Spring Leaf Flush in Aspen Trees

According to McGrath et al. (2010), "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." 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), these three researchers 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. So what did they find?

McGrath et al. 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%. Given these findings, atmospheric CO₂ enrichment appears to provide youthful aspen trees with a great head start to the growing season.

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