Increased leaf area expansion of hybrid poplar in elevated CO2. From controlled environments to open-top chambers and to FACE

We examined the response of hybrid poplar to elevated CO2 in contrasting gr owth environments: controlled environment chamber (CE), open-top chamber (O TC) and poplar free air CO2 enrichment (POPFACE) in order to compare short versus longterm effects and to determine whether generalisations in respons e are possible for this fast growing tree. Leaf growth, which for poplar is an important determinant of stemwood productivity was followed in all envi ronments, as were the determinants of leaf growth-cell expansion and cell p roduction. Elevated CO2 (550-700 mu mol mol(-1), depending on environment) resulted in an increase in final leaf size for Populus trichocarpa x Populu s deltoides (Populus x interamericana) and P. deltoides x Populus nigra (Po pulus x euramericana), irrespective of whether plants were exposed during a short-term CE glasshouse study (90 days), a longterm OTC experiment (3 yea rs) or during the first year of a POPFACE experiment. An exception was obse rved in the closed canopy POPFACE experiment, where final leaf size remaine d unaltered by CO2. Increased leaf extension rate was observed in elevated CO2 in all experiments, at some point during leaf development, as determine d by leaf length. Again the exception were the POPFACE experiment, where ef fects were not statistically significant. Leaf production and specific leaf area (SLA) were increased and decreased, respectively, on five out of six occasions, although both were only statistically significant on two occasio ns and interestingly for SLA never in the FACE experiment. Although both ce ll expansion and cell production were sensitive to CO2 concentration, effec ts appeared highly dependent on growth environment and genotype. However, i ncreased leaf cell expansion in elevated CO2 was often associated with chan ges in the biophysical properties of the cell wall, usually increased cell wall plasticity. This research has shown that enhanced leaf area developmen t was a consistent response to elevated CO2 but that the magnitude of this response is likely to decline, in long-term exposure to elevated CO2. Effec ts on SLA and leaf production suggest that CE and OTC experiments may not a lways provide good predictors of the 'qualitative' effects of elevated CO2 in long-term ecosystem experiments. (C) 2001 Elsevier Science Ltd. All righ ts reserved.