Canopy Geometry Research Articles

Multiple-scattering scheme useful for geometric optical modeling

Geometrical optical (GO) models have been widely used in remote sensing applications because of their simplicity and ability to simulate angular variation of remote sensing signals from the Earth's surface. GO models are generally accurate in the visible part of the solar spectrum, but less accurate in near-infrared (NIR) part in which multiple scattering in plant canopies is the strongest. Although turbid-media radiative transfer (RT) methods have been introduced to GO models to cope with the second-order and higher order scattering, the problem of canopy geometrical effects on multiple scattering still remains and becomes the main obstacle in GO model applications. In this paper, we propose and test a multiple scattering scheme to simulate angular variation in multiply scattered radiation in plant canopies. This scheme is based on various view factors between sunlit and shaded components (both foliage and background) in the canopy and allows the geometrical effects to propagate to the second-order and higher order scattering simulations. As the view factors depend on the canopy geometry, the scheme is particularly useful in GO models. This new scheme is implemented in the 4-Scale Model, which previously used band-specific multiple scattering factors. After the use of the scheme, these factors are removed and the multiple scattering at a given wavelength and angle of observation can be automatically computed. Improvements made with this scheme are shown in comparison with the top-of-canopy (i.e., PARABOLA) and airborne (i.e., POLDER) measurements with modeled results with and without the scheme. Examples of canopy-level hyperspectral signatures simulated using the scheme are also shown.

Estimation of leaf area index in open-canopy ponderosa pine forests at different successional stages and management regimes in Oregon

Leaf area and its spatial distribution are key parameters in describing canopy characteristics. They determine radiation regimes and influence mass and energy exchange with the atmosphere. The evaluation of leaf area in conifer stands is particularly challenging due to their open nature and clumping on the needle, shoot and tree scale. The overall objective of our study was to characterize leaf area index (LAI) ( L h, m 2 half-surface area foliage m −2 ground) in the vicinity of our old-growth and 14-year-old ponderosa pine ( Pinus ponderosa, var. Laws) eddy covariance flux sites, with future plans to scale from the flux sites to the pine region using ecosystem models and remote sensing. From the combination of optical and canopy geometry measurements, sapwood and litter-fall measurements, and one- and three-dimensional (3-D) models, we evaluated the variation in estimates of L h in a mixed-age stand at the old-growth flux site. We also compared sapwood area estimates from a local allometric equation with LAI-2000 estimates that have been corrected for clumping and the interception of light by stems and branches ( L hc, m 2 half-surface area m −2 ground) across a range of age classes and stand densities of ponderosa pine forests along a 15 km swath in Central Oregon that encompassed the flux sites. In the old-growth stand, the litter-fall and sapwood estimates tended to be higher than the optical and 3-D radiative transfer model estimates. Across the 15 km east–west gradient from the crest of the Cascade Mountains, L hc was typically lower than the sapwood estimates (L hsw; slope 0.38). The L hc data, as well as aboveground production estimates for the 17 pine plots will be useful for scaling flux measurements to the region using ecosystem models that have been validated with these data.

Pollutant dispersion in urban street canopies

The effects of building configurations on pollutant dispersion around street canopies were studied numerically. The dispersion of pollutants emitted from ground sources was simulated by continuously discharging large number of particles into the computation domain. The mean wind velocities at each time-step were firstly computed by solving the time-dependent incompressible Navier–Stokes equations, while the fluctuated velocities were determined using a statistical procedure. The trajectories of the discharged particles were obtained from a Lagrangian particle model. Three categories of numerical simulation were conducted to study the effect of different canopy geometries on the pollutant dispersion. The computed wind field data were consistent with the wind field characteristics described in the previous wind tunnel studies. A counter-clockwise vortex was found resulting in high pollutant concentration at the windward side of the downstream building of the street canopy and low pollutant concentration at the leeward side of the upstream building. The increase in height of the urban roughness buildings would facilitate the pollutant dispersion in urban street canopy under certain building configurations. Two or more vortices stacked vertically in a street canopy were found when height of the upstream and downstream buildings of a street canopy was increased, preventing pollutants from escaping out of the canopy.

