What was done
The authors grew well watered and fertilized black spruce (Picea mariana Mill. BSP) plants from seed in containers filled with a 3:1 mixture of peat:vermiculate in mini-greenhouses located within a larger greenhouse where they were exposed - at ambient and elevated (370 and 710 ppm) concentrations of atmospheric CO2) - to environmental conditions that simulated "a growth period (Summer, April 17-September 15) followed by hardening (Fall, September 16-November 5; Winter, November 6-February 5) and dehardening (Spring, February 6-February 28)," during which periods a number of plant properties and physiological processes were measured.

What was learned
In October, during the process of hardening, Bigras and Bertrand found that "bud set began earlier in elevated CO2 than in ambient CO2," and that "an increase in seedling cold tolerance in early fall was related to early bud set in elevated CO2." In addition, they say their results "showed that reductions in dark respiration and light compensation point during cold acclimation and de-acclimation can contribute to increased productivity in elevated CO2." In fact, they "observed an increase in light-saturated photosynthetic rate in response to elevated CO2 during the growth, hardening and de-hardening periods" that "varied seasonally between 12 and 50%." The net result of these and other CO2-induced plant benefits was a 38% increase in total plant mass at the end of the growing season that slowly declined to a value of 18% at the end of the winter.

What it means
In a CO2-enriched world of the future, black spruce seedlings should fare better than they do today throughout the entire year, which would appear to make carbon dioxide a beneficent "gas for all seasons."