A METAANALYSIS OF LEAF GAS-EXCHANGE AND NITROGEN IN TREES GROWN UNDERELEVATED CARBON-DIOXIDE

The response of trees to rising atmospheric CO2 concentration ([CO2]) is of concern to forest ecologists and global carbon modellers and is the focus of an increasing body of research work, I review studies pub lished up to May 1994, and several unpublished works, which reported a t least one of the following: net CO2 assimilation (A), stomatal condu ctance (g(s)), leaf dark respiration (R(d)), leaf nitrogen or specific leaf area (SLA) in woody plants grown at <400 mu mol mol(-1) CO2 or a t 600-800 mu mol mol(-1) CO2, The resulting data from 41 species were categorized according to growth conditions (unstressed versus stressed ), length of CO2 exposure, pot size and exposure facility [growth cham ber (GC), greenhouse (GH), or open-top chamber (OTC)] and interpreted using meta-analytic methods, Overall, A showed a large and signifcant increase at elevated [CO2] but length of CO2 exposure and the exposure facility were important modifiers of this response, Plants exposed fo r <50 d had a significantly greater response, and those from GCs had a significantly lower response than plants from longer exposures or fro m OTC studies, Negative acclimation of A was significant and general a mong stressed plants, but in unstressed plants was influenced by lengt h of CO2 exposure, the exposure facility and/or pot size, Growth at el evated [CO2] resulted in moderate reductions in g(s) in unstressed pla nts, but there was no significant effect of CO2 on g(s) in stressed pl ants, Leaf dark respiration (mass or area basis) was reduced strongly by growth at high [CO2], 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 res ponses of trees to elevated [CO2]. It also highlights important areas of continued uncertainty in our understanding of these responses.