RELATING INTERFEROMETRIC SIGNATURE OF REPEAT PASS ERS-1 SAR SIGNALS TO DYNAMIC LAND COVER CHANGES

Radar interferometry opens a very wide field of new applications especially to spaceborne Synthetic Aperture Radar (SAR) data. While repeat-pass SAR interferometry provides the possibility of producing topographic maps and geocoded as well as radiometrically calibrated radar images, interferometric coherence due to non-topographic phase change is a powerful tool to analyse the changes taking place in land covers. The degradation of coherence between two images is due to many effects and is not well understood. This degradation is originated either due to changing characteristics of the scatterers between the two images or due to the changes in permittivity driven by moisture migration. It is important to understand the process that determines the degree of coherence between images in repeat-pass SAR interferometry over different surface types. Considering the importance of this aspect, ERS-1 SAR repeat-pass data has been used to derive interferometric correlations between two images and analysis of these correlations with temporal changes, especially in surface soil moisture conditions, provides valuable thematic information. It is quite interesting to note that the changes in phase (represented by interferometric correlation) and intensity of backscatter are not uniform. While change in permittivity driven by moisture migration from bare soil conditions brings in changes in both phase and intensity, volume scattering as a function of canopy geometry, coupled with changes in permitivity from vegetated surfaces is more sensitive to phase than change in backscatter intensity especially between the two passes of 9 days. This reveals the sensitivity of phase information towards extremely dynamic land cover change in terms of soil moisture variations. A quantitative analysis on the interferometric correlation in combination with the backscatter intensity and the backscatter intensity change between the two passes reveals the crucial information which may be used for landuse/land cover classifications and monitoring and assessment of agricultural droughts in tropical regions.

測定角補正を施した衛星赤外放射温度による植生地に対する顕熱フラックス分布の推定

The spatial distribution of sensible heat flux over rice paddies is estimated on a several tens kilometer scale, together with possible error estimation using satellite infrared data. It is found that three types of correction of satellite infrared temperature (correction for atmospheric absorption, satellite viewing angle, and the surface emissivity) over rice paddy yield estimation errors of 25Wm-2 for sensible heat flux on a half-hourly basis as root mean squares errors (RMSEs). These errors are sufficiently small compared with general observational errors. In particular, the viewing angle correction is so significant that the RMSE is 46Wm-2 without the correction. Atmospheric and emissivity corrections using the LOWTRAN7 with radio sounding data yield a good estimation of surface infrared temperature, with RMSE being 1.0°C. An empirical and linear parameterization proposed by Troufleau et al. (1997) is applied for the viewing angle correction. In the approach, parameters are refitted based on the surface infrared temperature data. The Troufleau et al. (1997) parameterization is applied over a wide range of leaf area index (LAI) from 0.01 through 5. This parameterization efficiently corrects the temperature deficit by the viewing angle effect caused by canopy geometry. However, this parameterization is insufficient when the sensible heat flux is large because the temperature deficit is essentially nonlinear with regards to the difference between surface infrared temperature and air temperature.

Modeling lidar returns from forest canopies

Remote sensing techniques that utilize light detection and ranging (lidar) provide unique data on canopy geometry and subcanopy topography. This type of information will lead to improved understanding of important structures and processes of Earth's vegetation cover. To understand the relation between canopy structure and the lidar return waveform, a three-dimensional (3D) model was developed and implemented. Detailed field measurements and forest growth model simulations of forest stands were used to parameterize this vegetation lidar waveform model. In the model, the crown shape of trees determines the vertical distribution of plant material and the corresponding lidar waveforms. Preliminary comparisons of averaged waveforms from an airborne lidar and model simulations shows that the shape of the measured waveform was more similar to simulations using an ellipsoid or hemi-ellipsoid shape. The observed slower decay of the airborne lidar waveforms than the simulated waveforms may indicate the existence of the understories and may also suggest that higher order scattering from the upper canopy may contribute to the lidar signals. The lidar waveforms from stands simulated from a forest growth model show the dependence of the waveform on stand structure.

