Showing papers on "Leaf area index published in 1986"

Temporal and spatial patterns of specific leaf weight in successional northern hardwood tree species

Abstract: Temporal and spatial patterns of specific leaf weight (SLW, g/m2) were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan to evaluate the utility of SLW as an index of leaf photosynthetic capacity. No significant diurnal changes in SLW were found. Specific leaf weight decreased and then increased during leaf expansion in the spring. Most species, especially those located in the understory, then had relatively constant SLW for most of the growing season, followed by a decline in SLW during autumn. Specific leaf weight decreased exponentially down through the canopy with increasing cumulative leaf area index. Red oak (Quercus rubra), paper birch (Betula papyrifera), bigtooth aspen (Populus grandidentata), red maple (Acer rubrum), sugar maple (A. saccharum), and beech (Fagus grandifolia) generally had successively lower SLW, for leaves at any one level in the canopy. On a given site, comparisons between years and comparisons of leaves growing within 35 cm of each other showed that differences in SLW among species were not due solely to microenvironmental effects on SLW. Bigtooth aspen, red oak, and red maple on lower-fertility sites had lower SLW than the same species on higher-fertility sites. Maximum CO2 exchange rate, measured at lightsaturation in ambient CO2 and leaf temperatures of 20 to 25 C, increased with SLW. Photosynthetic capacities of species ranked by SLW in a shaded habitat suggest that red oak, red maple, sugar maple, and beech are successively better adapted to shady conditions.

Remote Sensing of Coniferous Forest Leaf Area

Abstract: Many important ecological questions concern eco? system processes occurring over large areas. However, our understanding of ecosystem functions is derived primarily from research executed on small, intensively studied sites, and extrapolation to large areas is diffi? cult. For example, it is not known definitively whether the land biota act as a source or a sink in the global carbon cycle, or whether increases in carbon dioxide concentrations and the subsequent predicted global warming would stimulate or suppress land vegetation (Bolin 1977, Woodwell et al. 1983). Much of this indecision results from our inability to directly measure important vegetation properties on large spatial scales. Estimates of the global carbon content of terrestrial plant biomass range from 450 x 1015 to 1000 x 1015 g. These estimates are derived by extrapolation of data from sites of intensive study to the areal coverage of vegetation assumed to be equivalent; hence no direct means of verification exist. As a consequence, no defensible estimate of energy and mass exchange rates is possible for large areas of terrestrial vegetation. Four recent workshops have evaluated the potential of advanced satellite technology for direct measure? ment of critical vegetation characteristics over large areas (Botkin 1982, 1985,Goody 1982, Wittwer 1983). Each workshop ultimately identified leaf area index (LAI, the area of leaf over a given area of ground) as the single variable both amenable to measurement by satellite and of greatest importance for quantifying en? ergy and mass exchange by plant canopies over landscapes. Characterizing vegetation in terms of LAI, rath? er than species composition, is a critical simplification for regional and global comparison of different terres? trial ecosystems. Previous research in crops and grass? lands has shown leaf area and biomass to be correlated with reflectance values measured by satellite-based sensors (Wiegand et al. 1979). We report here the first attempt to measure the LAI of coniferous forests using optical scanners of satellite resolution.

Comparison of Methods of Estimating Leaf‐Area Index In Old‐Growth Douglas‐Fir

Abstract: Leaf-area index (LAI, the projected total surface area of foliage per unit ground area) of an old-growth Douglas-fir stand in western Oregon was estimated from litterfall, light interception, sapwood cross-sectional area, and tree diameter. Estimates made by the first three techniques were similar, but the estimate based on tree diameter was twice as high as the others. For large trees with variable amounts of live crown, estimates of leaf area based on tree diameter appear to be inaccurate; therefore, the exceedingly high leaf-area indices previously reported for Douglas-fir forests are unre- liable. Sapwood cross-sectional area varies in correspondence with the canopy area and therefore is a better estimator of leaf area on large trees. Maximum LAI estimates based on sapwood area are similar to those for other temperate coniferous forests.

