Forest Trees and Their Response to Atmospheric Carbon Dioxide Enrichment: A Compilation of Results

Forest ecosystems are widely recognized as key contributors to the global C cycle. Yet insights into the complex response of forests to rising CO2 concentrations and their capacity for C sequestration can only be gained through controlled-exposure studies on seedlings and young trees. This disparity between the scale of the experiment and that of the questions being posed presents a unique set of challenges for the scientific community. We explore these challenges by first compiling two data bases, one which describes the photosynthetic response of ∼40 forest tree species to elevated CO2 and another that tabulates the growth results from :60 controlled CO2-exposure studies. These analyses clearly show that as CO2 concentrations increase, so too does photosynthesis, growth, and C allocation to leaves, stems, and roots. Limiting supplies of N, P, and water only slightly restrict the growth response of trees to elevated CO2 concentrations. It is argued, however, that while these measures are useful in describing the response of individual trees to CO2 enrichment, they provide few insights into how such data can be extrapolated to the scale of mature trees, forest stands, and potentially forest ecosystems. One approach to improving our concept of extrapolation is to consider how small-scale physiological and morphological responses to elevated CO2 integrate to affect whole-plant performance and how that integration is likely to change as forest trees grow older and larger. Changes in phenology, leaf area index, leaf litter chemistry, and fine-root turnover are prime examples of how small-scale responses of trees to elevated CO2 can affect ecosystem-level processes. We conclude that controlled-exposure studies must be viewed as a means of obtaining guidance as to how forest ecosystems will respond to altered CO2 concentrations so that new insights can be gained into the processes that will ultimately shape the structure and composition of forests in a future, higher-CO2 world.