Parameters Affecting Reflectance of Coniferous Forests in the Region of Chlorophyll Pigment Absorption

Brian Curtiss1 and Susan L. Ustin2
1Center for Study of Earth from Space (CSES)/CIRES
University of Colorado Boulder, CO 80309
(303) 492-6715 (303) 492-5070 (FAX)
2Department of Botany
University of California Davis, CA 95616
(530) 752-0621

Abstract

The observed variation in the reflectance in the region of chlorophyll absorption (450 to 900 nanometers) of mature Pinus ponderosa (Ponderosa Pine) is explained in terms of pigment concentrations and stress associated with pollutant exposure Reflectance and chlorophyll concentrations were measured on each years growth on each of 20 trees at four sites along an ozone exposure gradient in the southern Sierra Nevada. Additionally, each of these sites was characterized in terms of stand density and age. Exposure to ozone, a strong oxidant, results in chloroplast injury. Granulation of the chloroplast stroma is an early metabolic response to ozone exposure. Thus, an examination of the reflectance changes observed along the ozone gradient provide a means for quantifying the spectral changes associated with chloroplast damage and reduced photosynthetic efficiency. Additionally, the spectral trends associated with increasing needle age were related to the effects of cumulative ozone exposure and changes in overall chlorophyll concentration. It was found that the within site variation of the reflectance spectra was directly related to total chlorophyll concentration; an increase in chlorophyll concentration resulted in both a shift in the "red-edge" inflection to longer wavelength and an increase in the depth of the chlorophyll absorption well. Similarly, an increase in chloroplast damage, associated with higher levels of ozone exposure, also produced shift to longer wavelengths of the "red edge" but a decrease in the depth of the absorption well. The absorption band broadening observed in the high ozone exposure sites is indicative of an increase in the disorder of absorbing medium. In the case of chlorophyll, the observed broadening is probably the result of damage to the thylacoid membranes within the chloroplasts.

Introduction

One of the major goals within the field of botanical remote sensing is the accurate assessment of ecosystem productivity. Such existing and proposed models for the determination of ecosystem productivity assume a linear relationship between APAR (absorbed photosynthetically active radiation) and vegetation indices such as NDVI (normalized difference vegetation index), (Sellers 1985 & 1987). Additionally, it is generally assumed that APAR is a direct measure of the maximum photosynthetic capacity of an ecosystem and that the photosynthetic efficiency of an ecosystem is constant. Thus, these models calculate ecosystem productivity by integrating APAR over the growing season. These models work quite well for ecosystems where the assumptions are valid.

Several factors can result in errors in NDVI based estimates of APAR. In ecosystems with incomplete canopy closure, the spectral signature received by the sensor has components from both the vegetation and from the substrate (ea. soil, rock, plant litter). Depending on the viewing and illumination geometries, canopy architecture, and on the albedo of the substrate in the region of chlorophyll absorption, errors in NDVI estimated APAR can be as high as 50 percent. The presence of a large proportion of shadow can also produce errors in the observed reflectance of a forest canopy (Smith et al. 1983; Curtiss and Ustin 1988), that, if they are not corrected for, can result in an underestimation of NDVI. Thus, calculated NDVI can vary within a scene due to changes in viewing geometry, and can change seasonally because of illumination geometry. These changes will occur independent of any real change in ecosystems photosynthetic capacity.

Ecosystems tend to adjust their maximum photosynthetic capacity in relation to available resources through changes in leaf area index and other architectural changes, chlorophyll content, and species composition. When the rate change in a stress is greater than the rate at which the ecosystem can adjust its photosynthetic rate, the assumption of constant photosynthetic efficiency is not valid. Examples of this are. short term stresses, such as diurnal water stress; longer term (36 months) stress for ecosystems with slow rates of photosynthetic capacity adjustment, such as coniferous forests.

In some instances, these assumptions produce errors that cancel each other, and, therefore, give the appearance that the model is working. Northern latitude boreal forests are an example of such an ecosystem. During the winter months, calculated NDVI for these ecosystems goes down as would be expected if NDVI is related to photosynthetic productivity. In reality, NDVI goes down not because of a reduction in APAR, but because of the increase importance of shadowing associated with the seasonal change in solar zenith angle. This underestimate of APAR is offset by a lowering of photosynthetic efficiency resulting from the lower winter temperatures. Because the predictions of ecosystem productivity are linked to solar zenith angle, it is likely that they will be insensitive to true changes in boreal forest productivity such as might be associated with longer, colder winters or more frequent spring freezes.

