Results and Data

• Fine-root production was stimulated by elevated CO₂
• Initial enhancement of net primary productivity was not sustained because of feedbacks through the nitrogen cycle.

• Stable isotope analysis indicated that N availability declined faster in plots exposed to elevated CO₂
• Carbon storage in the soil increased in CO₂-enriched plots, including in protected forms.
• Successional development of the understory community was accelerated in elevated CO₂

The trajectory of the NPP response was the most important single product from the ORNL FACE experiment. The dynamics of the sweetgum stand and its response to elevated CO₂ were dominated by interactions with nitrogen. The initial stimulation of aboveground growth by elevated CO₂ was quickly dissipated, but NPP was enhanced with the additional productivity accounted for by enhanced production of fine roots, especially deeper in the soil, which sustained increased nitrogen uptake to support increased NPP. However, nitrogen availability steadily declined, leading to a loss of photosynthetic enhancement and the NPP response. This decline occurred faster in elevated CO₂, supporting the premise of progressive nitrogen limitation.

Data Availability

Data files describing CO₂ concentrations, meteorological conditions, and ecosystem responses are posted through the FACE Data Management System maintained by the Carbon Dioxide Information and Analysis Center. We hope this will make our data more accessible to users worldwide and to facilitate cross-site synthesis research and modeling activities.

Supplemental Table S1 for “Forest stand and canopy development unaltered by 12 years of CO₂ enrichment”