The
overarching goal of a long-term, multi-investigator, regional study of
ecosystem-atmosphere carbon cycling in a mixed forest ecosystem in the upper
Midwest of the USA is to observe ecosystem-atmosphere exchange of carbon
dioxide at scales of relevance to the global carbon balance, while
simultaneously understanding the mechanisms governing this exchange. This
study, the Chequamegon Ecosystem-Atmosphere Study (ChEAS), brings together
multiple approaches to observing carbon fluxes, including chamber flux, sap
flux and biometric measurements at the plot scale (~1 m2), multiple
stand-level (~1 km2) eddy-covariance flux towers, landscape-scale (~10-100
km2) eddy-covariance flux measurements from the WLEF tall tower,
multiple regional (103-105 km2) atmospheric
boundary layer (ABL) budget approaches using tall tower mixing ratio
measurements, and a regional (~105 km2) ABL budget using
a network of CO2 mixing ratio measurements on communications towers.
Flux measurements have been up-scaled to the region using a variety of
approaches, and compared to the regional ABL budget methods. Top-down and
bottom-up methods fall within a range of values for growing-season flux
estimates that suggests a level of precision for regional flux estimates of
approximately 0.5 gC m-2 d-1. A multi-tower inverse study
should increase the level of precision of the ABL budget flux estimates. Interpreting
the mechanisms governing these fluxes requires plot- and stand-level data. These
data show that variability in seasonal and annual fluxes among flux towers is
large, refuting hypotheses that ecosystem-atmosphere exchange can be explained
simply by climate, or that a sparse flux tower network can be used to map
carbon fluxes over continental domains. Stand age and stand type (e.g. aspen,
wetland, northern hardwood forest) explain a large fraction but not all of the
observed variability among stands. More sophisticated land classification
schemes may be needed to improve the precision of bottom-up methods. Multi-year
records are used to examine interannual variability in the carbon balance of
the region and show that interannual variability at WLEF is clearly correlated
with climate variability. Limited multi-year records at the plot- and stand-level
partly support the hypothesis that year-to-year variability in carbon fluxes
are coherent across the region, and begin to describe the causes of the observed
interannual variability. Further study is needed to evaluate the network design
required to describe both the magnitude and mechanisms of interannual
variability in the regional carbon balance.
Author: K.J. Davis, A. Andrews, J.A. Berry, P.V. Bolstad, et al (kjd10 at psu dot edu)
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