Date of Graduation
I examined the effects of long-term CO2 enrichment on leaf physiology, chemistry and phenology of overstory sweetgum (Liquidambar styraciflua L.) trees that were naturally established within an unmanaged loblolly pine plantation at the Duke Forest Free Air CO2 Enrichment (FACE) experiment. This experiment consisted of three control plots and three plots that had been continuously fumigated with elevated CO2. Elevated CO2 consistently increased photosynthesis in sun and shade leaves. Photosynthesis, however, was stimulated to a greater degree in sun leaves than shade leaves. Elevated CO2 stimulated apparent quantum yield, but there was no interaction between CO2 treatment and leaf position. A simple leaf-level model indicated that net carbon uptake per day was stimulated more in sun leaves than the shade leaves. I found no evidence of photosynthetic acclimation to CO2 enrichment in sun or shade leaves over three years. CO2 enrichment did not affect leaf nitrogen per unit leaf area (Na), chlorophyll or total nonstructural carbohydrates of sun or shade leaves. However, I found a strong relationship between Na and the modeled components of photosynthetic capacity, VCmax and Jmax, suggesting that photosynthetic capacity will be affected by changes in Na as the Duke Forest FACE experiment continues. CO2 treatment had little effect on the phenology and late season decline in photosynthesis of sweetgum leaves and, therefore, did not affect seasonal carbon gain by extending or shortening the growing season. I found a sustained reduction in stomatal conductance (g s) under elevated CO2 over four years and this effect was not different between sun and shade leaves. There was little evidence of stomatal acclimation in sun and shade leaves. Despite decreased g s under CO2 enrichment, relative stomatal limitation of photosynthesis was lower at elevated CO2 than at ambient CO2. Stomatal density was not changed by CO2 enrichment and, therefore, not associated with the CO2 induced decline in gs. Overall, my studies indicated that the effects of elevated CO2 on leaf processes of sweetgum trees have complex interactions with environmental and temporal variables that must be taken into account when considering the effects of CO2 enrichment on larger canopy and ecosystem scales.
Herrick, Jeffrey David, "Integrated responses of overstory sweetgum (Liquidambar styracuflua L.) trees to elevated atmospheric carbon dioxide: A field experiment at the Duke Forest FACE site." (2002). Graduate Theses, Dissertations, and Problem Reports. 9032.