What was done
Black cottonwood (Populus trichocarpa Torr. & Gray) seedlings were rooted in the ground near Gunnarsholt, Iceland, and grown within closed-top environmental chambers maintained at ambient and twice-ambient atmospheric CO2 concentrations for three years.  In addition, half of the seedlings in each CO2 treatment were supplied with supplemental mineral nutrients.  Thus, the authors studied the effects of elevated CO2 and soil nutrition on phenological characteristics within this important non-native Icelandic tree species.

What was learned
Neither elevated CO2 nor fertilization significantly affected the onset of spring bud break and shoot extension.  However, unfertilized seedlings growing in CO2-enriched air reached their maximum shoot extension rates about eight days earlier than seedlings growing in ambient air, although elevated CO2 did not influence the time to reach maximum shoot extension rates in fertilized seedlings.

At the other end of the growing season, elevated CO2 led to the cessation of shoot extension in unfertilized seedlings six days, and in fertilized seedlings three days, before it did in unfertilized and fertilized control seedlings growing in ambient air.  Fertilization, however, extended the cessation of shoot extension by 11 and 14 days in ambient and twice-ambient CO2 environments, respectively.  Thus, elevated CO2 tended to hasten the end of the growing season, while fertilization prolonged it.

What it means
As the air's CO2 concentration increases, black cottonwood seedlings growing in frigid environments might be offered a certain degree of protection against cold-induced damage.  Although elevated CO2 did not alter the initiation of bud break in the spring, it shortened the time for new buds to reach their maximum extension rates, thereby allowing them to achieve maximum growth rates earlier in the season, so that preparations for winter dormancy may be completed earlier in the fall, thus enabling them to more likely avoid the severe cold-induced die-back of newly produced tissues that often occurs at that time of year in Iceland.