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Elevated CO2 Increases Cell Expansion and Production in Tree Leaves
Ferris, R., Sabatti, M., Miglietta, F., Mills, R.F. and Taylor, G.  2001.  Leaf area is stimulated in Populus by free air CO2 enrichment (POPFACE), through increased cell expansion and production.  Plant, Cell and Environment 24: 305-315.

What was done
The authors studied three Populus species - P. x euramericana (clone I-214), P. nigra (clone Jean Pourtet) and P. alba (clone 2AS-11) - within six FACE plots, each of which contained all three species and three of which had the air above them enriched to a CO2 concentration just short of 550 ppm.  The trees had been planted directly into the ground in the spring of 1999, and various measurements were made on young expanding leaves of the trees between 16 August and 3 September of that year, when the seedlings were experiencing rapid exponential growth.

What was learned
The approximate 50% increase in atmospheric CO2 concentration increased the area of fully expanded leaves by 61, 28 and 19% in P. x euramericana, P. nigra and P. alba, respectively.  In P. x euramericana and P. nigra, the increase in leaf size was due to increases in both epidermal cell size and cell number, whereas for P. alba it was due only to an increase in cell production.  The authors note that some of the CO2-induced increase in leaf cell size may have been due to the fact that leaf cell wall extensibility increased in all species at the higher atmospheric CO2 concentration.  Also, leaves in the CO2-enriched FACE plots exhibited increases in the activity of xyloglucan endotransglycosylase, a cell wall-loosening enzyme, the greater CO2-induced activity of which may have played a key roll in allowing the cells to expand to larger size.

What it means
As the air's CO2 content continues to rise, it is likely that leaves of the Populus species studied - and, by inference, those of many other trees as well - will grow ever larger, at least in the early stages of growth that precede canopy closure, due to CO2-induced changes in the biochemical and biophysical properties of their cell walls.  This consequence of atmospheric CO2 enrichment, in turn, should be a strong impetus for increased biomass production and accumulation, which other evidence suggests will likely be sustained throughout the lives of the trees.  Hence, in the words of the authors, "species with larger leaves will develop larger canopies more quickly and may become more productive in a carbon-rich environment," such as seems certain to be our happy fate, as anthropogenic CO2 emissions continue to pour into the atmosphere and stimulate the biosphere to ever greater levels of life-sustaining activity.