Experimental Design

FACE apparatus
Six 25-m diameter plots were laid out in 1997, and construction of the FACE facility began thereafter. One plot was found to have significantly different soil characteristics, and it was removed from the experiment. The subsequent experimental design comprises two plots with elevated [CO2] and three plots with ambient [CO2]. Each plot is surrounded by 24 vent pipes spaced 3.3 m apart suspended from 12 aluminum towers.

The vent pipes are connected to a plenum through which a radial blower pushes air at 2.0 m3 s-1. Pure CO2 is introduced to the air flow immediately after the blower so that it is diluted prior to reaching the vent pipes. The CO2 comes from a natural gas source (Linde/BOC Gases) and has a delta13C signature of -51‰. Regulation of the CO2 concentration is achieved using FACE equipment and software designed at Brookhaven National Laboratory.

CO2 Treatments
The CO2 treatment was initiated in April, 1998, prior to leaf, and has been maintained 24 hours per day every year between April and November. The [CO2] set-point in 1998 was a constant 565 ppm. In 1999 and 2000 a dual set-point (565 ppm day and 645 ppm night) was used to better represent the diurnal variation in ambient [CO2].  Nighttime fumigation was discontinued in 2001 because it interfered with soil respiration measurements. The averages below include periods when there was no CO2 enrichment because of equipment failure, interruptions in CO2 supply, high winds, or deliberate suspension for experimental purposes. The treatments are best characterized by the daytime concentrations given below. Note that ambient concentration is higher than the global average becasue of high values in early morning hours when wind is low.

[Daytime CO2] (ppmv)


Research Tasks
Ten tasks are organized around an interconnected set of hypotheses. The objectives of these tasks are:

  • characterize aboveground tree growth and leaf area in relation to CO2 concentration and interannual variation in other environmental factors;
  • determine the relationships between stand dynamics, photosynthesis, respiration, and tissue chemistry;
  • establish the importance of stomatal conductance, boundary layer conductance, and canopy conductance as factors regulating whole-plant transpiration response to elevated CO2;
  • measure stem respiration response in sweetgum to CO2 and partition observed responses into growth and maintenance respiration;
  • quantify fine root density and turnover;
  • determine the trajectory of response to CO2 in leaf litter chemistry, and begin long-term measurement of litter decomposition;
  • quantify the critical components of the nitrogen cycle and their response to CO2 enrichment;
  • identify sources and dynamics of soil organic matter and their response to CO2 enrichment;
  • construct an intra-annual carbon budget for the stands to determine the impact of elevated CO2 on net ecosystem production;
  • use a mechanistic, process-based soil-plant ecosystem model to develop and refine hypotheses of ecosystem response to elevated CO2.

New tasks were initiated in 2005 with a general objective to explore the implications of the dramatic fine root response to elevated CO2 because this response may have far reaching consequences for the long-term trajectory of this forest stand in a CO2-enriched atmosphere:

  • continue to quantify net primary productivity (NPP) because this integrative measure of ecosystem response provides the context for interpretation of other components of biogeochemical cycling, and the trajectory of the NPP response is the most important single product from this experiment;
  • determine the consequence of enhanced fine root production for soil C and N dynamics
  • explore implications of current results to ecosystem model implementation, particularly with regard to allocation, and use ecosystem models to generalize results to longer-term forest response

2009 is the final year of this experiment, the 12th year of treatment. Research tasks related to the shutsown of the experiment include:

  • maintain CO2 treatments and quantify net primary productivity (NPP) with minimum disruption from harvest activities;
  • dig two 80 x 80 x 90 cm soil pits in each plot to quantify root mass and length as a function of depth and provide access for detailed analysis of the root-soil interface’
  • harvest trees in July (with leaves) and November (leafless) to update allometric relationships and canopy structure
  • archive soil and plant samples for subsequent analyses