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Effects of Elevated CO2 on the Total Leaf Area of a Closed-Canopy Pine Forest with a Hardwood Component
McCarthy, H.R., Oren, R., Finzi, A.C., Ellsworth, D.S., Kim, H.-S., Johnsen, K.H. and Millar, B. 2007. Temporal dynamics and spatial variability in the enhancement of canopy leaf area under elevated atmospheric CO2. Global Change Biology 13: 2479-2497.

What was done
Hypothesizing that elevated CO2 "would have little impact on stand L [canopy leaf area], either in terms of magnitude or dynamics, once our forest canopy closed," the authors "reconstructed 8 years (1996-2003) of L at Duke [University's] Free Air CO2 Enrichment [FACE] experiment to determine the effects of elevated atmospheric CO2 concentration on L before and after canopy closure in a pine forest with a hardwood component," where "the dynamics of L were reconstructed using data on leaf litterfall mass and specific leaf area for hardwoods, and needle litterfall mass and specific leaf area combined with needle elongation rates, and fascicle and shoot counts for pines."

What was learned
In response to the 200-ppm increase in atmospheric CO2 concentration supplied to the CO2-enriched forest FACE plots during the daylight hours of the growing season over the 8 years of the study, it was determined that the extra CO2 "enhanced pine L and the total canopy L (combined pine and hardwood species)," such that "on average, enhancement following canopy closure was ~16% and 14%, respectively," which for a 300-ppm increase in the air's CO2 concentration roughly corresponds to pine-only and combined-species L increases of about 24% and 21%, respectively. In addition, whereas the CO2-induced increase in pine-only L rose from essentially zero at low soil nitrogen (N) content to a maximum at the highest soil N levels found within the heterogeneous plots (which levels the researchers describe as only moderately fertile), the CO2-induced increase in the combined-species L "was constant across the N gradient."

What it means
The upshot of the latter finding, in the words of McCarthy et al., is that "in mixed species stands, L of canopy hardwoods which develop on lower fertility sites may be sufficiently enhanced under elevated CO2 to compensate for the lack of response in pine L," thereby generating "an appreciable response of total canopy L (~14%)," where little to no L increase might otherwise occur. This response, as noted above, roughly equates to a 21% rise in closed-canopy leaf area for a 300-ppm increase in the air's CO2 concentration, which is a pretty impressive finding.

Reviewed 12 March 2008