2 concentration ([CO 2]) is of concern to forest ecologists and global carbon modellers and is the focus of an increasing body of research work. I review studies published up to May 1994, and several unpublished works, which reported at least one of the following: net CO 2 assimilation (A), stomatal conductance (gs), leaf dark respiration (Rd) leaf nitrogen or specific leaf area (SLA) in woody plants grown at <400 μmol mol−1 CO 2 or at 600–800 μmol mol−1 CO 2. The resulting data from 41 species were categorized according to growth conditions (unstressed versus stressed), length of CO 2 exposure, pot size and exposure facility [growth chamber (GC), greenhouse (GH), or open‐top chamber (OTC)] and interpreted using meta‐analytic methods. Overall, A showed a large and significant increase at elevated [CO 2] but length of CO 2 exposure and the exposure facility were important modifiers of this response. Plants exposed for < 50 d had a significantly greater response, and those from GCs had a significantly lower response than plants from longer exposures or from OTC studies. Negative acclimation of A was significant and general among stressed plants, but in unstressed plants was influenced by length of CO 2 exposure, the exposure facility and/or pot size. Growth at elevated [CO 2] resulted in moderate reductions in gs in unstressed plants, but there was no significant effect of CO 2 on gs in stressed plants. Leaf dark respiration (mass or area basis) was reduced strongly by growth at high [CO 2] > while leaf N was reduced only when expressed on a mass basis. This review is the first meta‐analysis of elevated CO 2 studies and provides statistical confirmation of several general responses of trees to elevated [CO 2]. It also highlights important areas of continued uncertainty in our understanding of these responses.