Measurement and modeling of spatially explicit variation in light transmission through interior cedar-hemlock forests of British Columbia

We have characterized canopy geometry and light transmission by the nine dominant conifer and broad-leaved tree species of the interior cedar-hemlock (ICH) forests of northern British Columbia. Our field data were used to parameterize a spatially explicit model of light transmission through mixed-species forests. That model, a component of the forest dynamics simulator SORTIE, was developed for eastern deciduous forests, and this paper presents a test of that model in a very different ecosystem. Our results show that individual crowns of the ICH forests intercepted much more light than species of eastern deciduous forests but that the canopy as a whole allowed greater light penetration, largely because of openings between the relatively narrow, conical crowns of the western conifers. Light transmission by individual crowns was correlated with shade tolerance among the conifers (as in eastern deciduous species), but crown depth was not (in contrast with eastern species). Despite the fundamental differences in the nature of light transmission in the two ecosystems, the SORTIE light model developed for eastern deciduous forests was effective at predicting spatial variation in understory light levels in these western coniferous forests. The goodness of fit of such a simple model suggests that the most important factors regulating spatial variation in understory light levels in these forests are simply the sizes and distribution of nearby trees, and the local sky brightness distribution. Discrete canopy gaps represent a special case in which a region of the canopy is not occupied by crowns.

Structure Analysis and Classification of Boreal Forests Using Airborne Hyperspectral BRDF Data from ASAS

A new approach is presented for deriving vegetation canopy structural characteristics from hyperspectral bidirectional reflectance distribution function (BRDF) data. The methodology is based on the relationship between spectral variability of BRDF effects and canopy geometry. Tests with data acquired with the Advanced Solid-State Array Spectroradiometer (ASAS) over Canadian boreal forests during the BOREAS campaign show that vegetation structural characteristics can be derived from the spectral variability of BRDF effects. In addition, the incorporation of both BRDF effects and hyperspectral resolution data substantially improve the classification accuracy. Best classification results are obtained when hyperspectral resolution and BRDF data are combined, but the improvement is not consistent for all classes. For example, adding BRDF information to hyperspectral data increases the overall classification accuracy for a six-class fen site from 37.8% to 44.7%. The addition, however, reduces the accuracy for the jack pine class from 43.6% to 28.8%. These new findings provide evidence for improved capabilities for applications of MISR and MODIS data. The spectral resolution of MODIS is expected to be sufficient to derive canopy structural information based on the spectral variability of BRDF effects, and for MISR a significant improvement of classification accuracies can be anticipated from the combination of nadir reflectance and off-nadir data.

Do Variations in Local Leaf Irradiance Explain Changes to Leaf Nitrogen within Row Maize Canopies?

Numerous studies have dealt with the relationship between leaf nitrogen content and leaf irradiance. However, most of them refer to dense stands presenting reduced horizontal heterogeneity of foliage distribution. Both gradients of leaf nitrogen and leaf irradiance related to canopy depth are significant under such conditions, and modelling radiative exchange using a turbid-medium analogy and dividing the canopy into vegetation layers is sufficient. Conversely, row crops such as maize are characterized by strong horizontal heterogeneity of foliage distribution and the one-dimensional (1D) approach may be unsuitable. We thus modelled the three-dimensional (3D) geometry of maize canopies with varying densities and at different developmental stages using plant digitizing under field conditions. The nitrogen content per unit area of each leaf part was obtained subsequently by nitrogen analysis. We next calculated radiative exchange using a 3D volume-based approach within the canopies in order to estimate local leaf irradiance on a daily integration scale. Vertical gradients in leaf nitrogen content per unit area observed in dense stands during the vegetative phase corresponded largely to those reported in the literature. We also identified significant gradients in nitrogen content along the leaves, which had not before been clearly demonstrated. Our study shows that local light climate during plant development plays a major role in leaf nitrogen distribution and remobilization. Moreover, brutal plant thinning involves rapid changes in leaf nitrogen partitioning. It is concluded that taking account of the 3D heterogeneity of nitrogen and irradiance distribution may have implications for modelling crop photosynthesis and production.