Many of the errors associated with NDVI based estimates of APAR can be overcome by using contiguous, high spectral resolution measurements made by sensors such as the High Resolution Imaging Spectrometer planed for the Eos polar platform, (Goetz and Herring 1989). These new sensors measure the reflected spectrum in many, contiguous, spectral bands in the visible and reflected infrared wavelengths; this data can be used for the direct identification and quantification of spectral features diagnostic of leaf constituents and, will thus permit a more physiologically based understanding of vegetation spectra. Spectral patterns within the visible and shortwave infrared regions are diagnostic for the presence of vegetation (Asrar et al., 1984). They may permit the identification of fundamental physical canopy properties (Goel and Thompson, 1984a,b; Sellers, 1985, 1986), and potentially define spectral patterns related to the assessment of environmental stress (Chang and Collins, 1983; Bauer, 1985). While minerals can be identified based on the presence of diagnostic spectral features (Goetz et al. 1985), vegetation spectra are generally very similar to one another due to their small base of shared constituents, e.g. pigments, cellulose, lignin, protein, starch. Variations in shape of the reflectance spectra of vegetation will be largely controlled by differences in leaf and canopy structure and foliar optical properties. Additionally, because of the high dimensionality of these datasets, spectral un-mixing models (Adams and Adams 1984; Smith et al. 1985; Adams et al. 1986) can be used to separate the spectral signatures present in a mixed pixel (ea. vegetation, substrate, shade, see Ustin et al. 1986).

To date, there have been few experimental studies concerning the remote detection of photosynthetic properties. Most experimental studies investigating spectral characteristics associated with physiological processes have examined environmental stress(es), such as drought or mineral excess, on the reflectance characteristics of individual leaves (Chang and Collins 1983; Ripple 1986). Stress-induced shifts in the red-edge have been reported, both toward longer (i.e. red) or shorter (blue) wavelengths, and have been attributed to changes in chlorophyll content. This red-shift of the chlorophyll absorption edge was first reported by Gates et al. (1965) and later by Collins (1978).

In a pilot study of ozone treated conifers, Ustin and Curtiss (1987) found a marked increase in reflectance in the chlorophyll absorbance bands of ozone treated conifers (Figure 1). Additionally, they report a small blue shift of 3-9 nm in the derivative of the inflection point wavelength of the 'red edge'. Such changes are consistent with reports of chlorophyll loss as a characteristic consequence of ozone injury. Further, Ustin and Curtiss (1987) suggest that subtle changes in the 590-690 nm spectral region indicate that chlorophyll a and b concentrations may be quantified in addition to total chlorophyll.

This paper presents a first step towards a better understanding of the parameters that affect that portion of the reflected light spectrum dominated by chlorophyll pigment absorption. It is hoped that this understanding will provide a basis for the development of more accurate techniques for the assessment of APAR and photosynthetic efficiency.

Methods

The reflectance spectra of ozone exposed Pinus ponderosa (Ponderosa Pine) seedlings and needles from mature trees were measured with the NASA Jet Propulsion Laboratories' PIDAS spectroradiometer (Portable Intelligent Display and Analysis Spectrometer), (Goetz 1987). This instrument makes reflectance measurements in 1 nm wide spectral bands in the visible and near infrared (400-900 nm) and in 5 nm wide spectral bands in the near and shortwave infrared (900-2450 nary). The foreoptics, with a 6° field of view, was placed approximately one meter above samples, giving about a 10 centimeter field of view. Illumination was provided by two 120 volt 1000 watt floodlamps operated from two 120 volt 10 ampere constant voltage supplies to provide a constant and stable light source. The lamps were placed opposite each other, to minimize shadows, and about 0.5 meters above the canopy. Irradiance at the canopy was approximately equivalent to full sunlight.

The Ponderosa Pine seedlings were exposed to a single seasons duration of realistic fumigation profiles having stochastic daily doses and episodic peaks. Dose levels were chosen to be representative of summer air quality currently experienced in many parts of the western United States. Reflectance measurements of the seedlings were made on 9 different areas on composite canopies formed from arranging about 30 seedlings in a tray. The elevation of the trays holding the seedlings were adjusted to maintain a constant distance between the canopy top and the PIDAS foreoptics. FiberFrax, a uniformly reflective ceramic fiber wool, was used as the reflectance standard. This standard was placed at the same distance from the foreoptics as the Ponderosa Pine canopies and was measured before and after each set of canopy measurements.

