02693nas a2200265 4500008004100000245008500041210006900126300001500195490000800210520191600218653000902134653002002143653001902163653000802182653002002190653001502210100002302225700002002248700001702268700001702285700001402302700002802316700001702344856006602361 2011 eng d00aLinking plant growth responses across topographic gradients in tallgrass prairie0 aLinking plant growth responses across topographic gradients in t a1131 -11420 v1663 a

Aboveground biomass in grasslands varies according to landscape gradients in resource availability and seasonal patterns of growth. Using a transect spanning a topographic gradient in annually burned ungrazed tallgrass prairie, we measured changes in the height of four abundant C4 grass species, LAI, biomass, and cumulative carbon flux using two closely located eddy flux towers. We hypothesized that seasonal patterns of plant growth would be similar across the gradient, but the magnitude of growth and biomass accumulation would vary by topographic position, reflecting spatial differences in microclimate, slope, elevation, and soil depth. Thus, identifying and measuring local growth responses according to topographic variability should significantly improve landscape predictions of aboveground biomass. For most of the growth variables measured, classifying topography into four positions best captured the inherent spatial variability. Biomass produced, seasonal LAI and species height increased from the upland and break positions to the slope and lowland. Similarly, cumulative carbon flux in 2008 was greater in lowland versus upland tower locations (difference of 64 g m−2 by DOY 272). Differences in growth by topographic position reflected increased production of flowering culms by Andropogon gerardii and Sorghastrum nutans in lowland. Varying growth responses by these species may be a significant driver of biomass and carbon flux differences by topographic position, at least for wet years. Using a digital elevation model to classify the watershed into topographic positions, we performed a geographically weighted regression to predict landscape biomass. The minimum and maximum predictions of aboveground biomass for this watershed had a large range (86–393 t per 40.4 ha), illustrating the drastic spatial variability in growth within this annually-burned grassland.

10aANPP10aEddy covariance10aFlux footprint10aLAI10aMesic grassland10atopography1 aNippert, Jesse, B.1 aOcheltree, T.W.1 aSkibbe, A.M.1 aKangas, L.C.1 aHam, J.M.1 aShonkwiler-Arnold, K.B.1 aBrunsell, N. uhttps://link.springer.com/article/10.1007%2Fs00442-011-1948-602627nas a2200145 4500008004100000245011100041210006900152300001500221490000700236520214100243100001702384700001402401700001702415856004902432 2011 eng d00aValidating remotely sensed land surface fluxes in heterogeneous terrain with large aperture scintillometry0 aValidating remotely sensed land surface fluxes in heterogeneous  a6295 -63140 v323 a

The Large Aperture Scintillometer (LAS) has emerged as one of the best tools for quantifying areal averaged fluxes over heterogeneous land surfaces. This is particularly useful as a validation of surface energy fluxes derived from satellite sources. We examine how changes in surface source area contributing to the scintillometer and eddy covariance measurements relate to satellite derived estimates of sensible heat flux. Field data were collected on the Konza Prairie in Northeastern Kansas, included data from two eddy covariance towers: one located on an upland, relatively flat homogeneous area, and the second located in a lowland area with generally higher biomass and moisture conditions. The large aperture scintillometer spanned both the upland and lowland areas and operated with a path length of approximately 1 km specifically to compare to Moderate Resolution Imaging Spectroradiometer (MODIS) derived estimates of surface fluxes. The upland station compares well with the LAS (correlation of 0.96), with the lowland station being slightly worse (correlation of 0.84). Data from the MODIS sensor was used to compute surface fluxes using the ‘triangle’ method which combines the remotely sensed data with a soil-vegetation-atmosphere-transfer scheme and a fully developed atmospheric boundary layer model. The relative contribution to the surface observations is estimated using a simple footprint model. As wind direction varies, the relative contribution of upland and lowland sources contributing to the LAS measurements varies while the MODIS pixel contribution remains relatively constant. With the footprint model, we were able to evaluate the relationship between the LAS observations and the remotely sensed estimates of the surface energy balance. The MODIS derived sensible heat flux values correspond better to the LAS measurements (percentage error: 0.04) when there was a larger footprint compared to a time with a smaller footprint (percentage error: −0.13). Results indicate that the larger the footprint, the better the agreement between satellite and surface observations.