Influence of Spray Application Parameters on Deposition in an Asparagus Crop

A commercial air-assisted sprayer was tested in asparagus crop using a water-soluble food dye as a tracer. The machine had been modified by lowering the nozzles and the air outlets by 20 cm, in order to match more close by the geometry of the asparagus crop canopies and to improve deposition at the base of the stems. Several tests were performed to investigate how deposition was affected by various treatments, including: the number, position and size of the nozzles; the spray volume rate; the presence or absence of air assistance; the spray output angle, relative to the travel direction; and the forward speed. The most uniform deposition was obtained with: four nozzles per side (the lowest at a height of 46 cm from the ground), directed horizontally; an air flow rate of 6·1 m3/s; and a forward speed of 1·69 m/s. This adjustment was eventually compared with the standard sprayer setting, showing that total deposition on the canopy could be increased from 48·4 to 65·6% of the dose applied, without substantially increasing spray losses to the ground.

A field goniometer system (FIGOS) for acquisition of hyperspectral BRDF data

A new field goniometer system (FIGOS) is introduced that allows in situ measurements of hyperspectral bidirectional reflectance data under natural illumination conditions. Hyperspectral bidirectional reflectance distribution function (BRDF) data sets taken with FIGOS nominally cover the spectral range between 300 and 2450 nm in 704 bands. Typical targets are small-growing, dense, and homogeneous vegetation canopies, man-made surfaces, and soils. Field BRDF data of a perennial ryegrass surface reveal a strong spectral variability. In the blue and red chlorophyll absorption bands, BRDF effects are strong. Less-pronounced bidirectional reflectance effects are observed in the green and in most of the near-infrared range there surface reflectance is high. An anisotropy index (ANIX), defined as the ratio between the maximum and minimum bidirectional reflectance over the hemisphere, is introduced as a surrogate measurement for the extent of spectral BRDF effects. The ANIX data of the ryegrass surface show a very high correlation with nadir reflectance due to multiple scattering effects. Since canopy geometry, multiple scattering, and BRDF effects are related, these findings may help to derive canopy architecture parameters, such as leaf area index (LAI) or leaf angle distribution (LAD) from remotely sensed hyperspectral BRDF data. Furthermore, they show that normalized difference vegetation index (NDVI) data are strongly biased by the spectral variability of BRDF effects.

Spectral Mixture Analysis and Geometric-Optical Reflectance Modeling of Boreal Forest Biophysical Structure