Canopy structure, light microclimate and leaf gas exchange of Quercus coccifera L. in a Portuguese macchia: measurements in different canopy layers and simulations with a canopy model

Abstract: The structural characteristics of a diverse array of Quercus coccifera canopies were assessed and related to measured and computed light attenuation, proportion of sunlit foliage, foliage temperatures, and photosynthesis and diffusive conductance behavior in different canopy layers. A canopy model incorporating all components of shortwave and longwave radiation, and the energy balance, conductance, and CO2 and H2O exchanges of all leaf layers was developed and compared with measurements of microclimate and gas exchange in canopies in four seasons of the year. In the denser canopies with a leaf area index (LAI) greater than 5, there is little sunlit foliage and the diffuse radiation (400–700 nm) is attenuated to 5% or less of the global radiation (400–700 nm) incident on the top of the canopy. Foliage of this species is nonrandomly distributed with respect to azimuth angle, and within each canopy layer, foliage azimuth and inclination angles are correlated. A detailed version of the model which computed radiation interception and photosynthetic light harvesting according to these nonrandom distributions indicated little difference in whole-canopy gas exchange from calculations of the normal model, which assumes random azimuth orientation. The contributions of different leaf layers to canopy gas exchange are not only a function of the canopy microclimate, but also the degree to which leaves in the lower layers of the canopy exhibit more shade-leaf characteristics, such as low photosynthetic and respiratory capacity and maximal conductance. On cloudless days, the majority of the foliage in a canopy of 5.4 LAI is shaded —70%–90% depending on the time of year. Yet, the shaded foliage under these conditions is calculated to contribute only about one-third of the canopy carbon gain. This contribution is about the same as that of the upper 13% of the canopy foliage. Computed annual whole-canopy carbon gain and water use are, respectively, 60% and 100% greater for a canopy of 5 LAI than for one of 2 LAI. Canopy water-use efficiency is correspondingly less for the canopy of 5 LAI than for that of 2 LAI, but most of this difference is apparent during the cool months of the year, when moisture is more abundant.

Satellite-derived leaf-area-index and vegetation maps as input to global carbon cycle models-a hierarchical approach

Abstract: A hierarchical procedure for developing a leaf area index (LAI) map of deciduous boreal forests is studied. The collection of spectral reflectance data from the Boundary Waters Canoe area in Minnesota using helicopter-, high-altitude aircraft-, and Landsat-mounted spectral sensors is described. The relationship between LAI and biomass and the reflectance ratio is analyzed. The sensitivity of canopy reflectance in the visible and infrared to the LAI of the canopy for various boreal forest species is evaluated. The data reveal that Landsat data are useful for producing LAI maps of deciduous forest areas and the maps provide data which clarifies the function of vegetation in the global carbon cycle models.

Light Interception and Leaf Area Estimates from Measurements of Grass Canopy Reflectance

Abstract: Grassland is a major component of the Earth's available land. The vast area and remoteness of this ecosystem makes it difficult to assess its condition and monitor productivity by traditional niethods. Remote sensing potentially offers a rapid nondestructive approach for monitoring such ecosystems. A study was carried out in a tallgrass prairie site near Manhattan, Kansas, during the 1983 and 1984 seasons to investigate the feasibility of estimating light interception and green leaf area index (LAI) from measurements of canopy multispectral reflectance. Greenness (G, i) index was found to be strongly correlated with intercepted photosynthetically active radiation ( PAR). Two methods, a direct regression (RGR) and an indirect approach (IND), were used to estimate LAI from Goi index. The LAI values estimated by RGR method were consistently lower than the measured ones; however, good agreement was obtained between the LAI values estimated by IND method and the measured LAI. This suggests that Goi transformation of canopy spectral reflectance is more closely related to the fraction of intercepted PAR by green foliage than the quantity of green LAI.

Environmental Productivity Indices for a Chihuahuan Desert Cam Plant, Agave Lechuguilla

