Stand development patterns at the Duke free air CO2 enrichment site

Background/Question/Methods Elevated CO₂ may impact many aspects of forest ecosystem development and function. Much attention has already been given to questions regarding stimulation of carbon uptake and growth. However, most elevated CO₂ experiments are short compared to the life of a forest. Thus, it remains unclear whether elevated CO₂ actually results in the accumulation of greater biomass in the long term, or whether it simply accelerates the process of stand development. To address this issue, we drew upon fourteen years of data from the Duke free-air CO₂ enrichment site (Duke FACE) to evaluate patterns in stand development, including tree size distribution, mortality, competition, and stand carrying capacity. Results/Conclusions Over the course of the study elevated CO₂ shifted the range of pine diameters upwards, but it did not noticeably change the shape of the distribution, implying an acceleration rather than a fundamental change to stand development.  During the same period, mortality rates (% trees dead per year) were similar under ambient and elevated CO₂ for pine (2.4% yr^-1) and hardwood species (1.2% yr^-1). Analysis of the progression of height/diameter relationships through the development of the stand also showed that elevated CO₂ did not result in taller trees for the same diameter, even at small tree sizes where a difference would indicate a change in the intensity of tree-to-tree competition. Finally, there was little evidence to suggest that elevated CO₂ modified the maximum stand carrying capacity although it may still be too early in the development of this forest stand to determine. Overall, elevated CO₂ appeared to have had minimal impact on stand development patterns.