Abstract Biophysical structural information such as biomass, LAI, and NPP are important inputs to regional scale models of ecosystem processes and photosynthetic activity within boreal forests. However, traditional methods such as NDVI for deriving these variables from remotely sensed data have been inconsistent and unsatisfactory due to factors such as the confounding influence of background reflectance and canopy geometry on the overall pixel signal. To address this problem, we present new results which use spectral mixture analysis to determine areal fractions of sunlit canopy, sunlit background, and shadow at subpixel scales for predicting these biophysical variables. Geometric-optical reflectance models are used to estimate sunlit canopy component reflectance for input to the analysis together with field measures of background and shadow reflectance. In this article, we compare cylinder, cone, and spheroid models of canopy geometry and evaluate the importance of solar zenith angle variations in reflectance estimates for mixture fractions. These are computed from helicopter MMR radiometer data for 31 stands of black spruce along a gradient of stand densities near the southern fringe of the North American boreal forest. Component fractions are evaluated against ground data derived from dense-grid point analyses of coincident high resolution color photography and also for predicting biophysical variables. In general, the Li–Strahler spheroid model was better than the cone and cylinder models and the importance of correcting for solar zenith angle (SZA) was illustrated, with significant improvements noted for higher SZA as a result of corrections for canopy mutual shadowing. The best overall results were obtained from the shadow fraction using a spheroid model of canopy geometry at SZA 45°. Linear regression analyses showed biomass could be estimated with r2 values of 0.83 and a standard error (S.E.) of 1.7 kg/m2; LAI: r2=0.82, S.E.=0.46; and NPP: r2=0.86, S.E.=0.05 kg/m2/yr. These results were significantly higher than with NDVI for estimating biomass (r2=0.44), LAI (r2=0.60), and NPP (r2=0.56). Current and future areas of research are outlined towards improving our understanding of carbon cycling in large forested ecosystems as a function of variability in the physical climate system and environmental change.

Radar backscattering from alfalfa canopy: A clump modelling approach

Vegetation in the microwave regime can be modelled by replacing vegetation scattering elements (stems, leaflets, etc.) by their dielectric equivalents. Unlike most vegetation, the scattering elements in the alfalfa canopy are not independent of one another at L-band and they also exhibit a unique geometrical structure. This structural feature of the alfalfa canopy has been incorporated into a modelling procedure to predict radar response from the canopy. An alfalfa plant is modelled as a conical clump of stems that are clustered with leaflets. The scattering amplitude of this structural unit (clump) is calculated by the coherent addition of amplitudes from the individual stems. The calculations are done for L-band, where the leaflet size as well as the average distance among the leaflets are small compared to the wavelength. Careful ground and canopy geometry data were collected during the WASHITA'92 experiment conducted in Chickasha, Oklahoma in June 1992. The clump characteristics such as the number of stems with a common root, clump geometry, clustering factor of the leaves, etc., were measured in addition to sizes, density distributions of the stems and the leaves. These measured parameters of the alfalfa canopy have been used in a distorted Born approximation to compute the backscattering coefficient. For comparison, the backscatter from the alfalfa canopy is also computed using a conventional modelling procedure. In this approach, the vegetation is modelled as a random, independent, non-clump distribution of stems and leaflets. The theoretical results with and without the clumping structure are compared with one another and also with the experimental data obtained by the NASA truck based radar. There is a reasonable agreement between the clump model and the data. More importantly, the present study emphasizes the importance of coherence effects among different scattering elements in a canopy and the need to model them by identifying a respective structural unit for a particular canopy.

Physical Mechanisms in Hyperspectral BRDF Data of Grass and Watercress

A hyperspectral bidirectional reflectance data set of grass lawn and watercress canopies acquired under controlled laboratory conditions at the JRC European Goniometric Facility, at Ispra, Italy was analyzed with respect to basic physical reflectance mechanisms and factors influencing reflectance anisotropy. By normalizing reflectance data with the nadir reflectance as a reference, a strong influence of reflectance intensity on observed reflectance anisotropy is demonstrated for both vegetation canopies. This is explained by multiple scattering effects inside vegetation canopies as a function of canopy spectral absorbance characteristics. Two quantities based on canopy reflectances are utilized for characterizing the spectrally variable dynamics of BRDF effects: the anisotropy factor (ANIF), which is simply a normalization with nadir reflectance, and the anisotropy index (ANIX) defined as the ratio between the maximum and minimum reflectance values in the principal plane (or defined azimuth plane) per spectral band. Using these two quantities the study revealed that the basic physical reflectance mechanisms described by Kimes (1983) for broad spectral bands are extendable to hyperspectral bidirectional reflectance data. In addition, the influences of canopy geometry and multiple scattering on BRDF are clearly demonstrated for the erectophile grass lawn and the planophile watercress canopy. Since multiple scattering effects and canopy geometry are dominant factors regulating BRDF effects, vegetation canopy architecture parameters such as the leaf area index might be derived from hyperspectral BRDF data.

