In the studies I present here, I examine the effects of CO2 on plants as they interact with other environmental variables, including nitrogen availability, herbivore damage (simulated and real) and temperature. Because of logistic and funding constraints, there are few truly long-term experimental studies of CO2 effects on plants. Most studies are conducted over the course of a subset of the plant’s life cycle and developmental stages. In the next two chapters, I present the results of studies I conducted on the response of an annual, Solanum ptycanthum (Fam. Solanaceae) to CO2 enrichment under varying levels of nitrogen availability and herbivore damage. The use of an annual allowed me to examine CO2 response over the entire life of the plant as well as assess CO2 effects on traits associated with fitness (i.e. fruit production). In Chapter 2, I describe experiments conducted at Duke University Phytotron in which I grew plants under greenhouse conditions, in artificial media and herbivore damage was simulated. The work I present in Chapter 3 repeats the basic design of the experiment in Chapter 2, but was conducted at UMBS under field conditions and in open-topped chambers. In the field experiment, plants were grown in natural soil, subject to damage from real herbivores (aphids), and were exposed to natural variations in sunlight, rainfall and temperature, factors which are typically less variable in greenhouse conditions. The use of both types of studies allowed me to make comparisons between them based on experimental technique: i.e. field vs. greenhouse. In Chapter 4, I present the results of a study conducted in collaboration with Rich Norby at Oak Ridge National Laboratories. As part of a much larger and long-term collaborative research effort, we examined the influence of CO2 and temperature on nitrogen retranslocation during autumn senescence in sugar maple and red maple (Acer saccharum and Acer rubrum). Our purpose was twofold: 1) to determine if nitrogen retained in senesced leaves could be predicted based on the nitrogen content of mature, photosynthetically active leaves, and 2) if CO2 and/or temperature affected the timing and/or efficiency of the nitrogen retranslocation process.