Abstract: Productivity of Agave lechuguilla, a commercially harvested plant that occurs over vast areas of the Chihuahuan Desert, was measured using conventional dry mass changes in the field and was predicted based on physiological responses to environmental variables in the laboratory. An environmental productivity index (EPI) was constructed as the product of indices for water status, leaf temperature, and photosynthetically active radiation (PAR). Each of these component indices was assigned a maximum value of unity when that variable was not limiting net CO2 uptake over a 24—h period. Soil water potential, daily air and leaf temperatures, and PAR in the planes of the leaves at the field site in Coahuila, Mexico, could thus be quantitatively described in terms of their effect on net CO2 uptake. Water status proved to be the most important of the three variables considered, nearly all leaf unfolding occurring during the relatively wet summer and early fall. The rosette habit and low leaf area index (1.23) for the plant groups monitored led to a relatively uniform and high level of PAR over the leaves. Seasonal changes in PAR proportionally affected both the modest daytime net CO2 uptake and the predominant nighttime net CO2 uptake for this crassulacean acid metabolism plant, while seasonal variations in temperature had relatively small effects on net CO2 uptake over a 24—h period. EPI was highly correlated with the number of new leaves unfolding each month in the field (°2 °0.83); counting unfolding leaves in a nondestructive method of estimating productivity. For the 1—yr study period EPI averaged 0.28, which led to a predicted annual dry mass gain per unit leaf area of 0.68 kg/m2. Field measurements indicated that the actual dry mass gain was about half this value, the difference representing photosynthate needed for constructing and maintaining folded leaves, stem, and roots. The productivity of A. lechuguilla per unit ground area explored by its roots was 0.38 kg.m—2.yr—1, which although much less than for agricultural crops, is still much greater than the average productivity for desert ecosystems.

Shoot and Root Responses to Water Deficits in Rainfed Lowland Rice

Abstract: A field study was conducted to determine the response of the rice cultivar IR54 to a gradient of soil moisture conditions imposed for 19 days at the vegetative stage using a line source sprinkler system. A mild plant water stress at the vegetative growth stage decreased tiller number, leaf area index (LAI), apparent canopy photosynthetic rates, leaf nitrogen per unit land area, shoot and total root dry mass, and total root length density. After complete stress relief by reflooding, LAI and crop growth remained below that of unstressed plants. The lower cumulative assimilation per unit land area in the stressed treatments resulted in reduced shoot and root dry matter yields and lower grain yields. Water stress increased the ratio of shoot dry mass to root dry mass, and the ratio of leaf area to total root length. The decrease in root length was attributed to increased soil mechanical impedance.

Spectral Characterization of Biophysical Characterstics in a Boreal Forest: Relationship between Thematic Mapper Band Reflectance and Leaf Area Index for Aspen

Abstract: Initial results from analysis of a data set of simultaneous measurements of Thematic Mapper band reflectance and leaf area index are presented. The measurements were made over pure homogenous stands of Aspen in the Superior National Forest of northern Minnesota. The analysis indicates that the reflectance may be sensitive to the leaf area index of the Aspen early in the season. The sensitivity disappears as the season progresses. Based on the results of model calculations, a possible explanation for the observed relationship is developed. The model calculations indicate that the sensitivity of the reflectance to the Aspen overstory is dependent on the amount of understory present.

Simulated Relationships Between Spectral Reflectance, Thermal Emissions, and Evapotranspiration of a Soybean Canopy

Abstract: A canopy reflectance model is incorporated into a routine for simulating water and energy flows in the soil-plant-atmosphere system. The reflectance model is structured tocalculate canopy albedo throughout each simulation period and to determine spectral reflectances at a specified time during the day. Spectral vegetation indices are then calculated from the reflectances and related to the evapotranspiration and thermal response of the canopy. The canopy reflectance model is also used to establish the photo-sytheticaily active radiation load at various depths in the canopy. Stomatal resistances are calculated using these radiation values and integrated to give the minimum canopy resistance. Actual canopy resistance is obtained by adjusting minimum canopy resistance for environmental stresses such as leaf water potential and leaf temperature. Using data for a soybean canopy, canopy evapotranspiration and temperatures are simulated for a range of leaf area index values and compared with the corresponding spectral vegetation indices. The resuits indicate that the normalized difference spectral index has an inverse linear relationship with canopy temperature, concurring with results obtained from satellite observations. The possibility of using a spectral vegetation index and thermal observations together to parameterize surface moisture availability for evapotranspiration is considered.

Physiological causes of yield variation in cassava (Manihot esculenta Crantz)