The carbon balance of grazed and non‐grazed Spartina anglica saltmarshes at Skallingen, Denmark

1 Canopy‐level CO2 exchange and biomass were measured monthly for 9 consecutive months during 1990 on grazed and non‐grazed portions of a Danish salt marsh dominated by Spartina anglica. The empirical measurements were used to calibrate a model of canopy photosynthesis and soil respiration that was subsequently integrated to provide estimates of annual net ecosystem production (NEP). 2 Standing total and green biomass on the non‐grazed site reached maxima during September of 1.9 ± 0.2 and 1.1 ± 0.2 kg m–2 ( ± 1 SD, n = 3), respectively. Corresponding maxima (0.8 ± 0.1 and 0.5 ± 0.05 kg m–2), were achieved during July at the grazed site. 3 Sediment organic matter as a percentage of dry weight was 31.6% and 5% at the non‐grazed and grazed sites, respectively. Accounting for the higher sediment bulk density at the grazed site meant that concentrations of organic carbon were 0.027 and 0.021 g cm–3 at non‐grazed and grazed sites, respectively. Thus, the fraction of NEP removed by grazing animals significantly reduced the accumulation of sediment organic matter. 4 Estimate of NEP varied between 17 and 38 mol m–2 year–1 of carbon at the grazed site and between 5 and 38 mol m–2 year–1 at the non‐grazed site, depending on assumptions about canopy geometry. Soil respiration, 36 mol m–2 year–1, did not differ between sites. Canopy respiration was less (21 mol m–2 year–1) at the grazed site than at the non‐grazed site (28 mol m–2 year–1), and accounted for 22–29% and 28–41% of gross photosynthesis at grazed and non‐grazed sites, respectively. The decrease in canopy respiration compensated the grazed canopy for losses to herbivores. 5 A canopy photosynthesis model was used to calculate a theoretical envelope of canopy biomass, termed the production window, inside of which NEP is positive and outside of which NEP is negative. The production window is bisected by the optimum canopy biomass, which is the theoretical biomass at which NEP is maximized. The seasonality of NEP is determined by the trajectory of actual canopy biomass through the production window. It was shown that canopy biomass on the grazed site was close to the theoretical optimum biomass late in the season, while the non‐grazed canopy biomass was supraoptimal.

Hierarchical characterization of canopy architecture for boreal forest

Canopy architecture, the geometric organization of aboveground vegetation, provides an essential link between patterns observable by remote sensing and fundamental ecological processes. As part of the international Boreal Ecosystem‐Atmosphere Study (BOREAS); (1) we developed a hierarchical approach for sampling canopy architecture, and (2) we acquired a comprehensive data set for characterizing canopy architecture for the major BOREAS study sites. The approach involves a series of four sets of measurements at different spatial scales, ranging from the ecosystem to the leaf level: (1) regional characterization, involving measurements of the climate, ecosystem, and landscape features; (2) stand characterization, involving measurements of crown geometry (diameter at breast height (DBH), height, and crown extent), individual tree location, and understory cover; (3) tree vectorization, involving detailed sampling of the three‐dimensional distribution of canopy elements and crown form; and (4) characterization of canopy geometry as seen from beneath, involving acquisition of a multitemporal catalog of hemispherical photographs. The last three sets of measurements were then used to reconstruct the three‐dimensional geometry of the canopy. By comparing simulated hemispherical views upward from beneath this reconstructed canopy with in situ hemispherical photographs, the methodological approach was validated. Simulated photographs faithfully reproduced patterns observed for in situ hemispherical photographs, in particular, for gap fraction distributions of the middle ranges of zenith angles (20°–70°). Moreover, simulation of the gap fraction for this middle portion of the zenith angle was insensitive to exact mapping of the stand. The hierarchical data acquisition approach, involving mapping of tree locations and tree reconstruction, permits realistic representation of canopy material distribution. Our approach and our comprehensive data set provide a solid basis from which to integrate data gathered at the stand and tree scales, and a powerful tool for the simulation of the light regime anywhere in the canopy.

