Environmental variability and experimental method as factors influencing plant response to elevated atmospheric carbon dioxide.

Abstract

The amount of CO$\sb2$ emitted as a result of anthropogenic activities has been sufficient to significantly alter the chemical composition of the atmosphere over the last 150 years. There is cause for concern over the potential for CO$\sb2$ and other compounds with greenhouse properties to produce large-scale changes in global climate through latent warming of the atmosphere. However, CO$\sb2$ is unlike other greenhouse gases in its ability to influence ecosystem-level processes, including global carbon and nutrient cycles, via direct effects on photosynthesis. In my dissertation, I present the results of studies on the interaction of CO$\sb2$ enrichment with other environmental variables in two different systems. I studied the combined effects of CO$\sb2$ enrichment, nitrogen (N) availability and herbivory (simulated and real) on the response of a native annual weed, Eastern Black Nightshade (Solanum ptycanthum, Fam. Solanaceae), under two different experimental conditions: greenhouse and field. In the greenhouse, CO$\sb2$ enrichment had no effect on reproduction or biomass production or distribution, but in the field, plant biomass and leaf area production were reduced. Damage incurred from real or simulated herbivory had little effect in either experiment, but was negatively correlated with fecundity and fruit yield in the greenhouse. N increased fecundity in both experiments, but fruit yield was only enhanced in the greenhouse. N also increased the biomass of all vegetative plant components and reduced relative allocation to reproduction in the greenhouse, but in the field, N increased relative allocation to reproduction and biomass was unaffected. Differences in the results obtained between the experiments may be due to a higher level of environmental variation in the field compared to the greenhouse and indicate that careful consideration of experimental method should be given when interpreting results in terms of potential effects in natural systems. I also conducted a study which examined the effects of variable levels of CO$\sb2$ and temperature on foliar N distribution and retranslocation in two species of maples (Acer saccharum and A. rubrum) during autumn senescence. The results indicated that CO$\sb2$ and temperature affect foliar N differently: elevated temperature increased N on both mass (%) and area bases, but CO$\sb2$ enrichment reduced %N and had no effect on N distribution per unit area. Elevated temperature also significantly delayed the onset on N retranslocation and leaf abscission, but affected species differently. In addition, I found that the amount of N retained in senesced leaves could be predicted on the basis on the N content of green leaves by a single regression equation across all species and treatment combinations. This analysis should prove useful for more accurately representing N dynamics in models of ecosystem response to global change.