PARENT SESSION
Oral Session #53: Elevated CO₂: Communities, ecosystems, soils.
Presiding: G. Lin
Wednesday, August 7. 8:00 AM to 11:30 AM. Coconino Meeting Room, TCC.

Elevated CO₂ does not alter LAI of a forest stand.

Norby, Richard^*,1, Sholtis, Johnna², ¹ Oak Ridge National Laboratory, Oak Ridge, TN² Texas Tech University, Lubbock, TX

ABSTRACT- Despite many years of experiments on the responses of woody plants to elevated CO₂, there are hardly any data from which we can predict the response of leaf area index (LAI) of a forest stand, a critical determinant of forest productivity. Reasonable hypotheses can be posed suggesting either increased or decreased LAI in response to CO₂ enrichment, but these cannot be tested with small groups of seedlings or saplings undergoing exponential growth. The response of LAI to elevated CO₂ can be measured directly in the Oak Ridge Experiment on CO₂ Enrichment of Sweetgum, a free-air CO₂ enrichment (FACE) experiment in a closed-canopy stand of the deciduous tree Liquidambar styraciflua. Annual leaf mass production was measured by collecting leaves in litter baskets as they fell. Leaf mass was converted to leaf area using canopy-averaged values for leaf mass per area, with corrections for dry mass loss during senescence. The timing of leaf area production in the spring was tracked using PAR sensors above and below the canopy. Leaf production and leaf loss data sets were fit to an empirical model allowing estimation of the seasonal progression of LAI. Leaf mass production was significantly enhanced by CO₂, but there was no effect on LAI, which ranged from 5.2 to 7.0 in different years. Leaf area duration, the integration of LAI with time, also was not altered by CO₂ enrichment. Analysis of hemispherical photographs revealed no effect of CO₂ on canopy gap fraction, which was about 15%. Unlike the responses of exponentially-growing tree seedlings or saplings, the stimulation of NPP by CO₂ enrichment in this stand is not associated with increasing leaf area, and there has been no evidence of a compensatory reduction in leaf area as previously observed in open-top chamber studies. Other forest stands may differ in response, but the evidence from this sweetgum stand is that elevated CO₂ does not affect LAI.

KEY WORDS: FACE, leaf area index, Liquidambar styraciflua, elevated CO₂