1 aBrunsell, N.1 aHam, J.M.1 aArnold, K.A. uhttps://doi.org/10.1080/01431161.2010.50805802661nas a2200205 4500008004100000245007200041210006900113300001100182490000700193520203000200653002502230653000902255653001402264653001802278653002002296653002702316100001502343700001402358856008302372 2010 eng d00aNet carbon fluxes over burned and unburned native tallgrass prairie0 aNet carbon fluxes over burned and unburned native tallgrass prai a72 -810 v633 a

Prescribed burning of aboveground biomass in tallgrass prairie is common and may influence dynamics and magnitudes of carbon (C) movement between the surface and atmosphere. Carbon dioxide (CO2) fluxes were measured for 2 yr using conditional sampling systems on two adjacent watersheds in an ungrazed tallgrass prairie near Manhattan, Kansas. One watershed was burned annually (BA) and the other biennially (BB). Leaf and soil CO2 fluxes were measured in the source area. Net ecosystem exchange (NEE) of CO2 reached a maximum daily gain of 26.4 g CO2 · m−2 · d−1 (flux toward surface is positive) in July 1998 (year when both sites were burned and precipitation was above normal); gains were similar between sites in 1998. The maximum daily NEE loss of CO2 was −21.8 g CO2 · m−2 · d−1 from BA in September 1997 (year when only BA was burned and precipitation was below normal). When data were integrated over the two years, both sites were net sources of atmospheric CO2; NEE was −389 g C · m−2 · 2 yr−1 on BA and −195 g C · m−2 · 2 yr−1 on BB. Burning increased canopy size and photosynthesis, but the greater photosynthesis was offset by corresponding increases in respiration (from canopy and soil). Carbon losses from fire represented 6–10% of annual CO2 emissions (bulk came from soil and canopy respiration). Data suggest that annual burning promotes C loss compared to less-frequently burned tallgrass prairie where prairie is not grazed by ungulates. Greater precipitation in 1998 caused large increases in biomass and a more positive growing season NEE, indicating that C sequestration appears more likely when precipitation is high. Because C inputs (photosynthesis) and losses (canopy and soil respiration) were large, small measurement or modeling errors could confound attempts to determine if the ecosystems are long-term CO2 sources or sinks.

10aconditional sampling10afire10agrassland10aKonza Prairie10aLand management10anet ecosystem exchange1 aBremer, D.1 aHam, J.M. uhttps://www.sciencedirect.com/science/article/pii/S1550742410500101?via%3Dihub01970nas a2200241 4500008004100000245016300041210006900204300001500273490000800288520116300296653001201459653002301471653001801494653001601512653001001528653002601538653001501564653001301579100001701592700001401609700001801623856008701641 2008 eng d00aAssessing the multi-resolution information content of remotely sensed variables and elevation for evapotranspiration in a tall-grass prairie environment0 aAssessing the multiresolution information content of remotely se a2977 -29870 v1123 a

Understanding the spatial scaling behavior of evapotranspiration and its relation to controlling factors on the land surface is necessary to accurately estimate regional water cycling. We propose a method for ascertaining this scaling behavior via a combination of wavelet multi-resolution analysis and information theory metrics. Using a physically-based modeling framework, we are able to compute spatially distributed latent heat fluxes over the tall-grass prairie in North-central Kansas for August 8, 2005. Comparison with three eddy-covariance stations and a large aperture scintillometer demonstrates good agreement, and thus give confidence in the modeled fluxes. Results indicate that the spatial variability in radiometric temperature (a proxy for soil moisture) most closely controls the spatial variability in evapotranspiration. Small scale variability in the water flux can be ascribed to the small scale spatial variance in the fractional vegetation. In addition, correlation analysis indicates general scale invariance and that low spatial resolution data may be adequate for accurately determining water cycling in prairie ecosystems.

