Vern C. Vanderbilt

Hyperspectral remote sensing of foliar nitrogen content

A strong positive correlation between vegetation canopy bidirectional reflectance factor (BRF) in the near infrared (NIR) spectral region and foliar mass-based nitrogen concentration (%N) has been reported in some temperate and boreal forests. This relationship, if true, would indicate an additional role for nitrogen in the climate system via its influence on surface albedo and may offer a simple approach for monitoring foliar nitrogen using satellite data. We report, however, that the previously reported correlation is an artifact—it is a consequence of variations in canopy structure, rather than of %N. The data underlying this relationship were collected at sites with varying proportions of foliar nitrogen-poor needleleaf and nitrogen-rich broadleaf species, whose canopy structure differs considerably. When the BRF data are corrected for canopy-structure effects, the residual reflectance variations are negatively related to %N at all wavelengths in the interval 423–855 nm. This suggests that the observed positive correlation between BRF and %N conveys no information about %N. We find that to infer leaf biochemical constituents, e.g., N content, from remotely sensed data, BRF spectra in the interval 710–790 nm provide critical information for correction of structural influences. Our analysis also suggests that surface characteristics of leaves impact remote sensing of its internal constituents. This further decreases the ability to remotely sense canopy foliar nitrogen. Finally, the analysis presented here is generic to the problem of remote sensing of leaf-tissue constituents and is therefore not a specific critique of articles espousing remote sensing of foliar %N.

Opportunities for using the EOS Imaging Spectrometers and synthetic aperture radar in ecological models

Several promising approaches to assessing biochemical and architectural properties of landscapes are outlined. Strategies for using new EOS sensors in ecological models are examined. Ways in which ecological and remote sensing models can utilize information provided by the new sensors to characterize ecological properties at coarse scales and to estimate within-ecosystem properties are addressed.

Use of spectral analogy to evaluate canopy reflectance sensitivity to leaf optical properties

Abstract The spectral variation of canopy reflectance is mostly governed by the optical properties of the elements such as the leaves. Since leaf intrinsic scattering properties show very little spectral variation, leaf optical properties are related to their absorption properties. Spectral analogies are thus observed between two wavelengths for which the optical properties (absorption, reflectance, or transmittance) of the elements are similar. The red edge for green plants shows the full range of variation of leaf optical properties. The relationship between canopy reflectance and leaf reflectance measured concurrently at the red edge over sugar beet canopies was thus used to simulate canopy reflectance over the whole spectral domain from leaf reflectance spectra measured over the whole spectral domain. The results show that the spectral analogies found allows accurate reconstruction of canopy reflectance spectra. Explicit assumptions about the very low spectral variation of leaf intrinsic scattering properties are thus indirectly justified. The sensitivity of canopy reflectance (ρc) to leaf optical properties is then investigated from concurrent spectral variations of canopy (∂ρc/∂λ) and leaf reflectance (∂ρl / ∂λ): ∂ρc / ∂ρl = (∂ρc / ∂λ) (∂ρl / ∂λ)−1. This expression is strictly valid only when the optical properties of the soil background or of the other vegetation elements such as bark are either spectrally flat or do not contribute significantly to canopy reflectance. Simulations using the SAIL and PROSPECT models demonstrate that the sensitivity of canopy reflectance to leaf reflectance is significant for large vegetation cover fractions in spectral domains where absorption is low. In these conditions, multiple scattering enhances the leaf absorption features by a factor that can be greater than 2.0. To override the limitations of the SAIL model for the description of the canopy architecture, we tested the previous simulation results on experimental data. Concurrent canopy and leaf reflectance spectra were measured for a range of sugar beet canopies. The results show good agreement with the theoretical findings. Conclusions are drawn about the applicability of these findings, with particular attention to the potential detectability of leaf biochemical composition from canopy reflectance sensed from space.

Evidence of hot spot directional signature from airborne POLDER measurements

The POLDER instrument was flown during the BOREAS experiment over various sites and at various altitudes in the Canadian boreal forest and other nearby targets. The instrument design permits the acquisition of the directional signature of any surface cover. In particular, the high directional resolution of POLDER allows it to measure, with an unprecedented accuracy, the hot spot signature of natural targets. The authors present some typical examples of such highly anisotropic reflectance directional signatures. The ratio of the maximum reflectance (hot spot direction) to the minimum reflectance (broad area in the forward scattering hemisphere) varies with wavelength and canopy. It can be as large as six in the visible and three in the near IR.

