What was done
Silver birch seeds were germinated and the seedlings grown in open-top chambers located south of Edinburgh, England, for four years at ambient and twice ambient CO2 concentrations to determine the effects of elevated CO2 on whole tree growth and carbon budgeting.  This paper represents data collected over the fourth year of the study.

What was learned
Trees grown in elevated CO2 had 43% more leaves and displayed an annual net photosynthesis total that was 110% greater than that observed for trees grown in ambient CO2.  Despite this doubled carbon budget, trees grown in elevated CO2 only produced about 60% more biomass than control trees, indicating that a large portion of their photosynthate was likely transported belowground to maintain or increase symbiotic relationships with soil fungi and other rhizosphere organisms, as carbon losses via respiration by non-leaf tissues were similar regardless of CO2 concentration.

What it means
As the CO2 content of the air rises, silver birch seedlings will likely produce more leaves and greater amounts of photosynthetic sugars that can be used to increase their biomass and sustain greater populations of soil fungi and other rhizosphere organisms.  These carbohydrate sinks enable the trees to maintain a heightened state of physiological activity that staves off the photosynthetic acclimation to elevated levels of atmospheric CO2 that is often observed when plants are grown in containers and their root growth is restricted.