10aEntropy10ainformation theory10aKonza Prairie10aLatent heat10aMODIS10aSpatial heterogeneity10aSVAT model10awavelets1 aBrunsell, N.1 aHam, J.M.1 aOwensby, C.E. uhttps://www.sciencedirect.com/science/article/abs/pii/S0034425708000655?via%3Dihub02770nas a2200337 4500008004100000245006800041210006800109300001300177490000800190520183400198653001102032653001102043653002902054653001202083653001002095653001502105653002702120653001502147100001702162700001602179700001602195700001602211700001702227700001302244700001602257700001902273700001402292700001602306700001402322856009602336 2006 eng d00aEvaluation of MODIS NPP and GPP products across multiple biomes0 aEvaluation of MODIS NPP and GPP products across multiple biomes a282 -2920 v1023 aEstimates of daily gross primary production (GPP) and annual net primary production (NPP) at the 1 km spatial resolution are now produced operationally for the global terrestrial surface using imagery from the MODIS (Moderate Resolution Imaging Spectroradiometer) sensor. Ecosystem-level measurements of GPP at eddy covariance flux towers and plot-level measurements of NPP over the surrounding landscape offer opportunities for validating the MODIS NPP and GPP products, but these flux measurements must be scaled over areas on the order of 25 km2 to make effective comparisons to the MODIS products. Here, we report results for such comparisons at 9 sites varying widely in biome type and land use. The sites included arctic tundra, boreal forest, temperate hardwood forest, temperate conifer forest, tropical rain forest, tallgrass prairie, desert grassland, and cropland. The ground-based NPP and GPP surfaces were generated by application of the Biome-BGC carbon cycle process model in a spatially-distributed mode. Model inputs of land cover and leaf area index were derived from Landsat data. The MODIS NPP and GPP products showed no overall bias. They tended to be overestimates at low productivity sites — often because of artificially high values of MODIS FPAR (fraction of photosynthetically active radiation absorbed by the canopy), a critical input to the MODIS GPP algorithm. In contrast, the MODIS products tended to be underestimates in high productivity sites — often a function of relatively low values for vegetation light use efficiency in the MODIS GPP algorithm. A global network of sites where both NPP and GPP are measured and scaled over the local landscape is needed to more comprehensively validate the MODIS NPP and GPP products and to potentially calibrate the MODIS NPP/GPP algorithm parameters.10abiomes10aGlobal10aGross primary production10aLandsat10aMODIS10aMonitoring10aNet primary production10aValidation1 aTurner, D.P.1 aRitts, W.D.1 aCohen, W.B.1 aGower, S.T.1 aRunning, SW.1 aZhao, M.1 aCosta, M.H.1 aKirschbaum, A.1 aHam, J.M.1 aSaleska, S.1 aAhl, D.E. uhttp://knz.lternet.edu/content/evaluation-modis-npp-and-gpp-products-across-multiple-biomes02102nas a2200133 4500008004100000245007600041210006900117300001500186490000700201520163200208100001401840700001701854856009701871 2004 eng d00aOn the construction and calibration of dual probe heat capacity sensors0 aconstruction and calibration of dual probe heat capacity sensors a1185 -11900 v683 aDual-probe heat-capacity (DPHC) sensors can be used to measure soil heat capacity (C), water content, and temperature. Research was conducted to test design factors that affect sensor calibration, including: (i) calibration media, (ii) diameter and length of the needle probes, (iii) sensor body material, and (vi) duration and total power of the applied heat pulse. All sensors were calibrated in media with known C, including: agar (water), water-saturated glass beads, and dry glass beads. Calibration consisted of collecting heat pulse data in a given media and then calculating the apparent probe spacing (r app, distance between heater and detector needles) that yielded correct value of C An ideal sensor would have the same r app regardless of media type. The r app for all sensor designs increased as C decreased, on average changing by 6.7% between agar and dry beads. This undesirable result was consistent with previous studies that showed DPHC sensors calibrated in agar overestimated C in drier soils. Needle diameter (1.27 vs. 1.65 mm), sensor body material (urethane vs. high-conductivity epoxy), and shortening of the detector probe had a small effect on r app Sensors made with urethane bodies, 1.27-mm diam. needle probes, and shortened temperature probes showed less sensitivity to calibration media and are therefore recommended. The r app for this design only increased by 2.6% between dry and water-saturated wet beads. Apparent probe spacing was not affected by changes in total applied power (400–1600 J m−1) or heat pulse duration (2–16 s) when the correct analytical model was used to compute C1 aHam, J.M.1 aBenson, E.J. uhttp://knz.lternet.edu/content/construction-and-calibration-dual-probe-heat-capacity-sensors02349nas a2200133 