Oaks in a high-CO2 world

The concentration of carbon dioxide in the atmosphere is one environmental factor that is certain to influence the physiology and productivity of oak trees everywhere. Direct assessment of the impact of increasing CO2 is very difficult, however, because of the long-term nature of CO2 effects and the myriad potential interactions between CO2 and other environmental factors that can influence the physiological and ecological relationships of oaks. The CO2 responses of at least 11 Quercus species have been investigated, primarily in experiments with seedlings. The growth response varies considerably among these experiments, and there appears to be no basis for differentiating the response of oaks as a group from those of other woody plants. The more important challenge is to find a basis for addressing questions about the responses of oak forest ecosystems from experimental data on individual seedlings and saplings. A series of experiments with white oak (Quercus alba L) seedlings and saplings was focused toward larger-scale questions, such as whether N limitations would preclude growth responses to elevated CO2 and whether short-term physiological responses could be sustained over longer time scales. These experiments suggested three issues that are particularly important for addressing forest responses: leaf area dynamics, fine root production, and biotic interactions. By focusing seedling and sapling experiments toward these issues, we gain insight into the important processes that will influence ecosystem response and, at least in a qualitative sense, the sensitivity of those processes to elevated CO2.