Background
The authors write that "early spring leaf-out is critical to the growth and survival of competing trees in deciduous forests (Augspurger, 2008)," and that "individuals or genotypes that more quickly reach high LIA [leaf area index] will more successfully compete with neighbors for light energy and space."

What was done
Working at the Aspen FACE facility near Rhinelander, Wisconsin (USA), where aspen clones had been grown since 1997 in conditions simulating CO2 and O3 concentrations predicted for the mid-21st century (560 ppm CO2 and 1.5 times current-ambient O3), McGrath et al. documented the history of leaf area development and leaf photosynthetic operating efficiency over the first month of spring leaf-out eleven years later in 2008, at a time when the yearly growth-season CO2 fumigation had begun, but before O3 fumigation was started.

What was learned
The three researchers report that the trees in the elevated CO2 plots showed a 36% stimulation of leaf area index, while the trees in the elevated O3 plots actually showed a 20% reduction in LAI. In addition, they say that the photosynthetic operating efficiency of the CO2-enriched aspen leaves was significantly enhanced by a whopping 51%.

What it means
Like a good breakfast for children before heading off to school for the day, atmospheric CO2 enrichment appears to provide youthful aspen trees with a great head start to the growing season.

Reference
Augspurger, C.K. 2008. Early spring leaf out enhances growth and survival of saplings in a temperate deciduous forest. Oecologia 156: 281-286.