## 2 Materials and Methods
### NDVI Measurements  
The Normalized Difference Vegetation Index (NDVI), a measure of green vegetation derived from measurements of reflected radiation, was calculated several ways. Individual sensor NDVI methods are indicated by distinct subscripts, as listed in Table 2 and explained below. For ground NDVI (indicated by \"NDVI\({}_{\text{G}}\)\"), measurements of the treatment plots were taken immediately before harvesting using a dual-detector field spectrometer (Uni-SpecDC, PP Systems, Amesbury, MA, USA) fitted with fiber optics. One fiber (UNI686, PP Systems, Amesbury, MA, USA) was attached to an upward-looking cosine-corrected foreoptic (UNI435, PP Systems) for downwelling irradiance. The downward-looking fiber (UNI684, PP Systems) was fitted with a field-of-view restrictor (\"hypotube\"--UNI688, PP Systems) that limited radiance sampling to a field-of-view to approximately 20 deg, providing a ground sampling diameter (IFOV) of approximately 0.5 m. These two detectors and foreoptics were periodically cross-calibrated by sampling over a 99% reflective reference standard (Spectralon, Labsphere, North Sutton, NH, USA). Reflectance (\(\rho\)) at each wavelength (\(\lambda\)) was then calculated from radiance (\(R\)) and irradiance (\(l\)) using the following equation:  
\[\rho_{\lambda}=\frac{\left(R_{\text{plot},\lambda}/I_{\text{plot},\lambda} \right)}{\left(R_{\text{panel},\lambda}/I_{\text{panel},\lambda}\right)}, \tag{1}\] \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Green Forbs** & **Dead Forbs** & **Green Grass** & **Dead Grass** \\ \hline Total Biomass & YES & YES & YES & YES \\ Green Biomass & YES & NO & YES & NO \\ Standing Biomass & NO & NO & YES & YES \\ Green Standing Biomass & NO & NO & YES & NO \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The vegetation types included in each category of harvested biomass. Graminoids are designated “grass” and non-graminoids are designated “forbs.” \begin{table}
\begin{tabular}{c c} \hline \hline
**Type** & **Description** \\ \hline NDVI\({}_{\text{G}}\) & NDVI from ground spectrometry methods \\ NDVI\({}_{\text{T}}\) & NDVI from the MODIS Terra platform \\ NDVI\({}_{\text{A}}\) & NDVI from the MODIS Aqua platform \\ NDVI\({}_{\text{S}}\) & NDVI calculated from the SPOT system \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Description of the versions of the NDVI derived from the different sensors used in this study. where the subscript \"plot\" indicates the plot measurement, and the subscript \"panel\" indicates measurements made over the 99% reflective reference panel. From these reflectance (\(\rho\)) measurements, we calculated NDVI for each plot, using reflectance in the red and near-infrared wavebands as follows:  
\[\mathrm{NDVI_{G}}=\frac{\rho_{\mathrm{NIR}}-\rho_{\mathrm{RED}}}{\rho_{ \mathrm{NIR}}+\rho_{\mathrm{RED}}}, \tag{2}\]  
where the subscript \"NIR\" indicates the near-infrared waveband (800 nm for \(\mathrm{NDVI_{G}}\)) and the subscript \"RED\" indicates the red waveband (680 nm for \(\mathrm{NDVI_{G}}\)). For a more direct comparison with satellite NDVI, we also collected reflectance from 100 points from a 1-ha sampling grid (Figure 2). In this comparison, we simulated each satellite band from ground spectral reflectance by convolving the reflectance spectra against the band passes for each satellite sensor (MODIS Aqua, Terra, and SPOT). These 100 simulated NDVI values were then averaged to provide an equivalent, ground-measured index to directly compare with the individual satellite pixel NDVI values as indicators of annual grassland productivity. NDVI values derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) were downloaded as ASCII subsets from the Oak Ridge National Laboratory Distributed Active Archive Center website ([[http://daac.ornl.gov/MODIS/](http://daac.ornl.gov/MODIS/)]([http://daac.ornl.gov/MODIS/](http://daac.ornl.gov/MODIS/))) for both the Terra (\"\(\mathrm{NDVI_{I}}\)\") and Aqua (\"\(\mathrm{NDVI_{A}}\)\") satellite platforms.