What was done
Seedlings of paper and yellow birch were grown individually and in mixed species stands for four months in controlled environment facilities receiving atmospheric CO2 concentrations of 375 and 700 ppm.  In addition, the authors subjected seedlings to one of four irrigation regimes to study the interactive effects of elevated CO2 and soil moisture on photosynthesis and growth in these co-occurring temperate forest species.

What was learned
For individually grown paper birch, which commonly is found on more xeric (dry) soils, elevated CO2 proportionally increased photosynthesis and total seedling biomass as soil moisture became increasingly less available.  In the wettest irrigation treatment, for example, seedlings grown in elevated CO2 had 27% more biomass than those grown at 375 ppm CO2, while those grown in the driest regime exhibited 130% more biomass than their respective controls.

For individually grown yellow birch, which typically is found on more mesic (wet) soils, the CO2-induced photosynthetic enhancement was more stable across the range of soil moisture availability, and averaged about 30% in all but the wettest treatment, where no significant difference was found.  Similarly, atmospheric CO2 enrichment increased total seedling biomass by about 40% across all moisture regimes, but this effect was only significant in the two wettest treatments.

When grown in a mixed species stand, elevated CO2 significantly increased the biomass of paper birch by over two-fold, but only in the drier soil moisture regimes, and had no effect on seedling survivorship, regardless of irrigation treatment.  For yellow birch, elevated CO2 increased total biomass by 83%, but only in the wetter soil moisture regimes, and significantly increased the survivorship of yellow birch seedlings in the drier irrigation treatments.

What it means
As the atmospheric CO2 content continues to increase, the current distributions of these co-occurring temperate forest species should be reinforced, leading to significant improvements in the regeneration of paper and yellow birch in more xeric and mesic habitats, respectively.  In addition, because elevated CO2 increased seedling survivorship of yellow birch in drier soil moisture regimes, it is likely that yellow birch will also experience range expansions into regions that receive less rainfall as the CO2 content of the air rises.  Thus, elevated CO2 should allow these species to better persist in the warmer and drier climates of their southern ranges, even if the climate warms, thereby maintaining biodiversity in such habitats; and it may actually expand the northern ranges of yellow birch into wetter areas, thereby increasing biodiversity in those areas.