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Elevated CO2 Effects on Canopy Transpiration in a Mature Sweetgum Stand
Reference
Wullschleger, S.D. and Norby, R.J.  2001.  Sap velocity and canopy transpiration in a sweetgum stand exposed to free-air CO2 enrichment (FACE).  New Phytologist 150: 489-498.

What was done
A FACE study was established within a ten-year-old stand of sweetgum (Liquidambar styraciflua L.) trees growing in a forest plantation on nutrient-rich soils in Tennessee, USA.  These trees, which were in a linear growth phase at the onset of the experiment, were exposed to atmospheric CO2 concentrations of 390 and 540 ppm.  This paper reports on sap velocity data collected to assess transpiration and water-use efficiency within this stand after two years of atmospheric CO2 enrichment.  Measurements were made during the five-month growing season common to this species.

What was learned
Elevated CO2 reduced sap flow rates in sweetgum trees.  The reductions ranged from 5 to 20%, averaging 13% over the growing season.  It also reduced transpiration rates by a maximum of 21%, with a seasonal reduction of 12%.  After dividing each treatment's seasonal dry matter production by its seasonal transpiration, the authors calculated a stand-level increase in water-use efficiency of 28% for the CO2-enriched trees.

What it means
As the air's CO2 concentration increases, it is likely that transpirational water loss in sweetgum trees -- even at the stand-level -- will decrease.  In addition, because photosynthetic rates and biomass production will likely be enhanced by elevated levels of atmospheric CO2, the water-use efficiency of such stands should increase, possibly allowing this species to better deal with drought conditions and expand its range into drier areas.  Thus, carbon sequestration by sweetgum stands and plantations will also likely increase in a future high-CO2 world.