Abstract: Cassava (Manihot esculenta Crantz) is an important crop in many parts of the tropics, being mainly cultivated for its storage roots Farmers' yields are low and one of the constraints to higher yields is the lack of adequate clones At the beginning of the 1970s an extensive cassava research programme was started at CIAT (Colombia) One of its aims was to develop high-yielding clones by genetic modification of the plant habitusThis thesis begins with a literature review in which the available information on the physiological determinants of the yield of cassava storage roots is describedNext, a series of experiments carried out to deepen and to broaden this knowledge on physiological causes of yield variation in cassava is described MCol 1684 (the best cultivar of the CIAT cassava germ plasm bank) and MPtr 26 were used as the reference cultivars in the experiments, both in the field and in the greenhouseRate of leaf photosynthesis was measured by infrared gas analysis Measurements of the photosynthetic rate were carried out using the youngest fully expanded leaf from plants growing outdoors that were 35 - 45 days old Maximum photosynthetic rates varied from 074 x 10 -6 to 081 x 10 -6 kg CO 2 m -2 leafs -1 MCol 22 had the highest leaf photosynthetic rate A relatively low photonflux density level was required for light saturation of the photosynthetic rate This is characteristic for a plant species with a C 3 cycle Photosynthesis increased only slightly from 1000 to 1500 μE m -2 s -1 PAR (photosynthetically active radiation) Light efficiency at low light intensities (α) varied from 90 x 10 -9 to 124 x 10 -9 kg CO 2 J -1 The CO 2 concentration remained at an approximately constant level in the intercellular spaces, independent of the light level, being 212 vppm (0387 x 10 -3 kgm -2 ) At a photonflux density of 1500 μEm -2 s -1 mesophyl resistance was higher than leaf resistance to CO 2 (335 sm -1 compared with 185 sm -1 ) Transpiration rates did not differ between clones, but increased with light intensity water use efficiency (WUE)varied from 151 to 171 mg CO 2 uptake per g H 2 O, and was most efficient for MCol 22Linear relationships were found between total dry matter yield and the amount of intercepted PAR Photosynthetic efficiency varied from 19% to 25%, based on PAR during the first six months of the growth period, and decreased markedly in older plants The fraction of incoming intercepted PAR varied from 43% to 69% during the first six months Cultivars had an extinction coefficient (K) of 072 to 088 and their leaves were dominantly planophileA leaf area index (LAI) of 10 (about 50% light interception) was attained at 60 to 90 days from planting An LAI = 3, which coincides with a light interception by the leaf canopy of approximately 90% was reached 120 to 150 days after planting, so about 40% of a growth period of one year had elapsed before complete ground cover was achieved Genotypes with very different canopy characteristics reached an LAI of 3 in approximately the same timeCassava has an indeterminate habit with sympodial branching The length of the period until first branching depended on genotype and planting date Large genetic differences were found in leaf life, leaf size, plant age at which maximum leaf size was reached and leaf formation rate per apex Small variations in environmental conditions caused significant differences in canopy characteristicsDuring the growth period of cassava two periods with constant dry matter partitioning could be distinguished, with the apparent initiation of the filling of storage roots (AISS) being the crucial point A constant proportion of the dry matter formed is distributed to the storage roots: this is the efficiency of storage root production (ESRP) Genetic differences were found for ESRP and AISSThe influence of daylength on growth and yield was studied Daylength was increased by light bulbs, while the other growth conditions remained similar Daylength had only a very slight influence on AISS value ESRP was negatively influenced by long days Differences in ESRP were the main cause of differences in yield of storage roots for plants grown at different daylengths For MCol 22, the ESRP value was also considerably lower under long-day conditions, but the yield of storage roots was nevertheless only slightly reduced, because of the higher total dry matter yield MCol 22 is the first detected cassava clone whose yield of storage roots is nearly dayneutral and thus it is suitable for cultivation at higher latitudes Long-day conditions caused a large increase in LAI The higher LAI values were caused by a higher leaf formation rate per apex and a larger number of apices per m 2 Photosynthetic efficiency was not influenced by daylengthThe implications of the findings are discussed Plant properties that could be useful for improving storage root yield are: a high ESRP value, a low AISS value, a high growth vigour (short period until 50% light interception), a light interception of about 90% as long as possible, and a good sink potentialPresent knowledge of cassava cultivation techniques is such that high yields are possible Potential yield of storage roots is about 30 tha -1 year -1 on dry weight basis and about 90 tha -1 year -1 on fresh weight basis The highest recorded yield of fresh storage roots at the CIAT experimental farm is of that order (namely 82 tha -1 ), and was achieved after a growth period of one year

Carbon dioxide, water vapor and sensible heat exchanges of a grain sorghum canopy

Abstract: With the aid of eddy correlation instrumentation, the components of the energy budget and CO2 flux were measured over grain sorghum grown at Mead, Nebraska. Diurnal patterns of sensible heat, latent heat, CO2 and momentum flux are examined for typical days. On a mostly clear day when the crop leaf area index was 3.7, net radiation reached a mid-day peak of 560 W m-2, while sensible and latent heat fluxes peaked at 50 and 460 W m-2, respectively. The peak CO2 flux occurring just prior to solar noon was 1.5 mg m-2(ground area) s-1. CO2 flux (respiration from plants, soil and roots) in the early evening was about -0.28 mg m-2 (ground area) s-1.

