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.  The trees, which were in a linear growth phase at the onset of the experiment, were exposed to atmospheric CO2 concentrations of 360 and 550 ppm.  This paper reports on gas exchange data collected to assess photosynthesis, stomatal conductance and water-use efficiency within the stand after three years of atmospheric CO2 enrichment.

What was learned
On average, the 190-ppm increase in atmospheric CO2 concentration increased rates of net photosynthesis by 46% in both upper- and mid-canopy foliage.  In addition, it reduced stomatal conductances by 24 and 14%, in upper- and mid-canopy leaves, respectively, leading to increases in instantaneous water-use efficiencies of 68 and 78% in upper- and mid-canopy leaves, respectively.

What it means
As the air's CO2 concentration increases, mature stands of sweetgum trees will likely exhibit enhanced rates of photosynthesis that should lead to increased biomass production.  Indeed, the authors stated that they observed "no decline in [photosynthetic] enhancement over the 3 year [study] period."  In addition, such enhancements in photosynthesis, coupled with CO2-induced reductions in stomatal conductance, should greatly enhance the water-use efficiencies of individual leaves.  Thus, this species should be more adept at dealing with conditions of reduced soil moisture as the CO2 content of the air increases.  These results clearly demonstrate that the CO2 responsiveness of sweetgum trees is not limited to the seedling and sapling stages, but to mature trees as well.