Yen-Ben Cheng

Linking foliage spectral responses to canopy-level ecosystem photosynthetic light-use efficiency at a Douglas-fir forest in Canada.

The light-use efficiency (LUE) of a mature Canadian Douglas-fir forest (DF49) was studied using high-resolution in situ temporal, spatial, and spectral measurements in conjunction with fluxes acquired from an instrumented tower. We examined the photochemical reflectance index (PRI), a spectral index responsive to high light conditions that alters reflectance at 531 nm, in combination with several alternative reference bands at 551, 570, and 488 nm. These indices were derived from directional reflectance spectra acquired by a hyperspectral radiometer system mounted on the DF49 tower, viewing the canopy through almost 360° rotations multiple times an hour daily throughout the 2006 growing season. From canopy structure information, three foliage sectors within the canopy were delineated according to instantaneous illumination conditions (sunlit, shaded, and mixed shaded–sunlit). On sunny days, the PRI indices for the sunlit foliage sector captured high light-induced stress responses, expressed as significantly different PRI values for sunlit versus shaded foliage. This difference was not observed on highly diffuse or overcast days. PRIs on sunny days tracked the diurnal photoregulation responses throughout the growing season in concert with illumination intensity. We computed the effective instantaneous LUE for the three foliage groups (LUEfoliage) using modeled and measured information. We provide convincing evidence that LUEfoliage can be well described and strongly related to all variations of the PRI within this coniferous forest under relatively clear skies (0.59 > r2 > 0.80, P < 0.0001). LUEfoliage varied through the growing season between 0.015 and 0.075 µmol C µmol–1 absorbed photosynthetically active radiation (APAR), and the lowest daily values were associated with the sunlit foliage group. The mixed sunlit-shaded foliage was the only group to exhibit monthly averages close to the maximum ecosystem LUE parameter (εmax) used in LUE models for evergreen needle forests (0.0196 µmol C µmol–1 APAR). Implications for remote sensing of carbon uptake dynamics and the interaction of canopy structure and physiology are discussed.

Integrating Solar Induced Fluorescence and the Photochemical Reflectance Index for Estimating Gross Primary Production in a Cornfield

The utilization of remotely sensed observations for light use efficiency (LUE) and tower-based gross primary production (GPP) estimates was studied in a USDA cornfield. Nadir hyperspectral reflectance measurements were acquired at canopy level during a collaborative field campaign conducted in four growing seasons. The Photochemical Reflectance Index (PRI) and solar induced chlorophyll fluorescence (SIF), were derived. SIF retrievals were accomplished in the two telluric atmospheric oxygen absorption features centered at 688 nm (O2-B) and 760 nm (O2-A). The PRI and SIF were examined in conjunction with GPP and LUE determined by flux tower-based measurements. All of these fluxes, environmental variables, and the PRI and SIF exhibited diurnal as well as day-to-day dynamics across the four growing seasons. Consistent with previous studies, the PRI was shown to be related to LUE (r2 = 0.54 with a logarithm fit), but the relationship varied each year. By combining the PRI and SIF in a linear regression model, stronger performances for GPP estimation were obtained. The strongest relationship (r2 = 0.80, RMSE = 0.186 mg CO2/m2/s) was achieved when using the PRI and SIF retrievals at 688 nm. Cross-validation approaches were utilized to demonstrate the robustness and consistency of the performance. This study highlights a GPP retrieval method based entirely on hyperspectral remote sensing observations.

Dynamics of spectral bio-indicators and their correlations with light use efficiency using directional observations at a Douglas-fir forest

The carbon science community must rely on satellite remote sensing to obtain global estimates of photosynthetic activity, typically expressed as net primary production (NPP), gross primary production (GPP) or light use efficiency (LUE). The photochemical reflectance index (PRI), calculated as a normalized difference reflectance index using a physiologically active green band (~531 nm) and another physiologically insensitive green reference band (~570 nm), denoted as PRI(570), has been confirmed in many studies as being strongly related to LUE. Here, we examined the potential of utilizing PRI(570) observations under different illumination conditions for canopy LUE estimation of a forest. In order to evaluate this, directional hyperspectral reflectance measurements were collected continuously throughout the daytime periods using an automated spectroradiometer in conjunction with tower-based eddy covariance fluxes and environmental measurements at a coastal conifer forest in British Columbia, Canada throughout the 2006 growing season. A parameter calculated as the PRI(570) difference (dPRI(570)) between shaded versus sunlit canopy foliage sectors showed a strong correlation to tower-based LUE. The seasonal pattern for this correlation produced a dramatic change from high negative (r ~ ?0.80) values in the springtime and early fall to high positive values (r ~ 0.80) during the summer months, which could represent the seasonality of physiological characteristics and environmental factors. Although the PRI(570) successfully tracked canopy LUE, one or both of its green bands (~531 and 570 nm) used to calculate the PRI are unavailable on most existing and planned near-term satellites. Therefore, we examined the potential to use 24 other spectral indexes for LUE monitoring that might be correlated to PRI, and thereby a substitute for it. We also continued our previous investigations into the influence of illumination conditions on the observed PRI(570) and other indexes. Among the 24 indexes examined, three PRI indexes using different reference bands (488, 551 and 705 nm) showed high correlations to the traditional PRI(570), especially PRI(551) and PRI(705). This indicates three additional PRI variations for LUE monitoring if the traditional reference band at 570 nm is not available but the 531 nm band is available. Five other indexes also yielded high correlations to PRI(570): Dmax and DM705, two indexes calculated from derivative reflectance spectra; a simple ratio of reflectance values at 685 nm and 655 nm (SR685_655); and a double-peak optical index (DPI). The diurnal and seasonal dynamics of these eight indexes and PRI(570) were explored. All of these indexes except DPI expressed linear dependence on available sunlight and more strongly expressed diurnal dynamics in April than in August during summer drought. The differences for shaded versus sunlit canopy foliage sectors were also calculated for the eight indexes, and their correlations to canopy LUE across the season were examined. The performances were similar for the most successful and seasonally stable indexes: dPRI(551), dPRI(705) and dPRI(570). The other five indexes showed good correlation to LUE in some but not all the months, and the months with high correlations varied among them.

