| Summary: | We used a novel system of branch enclosures to study the impact of elevated CO<sub>2</sub> (900 ppm) on the gas-exchange characteristics of developed and developing leaves of white poplar (<i>Populus alba</i> L.), as well as of leaves subsequently developing at ambient CO<sub>2</sub>, outside the enclosures in which the CO<sub>2</sub> concentration was raised. We found no significant effect of elevated CO<sub>2</sub> on photosynthesis, respiration and isoprene emission, as the rates of developed and developing leaves inside the enclosures, and of leaves developing outside the enclosures, were similar to those recorded using enclosures maintained at ambient CO<sub>2</sub>. The enclosure system, however, largely influenced the rates of gas-exchange. In fact, leaves already developed inside the enclosures showed rates of photosynthesis, stomatal conductance, and isoprene emission higher than leaves developing inside the enclosures, and also higher than leaves developing outside the enclosure. These differences were caused by a higher efficiency in the light use and by a higher Ribulose 1,5 bisphosphate carboxylase (Rubisco) activity in leaves fully developed inside enclosures than in the other leaf classes. The experiment overall suggests that branch enclosures may alter the physiology of the plants, reducing or counteracting the impact of elevated CO<sub>2</sub>, which we predicted to stimulate photosynthesis and uncouple isoprene emission from photosynthesis. This may be an important bias against the use of enclosure systems for studies of the impact of environmental constraints and global change factors on physiological features.
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