4500008004100000245010100041210006900142300001500211490000700226520182700233100001402060700001802074856012302092 2003 eng d00aExperimental test of density corrections on CO2 flux as measured using open-path eddy covariance0 aExperimental test of density corrections on CO2 flux as measured a1393 -14030 v953 aEddy covariance is the most direct technique for measuring water, C, and energy fluxes above crops and managed ecosystems. When using open-path gas analyzers, corrections for air density fluctuations due to heat and water vapor flux must be applied, and these corrections are often larger in magnitude than the CO2 fluxes. Lack of energy balance closure, a common problem when using eddy covariance, implies that CO2 fluxes often are underestimated. Research was conducted to evaluate performance of the density corrections by making eddy covariance measurements above a large parking lot where CO2 and water vapor fluxes were almost zero. Uncorrected and corrected flux measurements were compared to the “known” values to determine accuracy. Data also were collected from a tallgrass prairie and a cedar forest to examine how density corrections and adjustments for energy balance closure affected daily C balances. Raw measurements from the parking lot showed apparent, density-induced, downward CO2 fluxes (i.e., apparent photosynthesis) of approximately-0.4 mg m−2 s−1 that were correlated with sensible heat. On average, the daily uncorrected CO2 flux was-12.7 g m−2 d−1, but the density correction reduced and changed the direction of the flux to 1.8 g m−2 d−1, which was very close to independent chamber measurements of 2.8 g m−2 d−1 Density corrections in the forest and prairie changed average daily CO2 fluxes by 20 to 80%. Energy balance closure averaged 80 and 95% in the prairie and forest, respectively. Corrections based on energy balance closure changed daily C balances by 16 to 35%. A plethora of post-measurement corrections, coupled with lack of energy balance closure, signals the need for additional research before eddy covariance can be routinely applied in agronomic research.1 aHam, J.M.1 aHeilman, J.L. uhttp://knz.lternet.edu/content/experimental-test-density-corrections-co2-flux-measured-using-open-path-eddy-covariance02502nas a2200133 4500008004100000245010200041210006900143300001500212490000700227520198000234100001502214700001402229856012502243 2002 eng d00aMeasurement and modeling of soil CO2 flux in a temperate grassland under mowed and burned regimes0 aMeasurement and modeling of soil CO2 flux in a temperate grassla a1318 -13280 v123 aSoil-surface CO2 flux (Rs), which is a large component of the carbon (C) budgets in grasslands, usually is measured infrequently using static or dynamic chambers. Therefore, to quantify annual C budgets, estimates of Rs are required during days when no direct measurements of Rs are available. Other researchers have developed empirical models based on soil temperature, soil volumetric water content (θv), and leaf area index (LAI) that have provided reasonable estimates of Rs during the growing season in ungrazed tallgrass prairie. However, the effects of mowing and grazing, which are common in grasslands, on predictions of Rs from those models are uncertain. Predictions of Rs during dormancy (postsenescence to spring fire) also are uncertain. Data from a year-long mowing study, which simulated grazing, were used to refit these models. Output from the models then was compared to independent data collected from nearby prairie sites. Results showed that LAI must be included to accurately estimate Rs in mowed prairie ecosystems. When LAI was not included in the model, predicted daily Rs following mowing was nearly four times greater than measured Rs, and cumulative, annual Rs was overestimated by 95–102%. When LAI was included in the model, predictions of Rs were comparable to measured Rs in the mowing study. Annual estimates of cumulative Rs ranged from 3.93 to 4.92 kg CO2/m2. When comparing the model with independent chamber data from nearby sites, cumulative Rs during those studies was within ±9% of cumulative estimates calculated from measured Rs. The model overestimated daily Rs during a dry period, suggesting a nonlinear response of Rs to soil water content; soil water matric potential may be more appropriate than θv for modeling Rs. Data suggest that Rs, in addition to being dependent on soil temperature and soil water content, is dependent on the photosynthetic capacity of the canopy and the subsequent translocation of C belowground.1 aBremer, D.1 aHam, J.M. uhttp://knz.lternet.edu/content/measurement-and-modeling-soil-co2-flux-temperate-grassland-under-mowed-and-burned-regimes02654nas a2200169 4500008004100000245014000041210006900181300001300250490000600263520200200269653002202271100001802293700001402311700002002325700001502345856012402360 1999 eng d00aBiomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO20 aBiomass production and species composition change in a tallgrass a497 -5060 v53 aTo determine the long-term impact of elevated CO2 on