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Growth Response of a Closed-Canopy Sweetgum Forest to Atmospheric CO2 Enrichment
Norby, R.J., Sholtis, J.D., Gunderson, C.A. and Jawdy, S.S.  2003.  Leaf dynamics of a deciduous forest canopy: no response to elevated CO2Oecologia 10.1007/s00442-003-1296-2.

What was done
At the Oak Ridge National Environmental Research Park in Roane County, Tennessee, the authors measured leaf area index (LAI), leaf mass per unit area (LMA) and net primary productivity (NPP, see Norby et al., 2002) in a closed-canopy stand of Liquidambar styraciflua (sweetgum) trees for a period of four years (years 2-5 of the study) within four 25-m-diameter FACE plots, two of which were maintained at the ambient atmospheric CO2 concentration and two of which were maintained at ambient + 180 ppm CO2.

What was learned
The authors report "there was no effect of CO2 on any expression of leaf area, including peak LAI, average LAI, or leaf area duration."  However, LMA and, therefore, whole canopy leaf mass "were significantly increased by CO2 enrichment."  In addition, NPP was increased by 22% over the four years of the study.

What it means
There has long been concern that in a closed-canopy forest, where individual trees are constrained to occupy a smaller space than what they would occupy in an unrestricted environment, they may not be able to respond positively to atmospheric CO2 enrichment and sequester more biomass than trees growing in ambient air.  This study and that of Norby et al. (2002), however, demonstrate that this hypothesis is false, and that trees "bulk up" both their mass per unit leaf area and aboveground trunk and branch mass even in the face of real-world space restrictions provided by neighboring forest trees competing for soil, air and water resources.

Norby et al. additionally report that DeLucia et al. (2002) have observed the same thing in a similarly designed FACE experiment within a Pinus taeda stand of similar stature, where aboveground biomass increased by 27% in response to a 200-ppm increase in atmospheric CO2 concentration.  And there is also the sour orange tree study of Idso and Kimball (2001), where what are now mature trees are physically constrained to occupy much smaller spaces than what they are capable of expanding to fill, and where exposure to an extra 300 ppm of CO2 has for several years resulted in a near-constant 80% increase in trunk and branch mass and an even larger increase in fruit production.

In light of these several observations, we can confidently conclude with Norby et al. that "the current evidence seems convincing that LAI of non-expanding forest stands will not be different in a future CO2-enriched atmosphere and that increases in light use efficiency and productivity in elevated CO2 are driven primarily by functional responses rather than by structural changes."

DeLucia, E.H., George, K. and Hamilton, J.G.  2002.  Radiation-use efficiency of a forest exposed to elevated concentrations of atmospheric carbon dioxide.  Tree Physiology 22: 1003-1010.

Idso, S.B. and Kimball, B.A.  2001.  CO2 enrichment of sour orange trees: 13 years and counting.  Environmental and Experimental Botany 46: 147-153.

Norby, R.J., Hanson, P.J., O'Neill, E.G., Tschaplinski, T.J., Weltzin, J.F., Hansen, R.T., Cheng, W., Wullschleger, S.D., Gunderson, C.A., Edwards, N.T. and Johnson, D.W.  2002.  Net primary productivity of a CO2-enriched deciduous forest and the implications for carbon storage.  Ecological Applications 12: 1261-1266.

Reviewed 22 October 2003