Diurnal change in trees as observed by optical and microwave sensors: the EOS synergism study

The EOS Synergism Study examined the temporal variability of the optical reflectance and microwave backscatter due to diurnal change in canopy properties of interest to eco- system modelers. The experiment was designed specifically to address diurnal changes in canopy water status (including water potential and content) that relate to transpiration. Multispectral optical and multifrequency, multipolarization microwave measurements were acquired using boom-truck- based systems over a 2-week period in August 1987. Sensor and canopy properties were collected around the clock. The canopy under study was a walnut orchard in the San Joaquin Valley of California. The results demonstrate large diurnal variations in the di- electric properties of the tree (the stem, or bole, specifically) that in turn produce significant diurnal changes in the micro- wave backscatter. The change in dielectric constant is related to the diural change in water potential, although the specific physical or physiological relationship has not yet been estab- lished. A diurnal change in optical reflectance could be attrib- uted primarily to sun angle-view angle change. Although some changes in canopy spectral properties were also observed, these could not be distinguished from edge effects of the canopy. The results suggest that permanently orbiting spaceborne sensors such as those on EOS should be placed in orbits that are optimized for the individual sensor and need not be tied together by a tight simultaneity requirement on the order of minutes to hours for the purpose of monitoring ecosystem properties. Microwave sensors, although able to image day or night, are sensitive to the diurnal pattern of change in vegeta- tion canopies and, therefore, should be in sun synchronous or- bits with a node crossing time selected to optimize sensitivity to the diurnal patterns (4-6 p.m. for this canopy and season). Furthermore, comparison of the long-term spaceborne syn- thetic aperture radar (SAR) data sets collected with SAR in different equator crossing time orbits must consider potential diurnal variation in the surface properties and the effects on the backscatter. Optical reflectance is affected more by sun-an- glehiew angle variations than a diurnal change in canopy water properties. Therefore, based on the results of this study, opti- cal sensors should be placed in orbits which minimize the influ- ence of clouds on terrestrial data acquisition.

Polarization of Light by Vegetation

The amount of sunlight specularly reflected by plants such as sunflower, sorghum, ivy, ponderosa pine, American elm, California laurel, various oaks, and citrus is sometimes so large that canopies may appear white instead of green when viewed obliquely toward the sun. Surface-scattered light is often a significant part of the total light reflected by plants of many diverse species.

High spectral resolution reflectance of Douglas fir grown under different fertilization treatments : Experiment design and treatment effects

Douglas fir [Pseudotsuga menziesii (Mirb.) Franco.] seedlings were grown with different fertilization treatments in an experiment designed to investigate the effects of foliar biochemistry on needle and canopy reflectance measurements. Potentially confounding effects of the covariance of canopy structure with foliar biochemical concentration were minimized by fertilizing after leaf expansion. Seedlings showed no significant differences in specific leaf area, % needle moisture, biomass, or LAI, but showed significantly different total nitrogen concentrations, and some differences in chlorophyll concentrations. Measurements were made of needle optical properties, and bidirectional reflectance was obtained of needles and of canopies. Canopy reflectance was acquired under sky illumination using two field spectroradiometers. Needle and canopy treatment differences were highly significant in the visible region. Some treatment differences were also found in broad-band regions in the infrared in the canopy reflectances. Narrow-band infrared differences were detected in needle first derivative spectra, and coincided with known protein absorption features. Needle and canopy bidirectional reflectances were similar, but narrow-band features found in the needle infrared spectra were not found in the canopy data. Two possibilities for the failure to detect these narrow features in the canopy data are a confounding effect from variability caused by illumination angle changes and/or the low signal-to-noise characteristics of the field spectroradiometer relative to the laboratory instrument. The suite of optical and structural measurements taken from the same set of trees should provide a useful data set for parameterizing canopy models aiming to predict high spectral resolution reflectance of canopies.

Canopy architecture of a walnut orchard

A detailed dataset describing the canopy geometry of a walnut orchard was acquired to support testing and comparison of the predictions of canopy microwave and optical inversion models. Measured canopy properties included the quantity, size, and orientation of stems, leaves, and fruit. Eight trees receiving 100 percent of estimated potential evapotranspiration water use and eight trees receiving 33 percent of potential water use were measured. The vertical distributions of stem, leaf, and fruit properties are presented with respect to irrigation treatment. Zenith and probability distributions for stems and leaf normals are presented. These data show that, after two years of reduced irrigation, the trees receiving only 33 percent of their potential water requirement had reduced fruit yields, lower leaf area index, and altered allocation of biomass within the canopy.

Inundation discriminated using sun glint

Inundation is linked to water, carbon, and energy budgets at landscape to global scales. We describe a new remote-sensing technique for identifying inundated areas based on the properties of the glitter-the strong, angular signature reflection that is characteristic of surface water and uncharacteristic of other cover types. We discriminated three cover types-vegetation emergent above inundated soils, open water, and noninundated cover types-from analysis of directional data collected in the red spectral band by the airborne POLDER (Polarization and Directionality of Earths Reflectance) sensor. We found that values of the normalized difference vegetation index (NDVI) decreased dramatically in the glitter direction, providing an indication of surface water. Application of our new technique holds promise for mapping the seasonal and interannual extent of inundation, a key descriptor of wetlands hydrology.

Tree Canopy Radiance Measurement System

A system is described for obtaining both an estimate of the spatial mean bidirectional reflectance factor (BRF) for a tree canopy (displaying a horizontally heterogeneous foliage distribution) and the statistical significance of that estimate. The system includes a manlift support-ing a horizontal beam 7 m long on which are mounted four radiometers. These radiometers may be pointed, and radiance data acquired, in any of 11 view directions in the principal plane of the sun. A total of 80 data points, acquired in 3 min, were used to estimate the BRF of a walnut orchard 5 m tall and detect true differences of 12% of the mean approximately 90% of the time.