What was done
Seedlings of Alnus glutinosa (L.) Gaertn. were grown in pots placed within growth chambers receiving atmospheric CO2 concentrations of 360 and 720 ppm to determine the effects of elevated CO2 on this native British tree species.

What was learned
Elevated CO2 significantly increased the number of lateral branches from 9 to 19 in seedlings grown at 720 ppm CO2 relative to those grown at ambient CO2 concentrations.  Elevated CO2 also increased the average leaf size of seedlings by 17%.  However, atmospheric CO2 enrichment did not affect stomatal density in this species.  Nonetheless, seedlings grown in elevated CO2 displayed stomatal conductance values that were 25% lower than those exhibited by seedlings grown in ambient CO2 concentrations, indicating that this phenomenon resulted from CO2-induced reductions in stomatal apertures.  In spite of these reductions in stomatal apertures, maximum photosynthetic rates measured in CO2-enriched seedlings were 95% greater than those observed in ambiently-grown seedlings.  When combined with the previously mentioned CO2-induced reductions in stomatal conductance, this finding implies a significant CO2-induced increase in plant water-use efficiency.

What it means
As the air's CO2 content rises, it is likely that this native British tree species will respond by exhibiting enhanced photosynthetic rates and reduced values of stomatal conductance.  Together, these changes should lead to more efficient water use in this species, thereby allowing it to produce more biomass with less water input.  Thus, this species will likely grow more robustly in the future, regardless of soil moisture availability.