primary production of native tallgrass prairie, we compared the responses of tallgrass prairie at ambient and twice-ambient atmospheric CO2 levels over an 8-year period. Plots in open-top chambers (4.5 m diameter) were exposed continuously (24 h) to ambient and elevated CO2 from early April to late October each year. Unchambered plots were monitored also. Above-ground peak biomass was determined by clipping each year in early August, and root growth was estimated by harvesting roots from root ingrowth bags. Plant community composition was censused each year in early June. In the last 2 years of the study, subplots were clipped on 1 June or 1 July, and regrowth was harvested on 1 October. Volumetric soil water content of the 0–100 cm soil layer was determined using neutron scattering, and was generally higher in elevated CO2 plots than ambient. Peak above-ground biomass was greater on elevated CO2 plots than ambient CO2 plots with or without chambers during years with significant plant water stress. Above-ground regrowth biomass was greater under elevated CO2 than under ambient CO2 in a year with late-season water stress, but did not differ in a wetter year. Root ingrowth biomass was also greater in elevated CO2 plots than ambient CO2 plots when water stress occurred during the growing season. The basal cover and relative amount of warm-season perennial grasses (C4) in the stand changed little during the 8-year period, but basal cover and relative amount of cool-season perennial grasses (C3) in the stand declined in the elevated CO2 plots and in ambient CO2 plots with chambers. Forbs (C3) and members of the Cyperaceae (C3) increased in basal cover and relative amount in the stand at elevated compared to ambient CO2. Greater biomass production under elevated CO2 in C4-dominated grasslands may lead to a greater carbon sequestration by those ecosystems and reduce peak atmospheric CO2 concentrations in the future.10atallgrass prairie1 aOwensby, C.E.1 aHam, J.M.1 aKnapp, Alan, K.1 aAuen, L.M. uhttp://knz.lternet.edu/content/biomass-production-and-species-composition-change-tallgrass-prairie-ecosystem-after-long02506nas a2200205 4500008004100000245008200041210006900123300001100192490000700203520185800210653001602068653002302084653000902107653001402116653002002130653002302150100001502173700001402188856009802202 1999 eng d00aEffect of spring burning on the surface energy balance in a tallgrass prairie0 aEffect of spring burning on the surface energy balance in a tall a43 -540 v973 aSpring burning of dead biomass in tallgrass prairie is a common practice that may influence heat and water vapor transport from the landscape. Bowen ratio methods were used to measure the surface energy balances and evapotranspiration (ET) from burned (B) and unburned (UB) prairie near Manhattan, KS, USA. Data were collected from day of year (DOY) 109–258, 1997 following fire on the B site on DOY 107. Early in the growing season, differences in albedo and surface conductance to water vapor transport (gs, i.e., mulch effect) caused large variations in energy fluxes between B and UB sites. During a 44-day period immediately after the burn (DOY 109–152), albedo averaged 43% lower on the B compared with the UB site. Consequently, available energy (net radiation minus soil heat flux) was 8.6% higher on the B than on the UB site. The gs during that time was over three times higher on the B site, a result of dead biomass removal by fire. During the same period, the daytime Bowen ratios averaged 0.79 on the B site and 2.89 on the UB site, with ET rates of 2.97 mm per day (B site) and 1.40 mm per day (UB site). By DOY 152, canopy growth had moderated differences in albedo and available energy between sites. However, gs and ET remained higher on the B site between DOY 152 and 181. Green leaf area index averaged 71% higher on the B site, and was the primary cause for this mid-season effect (i.e., differences in transpiration). By DOY 182, the effects of the burn on energy fluxes were negligible. Cumulative estimates of ET during the 150-day period were 503 mm on the B site and 408 mm on the UB site, thus, burning increased seasonal ET by 23.3%. Results suggest that land management or environmental factors that affect dead litter, albedo, or green leaf area will have strong impacts on the water and energy balances of a grassland.10aBowen ratio10aEvapotranspiration10afire10agrassland10aLand management10aSeasonal variation1 aBremer, D.1 aHam, J.M. uhttp://knz.lternet.edu/content/effect-spring-burning-surface-energy-balance-tallgrass-prairie02436nas a2200265 4500008004100000245008500041210006900126300001500195490000800210520160100218653001801819653002801837653001401865653001801879653002201897653002001919100002001939700001601959700001801975700002101993700001502014700001402029700001802043856010902061 1999 eng d00aElevated CO2 and leaf longevity in the C4 grassland dominant Andropogon gerardii0 aElevated CO2 and leaf longevity in the C4 grassland dominant And a1057 -10610 v1603 aIn central