Project objectives :

To better understand the nature of soil-vegetation-atmosphere exchanges of carbon dioxide and other greenhouse gases, long-term (multi-year) measurements, analysis, and modeling of energy and mass exchanges in and over a deciduous forest in the midwestern United States are being undertaken. Long term objectives are to: i) determine the current dynamics of carbon cycling for a deciduous forest ecosystem in the Midwest by a concerted measurement effort with various approaches; ii) develop, evaluate and assess methods to scale-up from point measurements of fluxes to regional budgets; iii) quantify the regional contribution to, and impact on, global carbon dioxide; iv) develop and evaluate scenarios to predict and quantify the ecosystem impacts (physical/ chemical/ biological) of enhanced carbon dioxide concentrations; v) expand and enhance theoretical understanding of the mechanisms, rates and magnitudes of carbon dioxide exchange processes in deciduous forests; vi) create links with, and provide integral support to, other carbon cycle related projects being conducted in the region.

Approach :

The principal approach is to measure concentrations and fluxes of carbon dioxide (CO2) and water/latent heat (H2O/QE), sensible heat (QH) and momentum  directly, using eddy correlation equipment mounted at multiple levels on a 46 m fixed tower. These observations are complemented by measurements of the radiation balance components and photosynthetically active radiation (PAR) and by standard meteorological measurements. Currently, the full suite of eddy correlation and radiation balance measurements is carried out at two levels above the canopy (at 1.7 and 1.3 times the canopy height) and at one level in the understory, at 2 m above ground.

The rationale to maintain two levels of flux measurements above the canopy rests on the notion that they are separated far enough to result in different flux footprints. This approach thus facilitates estimates of spatial representativeness of fluxes within the naturally occurring heterogeneity of the deciduous forest ecosystem. Estimates of measurement representativeness and spatial variability are instrumental in the methodology for up-scaling from the measurement scale to the ecosystem scale that is being developed and tested. The sub-canopy level eddy correlation and energy balance measurements provide estimates of the roles of the understory and the soil in the carbon, water and heat balance of the ecosystem. These measurements are important to integrate and quantify our understanding of the interaction between soil, understory and main canopy in the competition for energy, water and carbon. In addition, sub-canopy measurements of air motions by a three-dimensional sonic anemometer is essential to evaluate low-level nighttime drainage flows that are suspected to affect the local carbon budget. The magnitude and dynamics of this effect are as yet unknown.

These three main observation levels are enhanced by profiles of carbon dioxide, temperature and humidity over the entire depth of the tower and by four micrometeorological stations that are distributed below canopy within the likely flux footprint of the tower. Profile measurements are achieved in two ways: (i) by ventilated temperature and relative humidity probes (at four levels) and (ii) by analyzing air sampled at eight levels in an infrared gas analyzer (IRGA) to obtain mean concentrations of CO2 and H2O. The sampling is switched on an automatic schedule between the eight levels by a computer controlled solenoid manifold. Hourly profiles of temperature, water vapor and CO2 allow the evaluation of heat, water and carbon storage changes in the canopy air volume that are not easily accessible by either flux measurements or modeling techniques.

Annual Reports  are available from the NIGEC Website (1996 onward, Midwestern center)