Specific leaf area and leaf area index distribution in a young Douglas-fir plantation

Abstract: The spatial distribution of specific leaf area and leaf area index of needles in different age classes has been investigated in a young and unthinned Douglas-fir (Pseudotsugamenziesii (Mirb.) Franc...

Diurnal patterns of bidirectional vegetation indices for wheat canopies

Abstract: The perpendicular vegetation index (PVI) and normalized difference vegetation index (NDVI) were calculated from Mark II radiometer RED (0.63-0.69 μm) and NIR (0.76–0.90 μ) bidirectional radiance observations for wheat canopies. Measurements were taken over the plant development interval flag leaf expansion to watery ripeness of the kernels during which the leaf area index (LAI) decreased from 40 to 2-5. Spectral data were taken on four cloudless days five times (09.30, 11.00, 12.30, 14.00 and 15.30 hours (central standard time, C.S.T.) at five view zenith, Zv (0, 15, 30,45 and 60°) and eight view azimuth angles relative to the Sun, Av (0, 45, 90, 135, 180, 225, 270 and 315°). The PVI was corrected to a common solar irradiance (PVIC) based on simultaneously observed insolation readings. The PVIC at nadir view (Ž=0°) increased as (l/cosZs) increased on all the measurement days whereas the NDVI changed little as solar zenith angle (Zs) changed. Thus, the PVIC responded to increasing path length thro...