U.S. grasslands, plant and ecosystem responses to elevated CO2 are most pronounced when water availability is limited. In a northeast Kansas grassland, responses to elevated CO2 in leaf area, number, development, and longevity were quantified for the tallgrass prairie dominant, Andropogon gerardii. Plants were grown in open‐top chambers (OTCs) modified to limit water availability and to maximize responses to elevated CO2. In OTCs with elevated (×2 ambient) levels of CO2, aboveground biomass production and leaf water potentials were increased significantly compared with those of plants in OTCs with ambient CO2. There were no differences in leaf area or leaf number per tiller in A. gerardii in elevated compared with ambient OTCs. However, leaf area in adjacent unchambered plots with greater water availability was significantly higher than in the OTCs. The time required for developing leaves to achieve maximum leaf area was reduced by 29%, and the period of time until leaves senesced was increased by 20% for plants exposed to elevated compared with ambient CO2. Thus, leaves of this C4 grass species expanded more rapidly (6 d) and remained green longer (9 d) when exposed to elevated CO2. Such CO2‐mediated increases in leaf longevity in the dominant species may allow this grassland to respond more opportunistically to temporally variable rainfall patterns in high‐CO2 environments. These responses should be included in leaf‐based simulation models that attempt to mechanistically link physiological alterations to predicted canopy responses to increased CO2.10aEcophysiology10aelevated carbon dioxide10agrassland10aleaf lifespan10atallgrass prairie10aWater relations1 aKnapp, Alan, K.1 aBargman, N.1 aMaragni, L.A.1 aMcAllister, C.A.1 aBremer, D.1 aHam, J.M.1 aOwensby, C.E. uhttp://knz.lternet.edu/content/elevated-co2-and-leaf-longevity-c4-grassland-dominant-andropogon-gerardii02449nas a2200181 4500008004100000245013300041210006900174300001000243490000700253520172600260653003401986653002302020653003202043653003202075100001402107700002002121856012602141 1998 eng d00aFluxes of CO2, water vapor, and energy from a prairie ecosystem during the seasonal transition from carbon sink to carbon source0 aFluxes of CO2 water vapor and energy from a prairie ecosystem du a1 -140 v893 aIn many temperate-zone ecosystems, seasonal changes in environmental and biological factors influence the dynamics and magnitude of surface-atmosphere exchange. Research was conducted to measure surface-layer fluxes of CO2, water vapor, and energy in a C4-dominated tallgrass prairie during the autumnal transition from carbon sink to carbon source. Data were collected between DOY 220 and 320, 1996 on the Konza Prairie Research Natural Area near Manhattan, KS, USA. Mass fluxes were measured with a tower-based conditional sampling (CS) system, and the surface energy balance was measured with Bowen ratio (BR) methods. Soil-surface CO2 fluxes were measured with a closed-chamber system. Carbon and energy fluxes decreased over the study period as the canopy senesced. When skies were clear, daily net CO2 exchange (NCE) varied from a maximum gain of 17.8 g CO2 m−2 day−1 on DOY 226 to a maximum loss of −10.3 g CO2 m−2 day−1 on DOY 290. Over the 100-day study period, the ecosystem had a net loss of −217 g CO2 m−2, with the change from sink to source occurring on about DOY 255. Soil-surface CO2 fluxes were −0.4 mg CO2 m−2 s−1 at the start of the study but declined to −0.04 mg CO2 m−2 s−1 on DOY 320. The Bowen ratio increased from 0.5 to 4 over the study period. The seasonal trend in NCE was governed by the senescence of the canopy and not abrupt changes in weather. Senescence also influenced canopy conductance, which caused a seasonal transformation in the surface energy balance. Data suggest that any climatic or management factors that affect the rate and timing of the autumnal sink-source transition can have a strong influence, on the carbon and water balance in the ecosystem.10aCarbon sink-source transition10aSeasonal variation10aSurface-atmosphere exchange10aTallgrass prairie ecosystem1 aHam, J.M.1 aKnapp, Alan, K. uhttp://knz.lternet.edu/content/fluxes-co2-water-vapor-and-energy-prairie-ecosystem-during-seasonal-transition-carbon-sink02131nas a2200169 4500008004100000245008100041210006900122300001500191490000700206520155800213653002201771100001501793700001401808700001801822700002001840856010101860 1998 eng d00aResponses of soil respiration to clipping and grazing in a tallgrass prairie0 aResponses of soil respiration to clipping and grazing in a tallg a1539 -15480 v273 aSoil-surface CO2 flux (Fs) is an important component in prairie C budgets. Although grazing is common in grasslands, its effects on Fs have not been well documented. Three clipping treatments: (i) early-season clipping (EC); (ii) full-season clipping (FC); and (iii) no clipping (NC); which represented two grazing strategies and a control, were applied to plots in a tallgrass prairie in northeastern Kansas, USA. Measurements of Fs