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 were posted through the FACE Data Management System maintained by the Carbon Dioxide Information and Analysis Center. All of these files listed below have been moved to ESS-Dive (https://data.ess-dive.lbl.gov/data). Search on “ORNL FACE” or follow the links below. We hope this will make our data more accessible to users worldwide and to facilitate cross-site synthesis research and modeling activities.

CO2, weather, and soil data

Riggs J S ; Tharp M L ; Norby R J (2007): ORNL FACE Weather Data. CDIAC, ESS-DIVE repository. Dataset. doi:10.15485/1463817

Riggs J S ; Tharp M L ; Norby R J (2007): ORNL FACE CO2 Concentration Data. CDIAC, ESS-DIVE repository. Dataset. doi:10.15485/1463842

Tharp M L ; Norby R J (2007): ORNL FACE Soil Moisture Data. CDIAC, ESS-DIVE repository. Dataset. doi:10.15485/1463846

Plant Response data

NPP, dry mass, carbon mass, nitrogen, leaf area, and litter mass from 1998-2008. Excel file.

Norby R J ; Iverson C M ; Tharp M L (2007): ORNL FACE Nitrogen Concentrations (Leaves, Litter, Fine Roots, Wood). CDIAC, ESS-DIVE repository. Dataset. doi:10.15485/1463845

Norby R J ; Tharp M L (2007): ORNL FACE Basal Area. CDIAC, ESS-DIVE repository. Dataset. doi:10.15485/1463816

Ledford J ; Norby R J ; Tharp M L (2007): ORNL FACE Fine Root Data. CDIAC, ESS-DIVE repository. Dataset. doi:10.15485/1463811

Norby R J ; Tharp M L (2007): ORNL FACE Leaf Area Index (LAI). CDIAC, ESS-DIVE repository. Dataset. doi:10.15485/1463810

Norby R ; Warren J ; Iversen C ; Childs J (2021): ORNL FACE Final Harvest, 2009, with data on leaf characteristics and tree mass. ORNL FACE Experiment, ESS-DIVE repository. Dataset. doi:10.15485/1779883

Norby R (2021): ORNL FACE Tree Height, with data on height of a subset of trees measured every year, 1997-2009. Free-Air CO2 Enrichment (FACE) Experiment Synthesis Activities. ORNL FACE Experiment, ESS-DIVE repository. Dataset. doi:10.15485/1798175

FACE Synthesis and Model-Data Synthesis Files:

Norby R ; Oren R ; Boden T ; De Kauwe M ; Kim D ; Medlyn B ; Riggs J ; Tharp M ; Walker A ; Yang B ; Zaehle S (2015): Phase 1 Free Air CO2 Enrichment Model-Data Synthesis (FACE-MDS): Meteorological Data. CDIAC, ESS-DIVE repository. Dataset. doi:10.3334/CDIAC/FACE-MDS/MET.01

Walker A P ; De Kauwe M G ; Fenstermaker L F ; Hungate B ; Medlyn B ; Megonigal J P ; Oren R ; Pendall E ; Talhelm A F ; Zaehle S ; Zak D R ; Boden T ; Brown A L ; Burton A J ; Butnor J R ; Day F P ; Drake B G ; Dijkstra P ; Evans R D ; Finzi A C ; Iversen C M ; Jackson R B ; LeCain D ; McCarthy H R ; Powell T L ; Nowak R S ; Riggs J S ; Smith S D ; Stover D B ; Tharp L M ; Warren J M ; Wullschleger S D ; Norby R J (2018): FACE-MDS Phase 2: Data from Six US-Located Elevated CO2 Experiments. Free Air CO2 Enrichment Model Data Synthesis (FACE-MDS), ESS-DIVE repository. Dataset. doi:10.15485/1480325

Walker A P ; Yang B ; Boden T ; De Kauwe M G ; Fenstermaker L F ; Medlyn B ; Megonigal J P ; Oren R ; Pendall E ; Zak D R ; Zaehle S ; Burton A J ; Drake B G ; Evans R D ; Hungate B ; Johnson D P ; Kim D ; LeCain D ; Lewin K F ; Lu M ; Mueller K F ; Nowak R S ; Riggs J S ; Smith S D ; Tharp L M ; Zelikova T J ; Norby R J (2018): FACE-MDS Phase 2: Meteorological Data and Protocols. Free Air CO2 Enrichment Model Data Synthesis (FACE-MDS), ESS-DIVE repository. Dataset. doi:10.15485/1480328

Walker A P ; De Kauwe M G ; Medlyn B ; Zaehle S ; Asao S ; Guenet B ; Harper A ; Hickler T ; Jain A K ; Luo Y ; Lu X ; Luus K ; Shu S ; Wang Y ; Werner C ; Xia J ; Norby R J (2018): FACE-MDS Phase 2: Model Output. Free Air CO2 Enrichment Model Data Synthesis (FACE-MDS), ESS-DIVE repository. Dataset. doi:10.15485/1480327

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