Virtually all rangelands are harvested at some point during the year. Removal of plant leaf area and biomass alters a host of ecosystem processes. An experiment was conducted from June-October of 2005 and 2006 to study the effects of clipping tallgrass prairie at different dates on water vapor and CO2 fluxes. A chamber was designed to measure net carbon exchange (NCE), ecosystem respiration (RE), and water vapor fluxes with attention to selecting material and technology to minimize alterations to the microclimate of the chamber. By summing NCE and RE, Gross Canopy Photosynthesis (GCP) was calculated. Throughout the two-year study, the chamber had a minimal effect on microclimate, i.e. average chamber temperature increased 2.9° C, while chamber pressure increased only 0.3-Pa during measurements, and photosynthetically active radiation attenuation was approximately 10%. The immediate effect of all clipping treatments was a loss of leaf area and reductions in GCP, NCE, and RE and in most cases a decreased water vapor flux. Further patterns of carbon flux appear to be governed by the amount of water stress during canopy development. Following a period of drought, canopies that developed during periods of low water stress quickly increased carbon flux rates when water availability increased. However, flux rates from canopies, which developed under water stress, responded considerably slower to subsequent water availability. A separate experiment was conducted from June-October of 2006 to estimate GCP, leaf area index (LAI) and total aboveground biomass with a hyperspectral radiometer. All indexes used to estimate LAI and biomass had poor correlations with measured.. However, GCP was significantly correlated to all six indexes derived in this study. While GCP measured from June-October was significantly correlated to all indices, removal of the October scans greatly increased the accuracy of the estimates derived using indexes calculated from hyperspectral scans.

Flux Chamber. Picture taken of the chamber from opposite the IRGA. The light colored raceway along chamber bottom is air plenum. The foam strip along bottom seals the chamber to the soil frame.

Thirty-year seasonal and average monthly temperatures for 2005 and 2006 (a) and 30-year seasonal and measured precipitation for 2005 and 2006 (b) measured from Manhattan, KS.

Growing season patterns of mid-day soil temperature measured at 50 mm depth from the initial clipping treatment for 2005 (a) and 2006 (b).

Growing season patterns of soil water content in the top 200 mm of the soil profile for 2005 (a) and 2006 (b).

Gross canopy photosynthesis flux rates (GCP) as calculated as the summation of net carbon exchange and ecosystem respiration during 2005 from plots clipped on day of year (DOY) 157 and DOY 170 (a), plots clipped on DOY 188, DOY 202, and DOY 213 (b) and plots clipped on DOY 230 (c). Error bars represent 1 SE (n=3). A negative value indicates the biosphere acting as a sink for CO2.

Ecosystem respiration flux rates (RE) measured during 2005 with an opaque chamber from plots clipped on day of year (DOY) 157 and DOY 170 (a), plots clipped on DOY 188, DOY 202, and DOY 213 (b) and plots clipped on DOY 230 (c). Error bars represent 1 SE (n=3). A positive value indicates movement of CO2 to the atmosphere.

Net carbon exchange flux rates (NCE) measured during 2005 with a transparent chamber from plots clipped on day of year (DOY) 157 and DOY 170 (a), plots clipped on DOY 188, DOY 202, and DOY 213 (b) and plots clipped on DOY 230 (c). Error bars represent 1 SE (n=3). A negative value indicates the biosphere acting as a sink of CO2.

Water vapor flux rates (WV) measured during 2005 with a transparent chamber from plots clipped on day of year (DOY) 157 and DOY 170 (a), plots clipped on DOY 188, DOY 202, and DOY 213 (b) and plots clipped on DOY 230 (c). Error bars represent 1 SE (n=3).

Gross canopy photosynthesis flux rates (GCP) calculated as the summation of net carbon exchange and ecosystem respiration during 2006 from plots clipped on day of year (DOY) 160 and DOY 177 (a), plots clipped on DOY 187, DOY 198 (b) and plots clipped on DOY 221 and DOY 227 (c). Error bars represent 1 SE (n=3). A negative value indicates the biosphere acting as a sink of CO2.

Ecosystem respiration flux rates (RE) measured during 2006 with an opaque chamber from plots clipped on day of year (DOY) 160 and DOY 177 (a), plots clipped on DOY 187, DOY 198 (b) and plots clipped on DOY 221 and DOY 227 (c). Error bars represent 1 SE (n=3). A positive value indicates movement of CO2 to the atmosphere.

Net carbon exchange flux rates (NCE) measured during 2006 with a transparent chamber from plots clipped on day of year (DOY) 160 and DOY 177 (a), plots clipped on DOY 187, DOY 198 (b) and plots clipped on DOY 221 and DOY 227 (c). Error bars represent 1 SE (n=3). A negative value indicates the biosphere acting as a sink of CO2.

Water vapor flux rates (WV) measured during 2006 with a transparent chamber from plots clipped on day of year (DOY) 160 and DOY 177 (a), plots clipped on DOY 187, DOY 198 (b) and plots clipped on DOY 221 and DOY 227 (c). Error bars represent 1 SE (n=3).

Relationships between [(Rnir/Rrededge)-1] * PAR (a), Normalized Difference Vegetation Index (NDVI) (b), the PRI (c), Simple Ratio (SR) (d), Water Band Index (WBI) (e), [(Rnir/Rgreen)-1] * PAR (f), and Gross Canopy Photosynthesis (GCP) as determined by linear regression from June through October. The circled data points indicate the October 12th scans (n=99).

Relationships between [(Rnir/Rrededge)-1] * PAR (a), Normalized Difference Vegetation Index (NDVI) (b), PRI (c), Simple Ratio (SR) (d), Water Band Index (WBI) (e), [(Rnir/Rgreen)-1] * PAR (f), and Gross Canopy Photosynthesis (GCP) as determined by linear regression from June through September (n=81).

Relationships between (Rnir/Rgreen)-1 (a), (Rnir/Rrededge)-1 (b), Normalized Difference Vegetation Index (NDVI) (c), Simple Ratio (SR) (d) and Leaf Area Index as determined by linear regression from June through October (n=36).

Relationships between (Rnir/Rgreen)-1 (a), (Rnir/Rrededge)-1 (b), Normalized Difference Vegetation Index (NDVI) (c), Simple Ratio (SR) (d), and Leaf Area Index as determined by linear regression on data collect from June through August (n=18).