were made with a portable gas-exchange system at weekly to monthly intervals for 1 yr. Concurrent measurements of soil temperature and volumetric soil water content at 0.1 m were obtained with dual-probe heat-capacity sensors. Measurements of Fs also were obtained in grazed pastures. Fs ranged annually from 8.8 × 10−3 mg m−2 S−1 during the winter to 0.51 mg m−2 s−1 during the summer, following the patterns of soil temperature and canopy growth and phenology. Clipping typically reduced Fs 21 to 49% by the second day after clipping despite higher soil temperatures in clipped plots. Cumulative annual Fs were 4.94, 4.04, and 4.11 kg m−2 yr−1 in NC, EC, and FC treatments, respectively; thus, dipping reduced annual Fs by 17.5%. Differences in Fs between EC and FC were minimal, suggesting that different grazing strategies had little additional impact on annual Fs. Daily Fs in grazed pastures was 20 to 37% less than Fs in ungrazed pastures. Results suggest that grazing moderates Fs during the growing season by reducing canopy photosynthesis and slowing translocation of carbon to the rhizosphere.10atallgrass prairie1 aBremer, D.1 aHam, J.M.1 aOwensby, C.E.1 aKnapp, Alan, K. uhttp://knz.lternet.edu/content/responses-soil-respiration-clipping-and-grazing-tallgrass-prairie01868nas a2200181 4500008004100000245014500041210006900186300001300255490000800268520125600276653002001532100002101552700002101573700002001594700001401614700001801628856004001646 1997 eng d00aPhotosynthetic gas exchange and water relations responses of three tallgrass prairie species to elevated carbon dioxide and moderate drought0 aPhotosynthetic gas exchange and water relations responses of thr a608 -6160 v1583 aUndisturbed tallgrass prairie was exposed to ambient and elevated (twice-ambient) levels of atmospheric CO2 and experimental dry periods. Seasonal and diurnal midday leaf water potential (Ψ leaf), net photosynthesis $(A_{\text{net}})$, and stomatal conductance (g s) responses of three tallgrass prairie growth forms—a C4 grass, Andropogon gerardii; a broad-leaved woody C3 shrub, Symphiocarpos orbiculatus; and a C3 perennial forb, Salvia pitcheri—were assessed. $\Psi _{\text{leaf}}$ in A. gerardii and S. orbiculatus was higher under elevated CO2, regardless of soil moisture, while $\Psi _{\text{leaf}}$ in S. pitcheri responded only to drought. Elevated CO2 always stimulated $A_{\text{net}}$ in the C3 species, while A. gerardii $A_{\text{net}}$ increased only under dry conditions. However, $A_{\text{net}}$ under elevated CO2 in the C3 species declined with drought but not in the C4 grass. Under wet conditions, g s reduced in elevated CO2 for all species. During dry periods, gs at elevated CO2 was sometimes higher than in ambient CO2. Our results support claims that elevated CO2 will stimulate tallgrass prairie productivity during dry periods and possibly reduce temporal and spatial variability in productivity in these grasslands.10aWater relations1 aHamerlynck, E.P.1 aMcAllister, C.A.1 aKnapp, Alan, K.1 aHam, J.M.1 aOwensby, C.E. uhttp://www.jstor.org/stable/247492101765nas a2200181 4500008004100000245008500041210006900126300001300195490000600208520115400214653002201368100001801390700001401408700002001422700001501442700001501457856011101472 1997 eng d00aWater vapor fluxes and their impact under elevated CO2 in a C4 tallgrass prairie0 aWater vapor fluxes and their impact under elevated CO2 in a C4 t a189 -1950 v33 aWe measured leaf-level stomatal conductance, xylem pressure potential, and stomate number and size as well as whole plant sap flow and canopy-level water vapour fluxes in a C4-tallgrass prairie in Kansas exposed to ambient and elevated CO2. Stomatal conductance was reduced by as much as 50% under elevated CO2 compared to ambient. In addition, there was a reduction in stomate number of the C4 grass, Andropogon gerardii Vitman, and the C3 dicot herb, Salvia pitcheri Torr., under elevated CO2 compared to ambient. The result was an improved water status for plants exposed to elevated CO2 which was reflected by a less negative xylem pressure potential compared to plants exposed to ambient CO2. Sap flow rates were 20 to 30% lower for plants exposed to elevated CO2 than for those exposed to ambient CO2. At the canopy level, evapotranspiration was reduced by 22% under elevated CO2. The reduced water use by the plant canopy under elevated CO2 extended the photosynthetically-active period when water became limiting in the ecosystem. The result was an increased above- and belowground biomass production in years when water stress was frequent.10atallgrass prairie1 aOwensby, C.E.1 aHam, J.M.1 aKnapp, Alan, K.1 aBremer, D.1 aAuen, L.M. uhttp://knz.lternet.edu/content/water-vapor-fluxes-and-their-impact-under-elevated-co2-c4-tallgrass-prairie00666nas a2200205 4500008004100000245006700041210006700108260002700175300001300202653002200215100001800237700001400255700002000269700001700289700001600306700001500322700001400337700001700351856009200368 1996 eng d00aEcosystem level responses of