What was done
The authors grew snow gum seedlings in open-top chambers located near Bungendore, NSW, Australia, for approximately eight months (from autumn through spring) at atmospheric CO2 concentrations of 350 and 700 ppm to investigate the seasonal effects of elevated CO2 on photosynthesis and growth in this Eucalyptus species.

What was learned
The effects of elevated CO2 on photosynthesis and growth in snow gum seedlings were strongly dependent upon ambient temperature, which varied seasonally, as was the case for mature loblolly pine trees (see our Journal Review: Seasonal Photosynthetic Response of Pine to Elevated CO2).  During cold winter months, for example, seedlings grown in elevated CO2 displayed sustained reductions in their photosynthetic utilization of absorbed light energy relative to seedlings grown in ambient CO2.  However, in the relatively warmer spring months that followed, newly-produced leaves exhibited no such reductions in their utilization of light energy relative to their ambiently-grown counterparts.  In addition, even though photoinhibition was exacerbated by elevated CO2 in the cold winter months, when growth was limited by low temperature, CO2-enriched seedlings overcame this obstacle once minimum temperatures consistently exceeded 0°C, as indicated by photosynthetic rates and final biomass measurements that were 30 and 53% greater, respectively, than those of seedlings grown at ambient CO2.

What it means
As the CO2 content of the air increases, snow gum seedlings will likely display enhanced rates of photosynthesis and growth that vary with air temperature.  In cold winter months, the growth-stimulating effects provided by greater concentrations of atmospheric CO2 will probably be reduced or negated due to low air temperatures that have the ability to suppress growth.  However, once temperatures warm in the spring, it is likely that the positive interaction between temperature and CO2 will result in significant growth increases that will more-than-compensate for any reduced growth resulting from low air temperatures.  Thus, the rising atmospheric CO2 concentration should enhance new growth in this species following the transition from cold winters to relatively milder spring.