Canopy Geometry Effect on the Aerodynamic Behavior of Cross-Type Parachutes

Cross-type parachutes have been traditionally used for the deceleration of a variety of moving projectiles. In recent years their use has been extended to include the roll of stabilizers. The static and dynamic characteristics of the parachute, in stand-alone or as part of a closely coupled payload-parachute cone guration, have gained increased attention. Cross-type parachutes produce higher drag when spinning. The spin characteristics and their dynamic effect on the stability have produced yet another variable that should be considered in the control restrictions of the parachute design. An experimental investigation of the canopy geometrical variations ine uence on the aerodynamic characteristic of a (W/L = 0.333) crosstype parachute was conducted in the subsonic wind tunnel at the Technion— Israel Institute of Technology. The results show that small geometrical variations produce consistent changes in the aerodynamic characteristics in a manner that allows the designer to optimize the canopy geometry to obtain a desired behavior.

The geometry of the canopy of a dipterocarp rain forest in Sumatra

A study was conducted in a lowland dipterocarp rain forest in Sumatra to assess the structure and dynamics of the forest canopy. The canopy structure was analysed at different scales, including a previous mapping of a 5 ha forest mosaic and an interpretation of the canopy architecture, along a horizontal gradient between the high forest and a 320 m2 treefall. Canopy geometry was ascertained by analysis of digitized zenithal hemispherical photographs sampled at 25 near-ground sites. Gap size, morphology and dispersion varied continuously from the high forest to the treefall. An asymmetric pattern of gaps about the zenith was characteristic of almost all forest sites. The spatial dispersion of gaps was random in the high forest. It tended to be dumped in the forest border and in the treefall. Plant area indices ranged from 6.5 in the high forest to 2.9 in the treefall. These values reflect a reduced leaf area during a severe dry spell, at the time of measurements. Inclination angles of canopy elements were highly correlated with gap dispersion and canopy openness, as observed from the high forest to the treefall. Implications of canopy heterogeneity on the distribution of light were analysed. The importance of lateral light penetrating from the forest border into the high forest is stressed. The canopy gradient of architecture and geometry from the high forest to the treefall and the very diverse structure of the high forest are emphasized.

Observations on the relationship between SIR-C radar backscatter and the biomass of regenerating tropical forests

SIR-C SAR data were related to the above ground biomass of regenerating tropical forests in Amazonia, Brazil. C- and L- band SAR data in the conventional polarization configurations showed no significant relationship with forest biomass, which were estimated in the field to range from 63.8-141.1 tha -1. However, the strength of the relationships was increased through the use of backscatter ratios and stratification of the forests by dominant species. These results support the view that backscatter ratios enhance the relationship between radar backscatter and biomass, perhaps beyond some quoted radar saturation levels, by reducing the effect of differences due to forest type. They also demonstrate that an ability to differentiate between forests of different species composition, and canopy geometry, increases the strength of the relationship between the SAR backscatter and biomass.

Measurement of laser‐induced fluorescence decay and reflectance of plant canopies

A new lidar fluorosensor, for the remote characterization of the geometry and of the physiological status of plant canopies, is described. It is based on the use of a laser delivering very short pulses (<100 ps) and on the temporal analysis of both fluorescence and backscattered signals. In a first step, the deconvolution of the backscattered signal gives the level and the relative area of leaves illuminated by the laser beam. In a second step, this information on the canopy geometry is used to deduce the mean chlorophyll fluorescence lifetime, that is considered as an indicator of the photo‐synthetic activity. Field and laboratory measurements performed with the prototype show that the results obtained agree quite well with classical near‐contact chlorophyll fluorescence measurements and can bring also information on plant canopy geometry.