## 2 Methods  
### Grassland Sites  
Surface CO\({}_{2}\) concentrations were evaluated from three sites with differences in land cover and data, summarized in Table 1. The Nelson Environmental Study Area (39\({}^{\circ}\)N, 94\({}^{\circ}\)W) is comprised of 560 acres within the Kansas Field Station (KFS) owned by the University of Kansas. KFS is located north of Lawrence, Kansas, in a tallgrass prairie and deciduous forest ecotone. The eddy covariance (EC) tower at this upland site is part of the AmeriFlux network and started collecting data on July 15, 2007. It is within an abandoned grassland currently dominated by C\({}_{3}\) grasses, such as smooth brome (_Bromus inermis_ [PERSON]), tall fescue (_Festuca arundinacea_ [PERSON]), and Kentucky bluegrass (_Poa pratensis_ [PERSON]). Native grasses, forbs, and woody species are also present, and the soils are fine, montmorillonite, mesic Aquatic Argiudolls. From 1987 to 2007 this area has been experiencing woody encroachment despite management and burning approximately every five years [32]. Mean annual temperature in this temperate, mid-continental climate is 13\({}^{\circ}\)C, and mean annual precipitation is 937 mm.  
\begin{table}
\begin{tabular}{l c c c c c c} \hline \multirow{2}{*}{**Site**} & \multicolumn{3}{c}{**Features**} & \multicolumn{3}{c}{**Data**} \\ \cline{2-7}  & **Dominant Veg** & **Topography** & **Management** & **Missing Values (\(\%\))** & **RMSE (ppmv)** & **RMSE* (ppmv)** \\ \hline KFS & C\({}_{3}\) grass/forbs & upland & 5 yBurn & 18.57 & 12.9 & 13.9 \\ KZU & C\({}_{4}\) grass & upland & 1 yBurn & 6.43 & 18.9 & 18.8 \\ K4B & C\({}_{4}\) grass/C\({}_{3}\) forbs & lowland & 4 yBurn & 5.16 & 13.9 & 14.2 \\ \hline \end{tabular}
\end{table}
Table 1: Eddy covariance tower sites in northeastern Kansas.  
The Konza Prairie Biological Station (39\({}^{\circ}\)N, 96\({}^{\circ}\)W) is a field research station on 8600 acres south of Manhattan, Kansas, jointly owned by The Nature Conservancy and Kansas State University. Konza is located within the Flint Hills region known for flint-bearing limestone layers, and soils are fine, mixed, mesic Udic Argiustolls. This area is composed of native tallgrass prairie with primarily perennial C\({}_{4}\) grasses, such as big bluestem (_Andropogon gerardii_ [PERSON]), little bluestem (_A. scoparius_ Michx.), and indiangrass (_Sorghum nutans_ ([PERSON]), and other sub-dominant grasses, forbs, and woody species. The AmeriFlux eddy covariance tower, designated as tower KZU, is located on upland watershed site 1D that has been burned annually for several decades and has not been grazed for more than 30 years [22]. Data from another EC tower located on lowland watershed site 4B, a mix of C\({}_{3}\) forbs and C\({}_{4}\) grasses burned every four years to allow for woody encroachment, are also used in the analysis. The different vegetation and burning regime at this site, referred to as K4B, broaden the scope of the study by adding a third land-cover type to the comparison between surface and mid-tropospheric CO\({}_{2}\). Mean annual temperature for the Konza Prairie area is also 13\({}^{\circ}\)C. The mean annual precipitation is 835 mm, slightly less than the more eastern KFS site and characteristic of the Kansas precipitation gradient decreasing from east to west.