Variability of Energy Vapor and CO2 Exchange over two Midwestern Deciduous Forests

Fluxes of CO2, H2O, and energy are measured on 46 m meteorological towers at two AmeriFlux sites. Turbulent fluxes are obtained by eddy-covariance, using 3-d sonic anemometers in conjunction with closed-path infrared gas analyzers. The experimental set-up, instrumentation, data processing and analysis are very similar between the two sites. However, there are distinct differences in their geographic and climatic setting. One site (MMSF, Indiana, 39 19' N) is located in an extensive forest (fetch < 4 km) with a wide variety of tree species (29 identified species) in a small-scale ridge-ravine topography. The other site (UMBS, 45 34' N) lies in the north of Michigan's lower peninsula in a northern hardwood forest dominated by only five species. The topography of the UMBS site is gently undulating, but lakes at a distance of 1 km or more may exert a mesoscale influence. Here we compare the seasonal variations of carbon dioxide, energy and vapor fluxes between the two sites for 1999. Differences in net ecosystem exchange of CO2 are discussed in terms of the energy and water regimes of the two sites. For the MMSF site, the results for 1999 will also be compared to those of 1998. The period from August to the end of November of 1999 was classified as a severe drought in the eastern United States. The effect of this drought on energy partitioning and carbon sequestration will be evaluated.