Application of Remote Sensing to Agricultural Field Trials

Abstract: Remote sensing techniques enable quantitative information about a field trial to be obtained instantaneously and non-destructively. The aim of this study was to identify a method that can reduce inaccuracies in field trial analysis, and to identify how remote sensing can support and/or replace conventional field measurements in field trials. In the literature there is a certain consensus that the best bands from which characteristic spectral information about vegetation can be extracted are those in the visible (green and red) and infrared regions of the electromagnetic spectrum. This was confirmed in the present study by an analysis of multispectral scanner data ('Daedalus scanner') from field trials with cereals. The optimal bands that were thereby selected for explaining grain yield mostly contained the channels 5 (550- 600 nm), 7 (650-700 nm), and 9 (800-890 nm). Multispectral aerial photography was found to be most appropriate for recording extensive field trials in a short period. In the present study, recordings were carried out with a single-engine aircraft, using two Hasselblad cameras for obtaining vertical photographs on black and white 70-mm aerial films. In this way, costs stayed within acceptable limits. The recording scale chosen, given the dimensions of the trials at the experimental farm of the Wageningen Agricultural University, where the research was carried out, was 1:8 000. Photographs were taken approximately fortnightly to keep in step with conventional field sampling. The film/filter combinations selected for obtaining a high spectral resolution and for matching bands 5, 7 and 9 of the Daedalus scanner, resulted in the following passbands: green : 555-580 nm; red :665-700 nm; infrared :840-900 nm, The densities of the objects on the film were measured by means of an automated Macbeth TD-504 densitometer. An aperture with a diameter of 0.25 mm was selected for the densitometer, in order to obtain a high spatial resolution at the scale of 1:8000, applicable to field trials with plots 3 metres wide. The measured densities were converted into exposure values, corrected for light falloff, and then a linear function was applied to convert them into reflectance factors. In this linear function the exposure time, relative aperture, transmittance of the optical system, irradiance, path radiance and atmospheric attenuation were incorporated. Reference targets with known reflectance characteristics were set up in the field during missions and recorded at the same camera setting and under the same atmospheric conditions as the field trials, in order to ascertain the parameters of the linear function. Information about crop reflectance obtained from the literature suggested that reflectances in the visible region of the electromagnetic spectrum (green or red) would be most suitable for estimating soil cover, whereas reflectances in the infrared might be most suitable for estimating leaf area index (LAI). Other plant characteristics, such as dry matter weight or yield, may be estimated indirectly from reflectances. Field trials with cereals analysed during the present study showed that treatment effects shown by green and red reflectances tended to be opposite to those shown by LAI. Treatment effects shown by infrared reflectance tended to be similar to those shown by LAI, even at large LAI (6-8). The treatment effects manifest in reflectances were more stable in time than those for LAI. Coefficients of variation of residuals resulting from analyses of variance were systematically smaller for reflectances than for the LAI in all experiments: those for the infrared reflectance were particularly small. In general, critical levels in testing for treatment effects were smaller for the infrared reflectance than for the LAI, which indicates that the power for infrared reflectance was larger than for LAI. Soil moisture content is not constant during the growing season and differences in soil moisture content greatly influence soil reflectance. Since a multitemporal analysis of remote sensing data was required, a correction had to be made for soil background when ascertaining the relationship between reflectances and crop characteristics. In the literature no index or reflectance model stood out as being suitable for estimating crop characteristics in agricultural field trials. Thus, in this monograph an appropriate simplified reflectance model is presented for estimating soil cover and LAI for green vegetation. First of all, soil cover is redefined as: the vertical projection of green vegetation and the relative area of shadows included, seen by a sensor pointing vertically downwards, relative to the total soil area (in this definition soil cover depends on the position of the sun). Then, the simplified reflectance model is based on the expression of the measured reflectance as a composite reflectance of plants and soil: the measured reflectance in the various passbands is a linear combination of soil cover and its complement, with the reflectances of the plants and of the soil as coefficients, respectively. By using this model, it should, theoretically, be possible to correct for soil background when estimating soil cover by combination of measurements in the green and red passbands. In practice, however, all the procedures derived yielded poor results because the difference between green and red reflectances was so small. Thus, attention was focussed on estimating LAI. For estimating LAI a corrected infrared reflectance was calculated by subtracting the contribution of the soil from the measured reflectance. Theoretically, combining the reflectance measurements obtained in the green, red and infrared passbands, enables the corrected infrared reflectance to be calculated, without knowing soil reflectances. The main assumption was that there is a constant ratio between the reflectances of bare soil in different passbands, independent of soil moisture content: this assumption is valid for many soil types. For the soil type at the experimental farm of the Agricultural University, the corrected infrared reflectance can be approximated by the difference between total measured infrared and red reflectances. Subsequently this corrected infrared reflectance was used for estimating LAI according to the inverse of a special case of the Mitscherlich function. This function contains two parameters that have to be ascertained empirically. Model simulations with the SAIL model (introduced by Verhoef, 1984) confirmed the potential of this simplified, semi-empirical, reflectance model for estimating LAI. Analogous derivations were applied for a generative canopy (cereals) with yellowing leaves. The estimation of LAI by reflectances yielded good results for the field trials with cereals analysed in this study. The presence of treatment effects could be shown with larger power and the coefficients of variation were smaller for this estimated LAI than for the one measured in the field. Regression curves of LAI on corrected infrared reflectance differed significantly in different trials with the same crop, particularly for the generative stage. This may have been caused by large systematic discrepancies between LAI measurements obtained with the conventional sampling techniques for two field trials, because of subjectivity in separating green from yellow leaves. To date, the best approach is to ascertain regression curves of LAI on corrected infrared reflectance for each field trial by incorporating a few additional plots, in which both the LAI and the reflectances are measured.

Field Spectroscopy of Agricultural Crops

Abstract: The design, implementation, and results of multisite multiyear experiments to measure and model the multispectral reflectance of agricultural crops in relation to their biophysical characteristics are described. The experimental approach involved multitemporal reflectance measurements together with detailed measurements of the agronomic characteristics of crop canopies. One result of the field measurements and analyses was a quantitative description of the complex relationships among crop canopy, soil, atmosphere, and illumination and sensor geometries. Leaf area index was identified as a key biophysical parameter linking crop physiology and multispectral remote sensing. Quantitative understanding and models of this relationship led to the development of spectral-temporal profile models for crop species identification and development stage estimation. A second key development has been the development of conceptual approaches and models for spectral estimation of leaf area index and light interception of crop canopies as inputs to crop growth and yield models. Other results include quantification of the effects of soil background, cultural practices, moisture stress, and nutrient deficiencies on crop reflectance, and the effects of sun angle and sensor view angle on measured canopy reflectance. The field measurements of canopy reflectance and geometry also provided data bases to test and validate canopy radiation models. In summary, the AgRISTARS field research on agricultural crops has provided a critical link between satellite and leaf spectral data.

