What was done
Closed chambers were constructed around 20-year-old Scots pine (Pinus sylvestris L.) trees growing on a low-nitrogen-containing soil.  For three years thereafter, the trees in the chambers were fumigated with air containing 350 and 700 ppm CO2 at ambient and elevated air temperatures (about 4°C above ambient temperatures), in order to study the effects of elevated CO2 and air temperature on stem growth in this coniferous species when growing on a soil low in nitrogen.

What was learned
Exposure to elevated CO2 extended the growing season by approximately one week, while elevated air temperature extended the growing season by about two weeks.  After three years of treatment, cumulative stem diameter growth in the CO2-enriched trees grown at ambient air temperature was 57% greater than that displayed by control trees grown at ambient CO2 and ambient air temperature.  In addition, trees subjected to elevated CO2 and elevated air temperature exhibited cumulative stem-diameter growth that was 67% greater than that displayed by trees grown in ambient air at ambient air temperatures.

What it means
As the atmospheric CO2 concentration increases, mature Scots pine trees will likely respond by increasing stem-diameter growth, even if growing on soils low in nitrogen, and even if air temperatures rise by as much as 4°C.  Thus, future increases in stem growth will likely facilitate enhanced carbon sequestration by Scots pine, especially if the growing season is extended by atmospheric CO2 enrichment or increased air temperatures as reported in this study.