Leaf area index (LAI) and leaf angle distribution are widely used indices of vegetative canopy structure that are difficult to measure directly. This study was conducted to test a commercially available instrument for rapidly determining LAI and foliage inclination information from “fisheye” measurements of light interception. The instrument's estimates of LAI are compared with direct measurements in soybean [Glycine max (L.) Merr.], winter wheat (Triticum aestivum L.), and prairie grass. The dominant grass species in the plots were indian grass [Sorghastrum nutans (L.) Nash], switchgrass (Panicum virgatum L.), and big bluestem (Andropogon gerardii Vitman). The instrument's LAI resolution was better than 3%, and its LAI error was generally less than 15%. Variations in sky brightness patterns caused variations in LAI estimates in winter wheat of less than 10%, and the presence of direct solar radiation increased LAI errors to more than 30% in canopies of differing species and LAI. In the presence of gaps in the canopy, the sensor's azimuthal view should be reduced. A simple test indicates if a canopy's gaps are significant.

Due to climate differences, an extreme range in productivity occurs along a 250—km, west—east transect at °44° north latitude in western Oregon, USA, where coniferous evergreen forests dominate. As part of the Oregon Transect Ecosystem Research (OTTER) project, our objective was to evaluate how climate constrains net primary production (NPP) by limiting the utilization of intercepted photosynthetically active radiation (IPAR). The forests measured along the transect intercepted from 22% to 99.5% of the incident PAR. With data collected from recording meteorological stations installed near each site, we defined the hourly conditions when photosynthesis was partly or completely limited by drought, extreme humidity deficits, or frost. From this analysis we calculated that the fraction of incident PAR that could be utilized throughout the year ranged from 92% in the coastal rainforests to <25% in the juniper woodland. NPP varied from 3 to 26 Mg°ha—1°yr—1 with the fraction of belowground NPP, estimated from litterfall, increasing from 20% to 60% of the total as the environment becomes harsher. Light—use efficiency (°u) calculated under conditions when the environment did not constrain photosynthesis, averaged 0.8 g/MJ for aboveground NPP and 1.3 g/MJ for total NPP.

A simple equation has been developed for describing the bidirectional reflectance of some vegetative canopies and bare soil surfaces. The equation describes directional reflectance as a function of zenith and azimuth view angles and solar azimuth angle. The equation works for simulated and field measured red and IR reflectance under clear sky conditions. Hemispherical reflectance can be calculated as a function of the simple equation coefficients by integrating the equation over the hemisphere of view angles. A single equation for estimating soil bidirectional reflectance was obtained using the relationships between solar zenith angles and the simple equation coefficients for medium and rough soil distributions. The equation has many useful applications such as providing a lower level boundary condition in complex plant canopy models and providing an additional tool for studying bidirectional effects on pointable sensors. The focus of past research has been on cause and effect relationships (i.e., characteristics of the surface

Estimation of the maximum chlorophyll fluorescence yield under illumination, or FmЈ, by traditional single saturation pulse (SP) methodology is prone to underestimation error because of rapid turnover within photosystem (PS) II. However, measurements of fluorescence yield during several single pulses of variable intensity describes the irradiance dependence of apparent FmЈ, from which estimates of FmЈ at infinite irradiance can be derived. While such estimates have been shown to result in valid approximations of FmЈ, the need to apply several single pulses limits its applicability. We introduce a novel approach that determines the relationship between apparent FmЈ and variable irradiance within a singlẽ 1 s multiphase flash (MPF). Through experiments and simulations, we demonstrate that the rate of variation in irradiance during an MPF is critical for achieving quasisteady-state changes in the proportions of PSII acceptor side redox intermediates and the corresponding fluorescence yields, which are prerequisites for accurately estimating FmЈ at infinite irradiance. The MPF methodology is discussed in the context of improving the accuracy of various parameters derived from chlorophyll fluorescence measurements, such as photochemical and non-photochemical quenchings and efficiencies. The importance of using MPF methodology for interpreting chlorophyll fluorescence, in particular for integrating fluorescence and gas exchange measurements, is emphasized.

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