Estimation of canopy parameters for inhomogeneous vegetation canopies from reflectance data. I: Two-dimensional row canopy

Abstract: A canopy-reflectance (CR) model for row-planted vegetation is presented. Its use of an estimation of important biophysical variables like leaf-area index (LAI) and average leaf angle (ALA) from bidirectional CR data is discussed. Using field-measured CR data for a partially covered soybean canopy, it is shown that one can accurately estimate LAI, ALA and extent of percentage of ground cover from CR data.

Spectral Radiance Estimates of Leaf Area and Leaf Phytomass of Small Grains and Native Vegetation

Abstract: Similarities and/or dissimilarities in radiance characteristics were studied among barley (Hordeum vulgare L.), oats (Avena fatua L.), spring and winter wheat (Triticum aestivum L.), and shortgrass prairie vegetation. The site was a Williams loam soil (fine-loamy mixed, Typic Argiborolls) near Sidney, Montana. Radiances were measured with a truck-mounted radiometer. The radiometer was equipped with four wavelength bands: 0.45 to 0.52, 0.52 to 0.60, 0.63 to 0.69, and 0.76 to 0.90 , ?m. Airborne scanner measurements were made at an altitude of 600 m four times during the season under clear sky conditions. The airborne scanner was equipped with the same four bands as the truck-mounted radiometer plus the following: 1.00 to 1.30, 1.55 to 1.75, 2.08 to 2.35, and 10.4 to 12.5 ?m. Comparisons using individual wave bands, the near IR/red ((0.76 to 0.90 ?m)/(0.63 to 0.69 ?m)) ratio and the normalized difference vegetation index (ND=(IR-red)/(IR+red)) showed that only only during limited times during the growing season were some of the small grains distinguishable from one another and from native rangeland vegetation. There was a common relation for all small grains between leaf area index and green leaf phytomass and between leaf area index or green leaf phytomass and the IR/red ratio.

Spectral Components Analysis: A Bridge between Spectral Observations and Agrometeorological Crop Models

Abstract: Spectral observations have been acknowledged to indicate general plant conditions over large areas but have yet to be exploited in connection with agrometeorological crop models. One reason is that it is not yet appreciated how periodic spectral observations of row-cropped and natural plant canopies, as expressed by vegetation indices (VI), can provide information on important crop model parameters, such as leaf area index (LAI) and absorbed photosynthetically active radiation (APAR). Two experiments were conducted under AgRISTARS sponsorship, one with cotton and one with spring wheat, specifically to determine the relationships for each term in the " spectral components analysis" identity \begin{equation*} LAI/VI \times APAR/LAI = APAR/VI.\end{equation*} LAI and APAR could, indeed, be well estimated from vegetation indices such as normalized difference(ND) and perpendicular vegetation index (PVI)?apparently because of the close relation between the VI and amount of photosynthetically active tissue in the canopy. APAR and VI measurements are similarly affected by solar zenith angle (SZA), and LAI can be divided by cos SZA at the time of the VI and APAR measurements to achieve correspondence. APAR, ND, and PVI plotted against LAI all asymptote to limiting values in the same way yield does as LAI exceeds 5, further linking canopy development to yield capability. In summary, the spectral components analysis results presented add credence to the information conveyed by spectral canopy observations about plant development and yield, and establish a bridge between remote observations and agrometeorological crop modeling through the variables of mutual concern, LAI, biomass, and yield.

A comparison of spectral reflectance and sward surface height measurements to estimate herbage mass and leaf area index in continuously stocked ryegrass pastures

Abstract: Measurements of sward surface height and of the ratio of light at 660 and 730 nm reflected from a pasture canopy were correlated with measurements of leaf area index (LAI) and herbage mass of two ryegrass dominant swards. Both pastures were continuously stocked by sheep to maintain a range of sward heights from 2 to 6 cm corresponding approximately to LAI 2 to 5. Sward height appeared to be linearly related to both LAI and herbage mass, whereas 660/730 reflectance displayed a non-linear relationship with both parameters. The accuracy of prediction by the two methods over the range LAI 1 to 3 or herbage mass 700 to 1800 kg DM ha-1 was very similar. However, reflectance measurements could not be used above about LAI 3–4 and to this extent were less useful. Nevertheless, reflectance measurements have a potential advantage in their ability to sample large areas of pasture very quickly.