Vegetation light use efficiency is a key physiological parameter at the canopy scale, and at the daily time step is a component of remote sensing algorithms for scaling gross primary production (GPP) and net primary production (NPP) over regional to global domains. For the purposes of calibrating and validating the light use efficiency (εg) algorithms, the components of εg– absorbed photosynthetically active radiation (APAR) and ecosystem GPP – must be measured in a variety of environments. Micrometeorological and mass flux measurements at eddy covariance flux towers can be used to estimate APAR and GPP, and the emerging network of flux tower sites offers the opportunity to investigate spatial and temporal patterns in εg at the daily time step. In this study, we examined the relationship of daily GPP to APAR, and relationships of εg to climatic variables, at four micrometeorological flux tower sites – an agricultural field, a tallgrass prairie, a deciduous forest, and a boreal forest. The relationship of GPP to APAR was close to linear at the tallgrass prairie site but more nearly hyperbolic at the other sites. The sites differed in the mean and range of daily εg, with higher values associated with the agricultural field than the boreal forest. εg decreased with increasing APAR at all sites, a function of mid‐day saturation of GPP and higher εg under overcast conditions. εg was generally not well correlated with vapor pressure deficit or maximum daily temperature. At the agricultural site, a εg decline towards the end of the growing season was associated with a decrease in foliar nitrogen concentration. At the tallgrass prairie site, a decline in εg in August was associated with soil drought. These results support inclusion of parameters for cloudiness and the phenological status of the vegetation, as well as use of biome‐specific parameterization, in operational εg algorithms.

Soil‐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.

Grazing by ungulates is common in grasslands and may influence evapotranspiration (ET). The Bowen ratio energy balance method (BREB) was used to measure ET from grazed (GR) and ungrazed (UGR) tallgrass prairie sites in northeastern Kansas, USA. Yearling steers were stocked on the GR site from day of year (DOY) 128 to 202 in 1999, and ET data were collected from DOY 141 to 295. Grazing reduced ET by 28% between DOY 179 and 207; mean ET values were 3.6 (GR) and 5.0 mm d−1 (UGR). During that period, leaf area index (LAI) was an average of 78% lower on the GR site, and below‐normal precipitation kept soil dry near the surface; hence, transpiration and evaporation of water from soil decreased. Lower ET during that period, conserved soil water in the 0‐ to 0.30‐m profile on the GR site. Before that (e.g., DOY 152–179), ET was similar between treatments, despite an average 70% lower LAI on the GR site compared with the UGR site. Above‐normal precipitation during that period probably maintained high evaporation of water from soil, thereby compensating for reductions in transpiration (via LAI removal) on the GR site. Cumulative ET values during the 155‐d study were estimated at 526 and 494 mm on the UGR and GR sites, respectively. Thus, grazing reduced seasonal ET by 6.1%. Late in the study, ET was higher on the GR site, despite a lower LAI compared with the UGR site. Younger leaves in regrowth after grazing resulted in delayed senescence, causing higher ET on the GR site.

We 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.

High temperature and drought stresses may reduce quality in cool-season turfgrasses during summer months in the transition zone. This growth chamber study was conducted to evaluate effects of high temperature and drought on physiology and growth of 'Apollo' Kentucky bluegrass (Poa pratensis L.) (KBG), 'Dynasty' tall fescue (Festuca arundincea Schreb.) (TF), and 'Thermal Blue', a hybrid (HBG) between KBG and Texas bluegrass (Poa arachnifera Torr.). Turfgrasses were exposed for 48 days to supra-optimal (high temperature; 35/25 o C, 14-h day/10-h night) and optimal (control; 22/15 o C, 14-h day/10-h night) temperatures under well-watered (100% evapotranspiration [ET] replacement) and deficit (60% ET replacement) irrigation. Heat resistance was greater in HBG, which had greater visual quality, gross photosynthesis (Pg), dry matter production, and lower electrolyte leakage and soil-surface temperatures than KBG and TF under high temperature. Cumulative Pg during the study was 16% and 24% greater in HBG than in KBG and TF, respectively. Green leaf area index (LAI) in HBG was not affected by high temperature, but LAI was reduced by 29 % in KBG and 38% in TF. Differences in drought resistance were negligible among species. The combination of high temperature and drought caused rapid declines in visual quality and dry matter production, but HBG generally performed better. Results indicated greater heat resistance, but not drought resistance, in HBG than in KBG or TF.1 High temperature and drought stresses are significant problems in cool-season turfgrasses during summer months in the U.S. transition zone, which covers 480 to 1120 km north to south between the northern regions where cool-season grasses are adapted and the southern regions where warm-season grasses are adapted (Dunn and Diesburg, 2004). High temperature and drought stresses often occur simultaneously during summer months and may limit growth and cause a severe decline in the visual quality of cool-season turfgrasses (Perdomo et al., 1996;Bonos and Murphy, 1999;Jiang and Huang, 2000;Wang and Huang, 2004). Recent increases in competition for water have resulted in restrictions in water use for irrigation of turfgrasses (EIFG, 2004), which further exacerbates the problem of drought stress in cool-season turfgrasses. Predictions of higher temperatures from global warming also suggest that heat stress in cool-season turfgrasses may become more common in some regions, including the transition zone (National Assessment Synthesis Team, 2000).Hybrid bluegrasses (HBG), which are genetic crosses between native Texas bluegrass and KBG, may have greater heat and drought resistance than other cool-season grasses. Hybrid bluegrasses have similar visual qualities to KBG, which is a fine-textured, cool-season turfgrass commonly used in lawns and golf courses in the U.S. (Read et al., 1999;Turgeon, 2002).Consequently, new cultivars of HBG are being investigated as potential water-saving, heatresistant alternatives to current cool-season turfgrasses. Abraham et al. (...

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Turfgrass A gronomy J our n al • Volu me 10 0 , I s sue 4 • 2 0 0 8 949 Published in Agron. J. 100:949-956 (2008).

Normalized difference vegetation index (NDVI, computed as [near infrared (NIR) -Red)]/[NIR +Red]) may provide an objective means to evaluate visual quality of turfgrass. The NDVI is influenced by red (visible) and NIR reflectance (invisible), but each may respond differently to environmental factors; basic information is lacking about the two components in relation to turf quality. In this 3-yr study near Manhattan, KS, we examined relationships of NDVI and its component reflectances along with visual quality ratings in Kentucky bluegrass {Poa pratensis L., 'Apollo'), two Kentucky bluegrass x Texas bluegrass {Poa arachnifera Torr.) hybrids (Thermal Blue' and 'Reveille'), and tall fescue {Festuca arundinacea Schreb., 'Dynasty'). Percentage green cover was measured with digital image analysis and shoot density was estimated visually to evaluate their impacts on turf quality and reflectance. Differences in NDVI and red and NIR reflectances were observed among turfgrasses at each level of quality. Across the range of turf quality, NDVI was influenced more strongly by red than NIR reflectance. Red reflectance was strongly affected by density (r = 0.85) and green cover (r = 0.86); NIR reflectance was affected by density (r = 0.63) but negligibly by green cover. Results suggest other fundamental factors that are poorly understood may be affecting NIR reflectance and, hence, NDVI in turf. These factors may confound relationships between NDVI and turf quality and require further study.