tallgrass prairie to elevated CO20 aEcosystem level responses of tallgrass prairie to elevated CO2 aLondonbAcademic Press a147 -16210atallgrass prairie1 aOwensby, C.E.1 aHam, J.M.1 aKnapp, Alan, K.1 aRice, C., W.1 aCoyne, P.I.1 aAuen, L.M.1 aKoch, G.W1 aMooney, H.A. uhttp://knz.lternet.edu/content/ecosystem-level-responses-tallgrass-prairie-elevated-co202903nas a2200217 4500008004100000245010500041210006900146300001100215490000800226520215900234653001702393653001602410653002502426653002202451653002002473100002002493700002102513700001402534700001802548856011902566 1996 eng d00aResponses in stomatal conductance to elevated CO2 in 12 grassland species that differ in growth form0 aResponses in stomatal conductance to elevated CO2 in 12 grasslan a31 -410 v1253 aResponses in stomatal conductance (g st ) and leaf xylem pressure potential (ψ leaf ) to elevated CO2 (2x ambient) were compared among 12 tallgrass prairie species that differed in growth form and growth rate. Open-top chambers (OTCs, 4.5 m diameter, 4.0 m in height) were used to expose plants to ambient and elevated CO2 concentrations from April through November in undisturbed tallgrass prairie in NE Kansas (USA). In June and August, ψ leaf was usually higher in all species at elevated CO2 and was lowest in adjacent field plots (without OTCs). During June, when water availability was high, elevated CO2 resulted in decreased g st in 10 of the 12 species measured. Greatest decreases in g st (ca. 50%) occurred in growth forms with the highest potential growth rates (C3 and C4 grasses, and C3 ruderals). In contrast, no significant decrease in g st was measured in the two C3 shrubs. During a dry period in September, reductions in g st at elevated CO2 were measured in only two species (a C3 ruderal and a C4 grass) whereas increased g st at elevated CO2 was measured in the shrubs and a C3 forb. These increases in g st were attributed to enhanced ψ leaf in the elevated CO2 plants resulting from increased soil water availability and/or greater root biomass. During a wet period in September, only reductions in g st were measured in response to elevated CO2. Thus, there was significant interspecific variability in stomatal responses to CO2 that may be related to growth form or growth rate and plant water relations. The effect of growth in the OTCs, relative to field plants, was usually positive for g st and was greatest (>30%) when water availability was low, but only 6–12% when ψ leaf was high. The results of this study confirm the importance of considering interactions between indirect effects of high CO2 of plant water relations and direct effects of elevated CO2 on g st , particularly in ecosystems such as grasslands where water availability often limits productivity. A product of this interaction is that the potential exists for either positive or negative responses in g st to be measured at elevated levels of CO2.10aelevated CO210aGrowth form10aStomatal Conductance10atallgrass prairie10aWater relations1 aKnapp, Alan, K.1 aHamerlynck, E.P.1 aHam, J.M.1 aOwensby, C.E. uhttp://knz.lternet.edu/content/responses-stomatal-conductance-elevated-co2-12-grassland-species-differ-growth-form01907nas a2200133 4500008004100000245008300041210006900124300001300193490000700206520142600213100001701639700001401656856010301670 1995 eng d00aMeasurements of water use by prairie grasses with heat balance sap flow gauges0 aMeasurements of water use by prairie grasses with heat balance s a150 -1580 v483 aDirect and continuous measurements of water use by range grasses are needed by both range scientists and land managers. This study tested a heat balance sap flow gauge on individual culms of the tallgrass prairie species big bluestem (Andropogon gerardii Vitman) and indiangrass [Sorghastrum nutans (L.) Nash]. Gauge performance was evaluated on potted plants in the laboratory, greenhouse, and field by comparing sap flow to gravimetric measurements of transpiration. In the laboratory, gauge-measured water loss was consistently within ±10% of gravimetric measurements for both species at flow rates ≤ 4 g $\text{hour}^{-1}$. The first-order time constant of the gauge was calculated to be <20 seconds. In the greenhouse, sap flow estimates were consistently below gravimetric water loss and negative flows were often computed because of suspected errors in the radial heat flux component. Laboratory data showed that despite the gauge being surrounded with insulation, errors in the heat balance could occur because of external air temperature changes. In the field, environmental alterations in the stem energy balance affected the accuracy of gauges placed outside a plant canopy, but accurate measurements did occur when the plants were placed within a plant canopy. Heat transfer analysis indicated that foam insulation should be 20 to 25 mm thick to minimize the effect of the environment on gauge performance.1 aSenock, R.S.1 aHam, J.M. uhttp://knz.lternet.edu/content/measurements-water-use-prairie-grasses-heat-balance-sap-flow-gauges