The Southern Sierra Nevada is an area of regional ozone exposure. Reflectance spectra of three branches from each of 20 Ponderosa Pine trees at four sites experiencing a range of ozone exposure levels were measured using a setup similar the one used for the seedlings. Chlorophyll analysis and reflectance measurements where made separately on each years growth. For these measurements, Halon was used as a reflectance standard. A more complete description of the site selection and sample analysis is presented in Ustin et al. (1988).

Results

Ozone tends to be regionally distributed and has been clearly implicated in forest decline (McLauglin 1986). Chronic exposure to ozone, a strong oxidant, results in direct injury of cell membranes, with chloroplast injury being an early metabolic response. General disruption of chloroplast function proceeds other cellular changes (Thompson et al. 1966). Thus, ozone exposure provides a means for examining the spectral changes associated with reductions in both photosynthetic capacity and efficiency.

The primary spectral changes observed in the seedings is that associated with chlorophyll loss, see Figure 1. Chlorophyll loss results in an increase in reflectance in the region of chlorophyll pigment absorption (450 - 700 nm). Additionally, this chlorophyll loss results in a shift in the red edge inflection to shorter wavelengths (a blue shift). The red edge inflection is defined as the center at half maximum of the first derivative of the spectrum, see Figure 2. The use of the wavelength of the red edge inflection (REI) is useful because it is much more insensitive to mixed pixel effects as compared to NDVI or other band ratio techniques for the quantification of light absorption by chlorophyll. The reflectance spectra in Figure 3 show that the reflectance spectrum of an ozone exposed seedling could be confused with the mixed signature of a control (no ozone exposure) seedling and granitic soil when using a broad band sensor such as TM or AVHRR. However, the wavelength of the REI of the mixture does not match the REI of the ozone exposed seedling.

While the relationship between chlorophyll concentration and maximum photosynthetic capacity is good within a single species, architectural variation between species and environmentally produced variation in photosynthetic efficiency result in a breakdown of this relationship. Thus, chlorophyll concentration can be used to asses the ability of parameters derived from reflectance data to predict maximum photosynthetic capacity only when the comparison is made within a species and when all samples have equivalent photosynthetic efficiency. Figure 4 show the relationship between REI wavelength and needle chlorophyll content for two site with similar ozone exposures. Each years growth (up to four) for 20 trees at 2 sites is plotted as a separate data point.

Some of the variance about the regression line observed in Figure 4 can be explained in terms of variance in photosynthetic efficiency. Within a single site there is considerable variation in the observed visual symptoms of ozone injury, such a chlorosis, low needle retention and fewer years of needles retained. The greater chloroplast damage in these trees produces a broadening of the chlorophyll absorption bands that results in a shift to longer wavelengths (a red shift) of the REI (see Fig. 5) that is indicative of a lower overall photosynthetic efficiency.

Summary

Photosynthetic properties are potentially measurable using parameters derived from remote sensing data such as the wavelength of the red edge inflection. The wavelength of the red edge inflection is quite insensitive to mixed pixel effects, and, therefore may be used for ecosystems with incomplete canopy closure. Shifts in the wavelength of the red edge inflection to shorter wavelength in conjunction with a reduction in the chlorophyll absorption well depth are indicative of a reduction in total chlorophyll, and, thus, a reduction in photosynthetic capacity. Actual photosynthetic rates are often less than the photosynthetic capacity due to ambient environmental conditions, such as chronic ozone exposure, drought or high temperature stress. Red edge shifts to longer wavelengths produced by ozone induced chloroplast membrane damage and are indicative of this reduction in photosynthetic efficiency.

Because of the complex interaction of chlorophyll absorption band broadening and band depth the ability to determine the components of the photosynthetic process using the remote sensing techniques discussed here is untried and problematic. However, current techniques relying on measures like NDVI have been demonstrated to be quite sensitive to canopy structure, variability of the substrate, and viewing and illumination geometry. Thus, more accurate means of predicting photosynthetic capacity and efficiency are required before remotely sensed techniques can be applied for quantitative scene-independent predictions of ecosystem productivity. If the specific chlorophyll absorption feature parameters identified in this paper can alone, or in combination, produce greater reliability of prediction, then the results of this study will have broad significance.

Acknowledgments

Although the information in this document has been funded in part by the United States Environmental Protection Agency under Grant #R-814274-01-0 and contract #7B0008NTEX, it does not necessarily reflect the view of the Agency and no official endorsement should be inferred. Additional funding for this study was provided under NASA/JPL contract #958039.

References Cited

1998, Center for Spatial Technologies and Remote Sensing (CSTARS)
University of California, Davis