Utilizing In Situ Directional Hyperspectral Measurements to Validate Bio-Indicator Simulations for a Corn Crop Canopy

Two radiative transfer canopy models, SAIL and the two-layer Markov-Chain Canopy Reflectance Model (MCRM), were coupled with in situ leaf optical properties to simulate canopy-level spectral band ratio vegetation indices with the focus on the photochemical reflectance index in a cornfield. In situ hyperspectral measurements were made at both leaf and canopy levels. Leaf optical properties were obtained from both sunlit and shaded leaves. Canopy reflectance was acquired for eight different relative azimuth angles (ψ) at three different view zenith angles (θv), and later used to validate model outputs. Field observations of PRI for sunlit leaves exhibited lower values than shaded leaves, indicating higher light stress. Canopy PRI expressed obvious sensitivity to viewing geometry, as a function of both θv and ψ. Overall, simulations from MCRM exhibited better agreements with in situ values than SAIL. When using only sunlit leaves as input, the MCRM-simulated PRI values showed satisfactory correlation and RMSE, as compared to in situ values. However, the performance of the MCRM model was significantly improved after defining a lower canopy layer comprised of shaded leaves beneath the upper sunlit leaf layer. Four other widely used band ratio vegetation indices were also studied and compared with the PRI results. MCRM simulations were able to generate satisfactory simulations for these other four indices when using only sunlit leaves as input; but unlike PRI, adding shaded leaves did not improve the performance of MCRM. These results support the hypothesis that the PRI is sensitive to physiological dynamics while the others detect static factors related to canopy structure. Sensitivity analysis was performed on MCRM in order to better understand the effects of structure related parameters on the PRI simulations. LAI showed the most significant impact on MCRM-simulated PRI among the parameters studied. This research shows the importance of hyperspectral and narrow band sensor studies, and especially the necessity of including the green wavelengths (e.g., 531 nm) on satellites proposing to monitor carbon dynamics of terrestrial ecosystems.

Impact of Spectral Resolution on Solar Induced Fluorescence and Reflectance Indices for Monitoring Vegetation

This study examines the impact of spectral resolution on red-edge reflectance (R) and Fraunhofer Line Depth (FLD) derived fluorescence (F) from vegetation. The goal of this investigation is to present data describing net canopy CO2 exchange (Anet) of corn (Zea mays L.) under variable N supply and present considerations for both fluorescence and reflectance sensing methodologies to remotely quantify this key regulator of ecosystem/biome productivity. A number of R indexes were investigated and consistent relationships were evident between red-edge R and R derivative (D) indexes to indicators of crop growth and condition. Through Gaussian FWHM spectral broadening of the native 3 nm data in intervals from 10 nm to 50 nm, it was determined that correlations were maintained between the top two performing indexes (Dmax/D744, R800/R750) and their respective measures of crop condition (Anet, C:Chl) up to a 20 nm spectral resolution. Adaxial corn leaf R was obtained from three spectrometers operating in unison with optical fibers bundled together enabling NADIR measurement of leaf R at five spectral resolutions ranging from 0.2 nm to 5 nm. In general, the increased band depth of high spectral resolution data allowed for more accurate SIF retrievals with improved relationships to plant biophysical parameters. From this investigation we conclude that indices calculated from both R and F data types supplied useful information for modeling nitrogen use for carbon sequestration by vegetation.

Using Reflectance Measurements to Determine Light use Efficiency in Corn

This study examines the ability of narrow band vegetation indexes to detect spectral changes associated with stress and relate them to light use efficiency (LUE) over the course of a day as well as through the growing season. In a corn field in Beltsville, MD, carbon flux measurements were made at a flux tower using eddy covariance techniques. At this site on six days during the 2007 growing season hyperspectral reflectance measurements were collected at hourly intervals. LUE was determined at the time of the reflectance measurements from carbon fluxes measured at the flux tower. The measured LUE values were compared with a number of spectral indices that have been shown to be related to a number of leaf characteristics including: the status of the xanthophyll cycle; solar induced fluorescence; leaf water content; and concentrations of pigments such as chlorophyll, carotenoids, and anthocyanins. We found that several of these indices provide good estimates of both diurnal and seasonal LUE variability.

Applications using EO-1 hyperion AT-sensor and surface reflectance: comparisons and case studies

The NASA EO-1 Hyperion observations were utilized to derive at-sensor Top-of-atmosphere (TOA) and ATREMcorrected surface reflectance over three study sites of different land use types. Direct comparisons between TOA and ATREM reflectance showed the most disagreement in the visible spectral region and regions that were affected by water absorption features. Nevertheless, as little as 3% overall differences were found for an arid dessert scene. Furthermore, example applications using Hyperion at-sensor TOA reflectance were investigated. Selected band ratio vegetation indices calculated from both TOA and surface reflectance were correlated (r∼0.6 to 0.94) but differed in magnitude. For instance, NDVI calculated from at-sensor TOA reflectance consistently showed lower values. A potential quick-look product using these indices to model relative vegetation stress was demonstrated in this study.

Foliar Bio-Physical and Spectral Properties Associated with Light Environment in a Mature Poplar Stand

This study evaluates the ability of reflectance (R) and chlorophyll fluorescence (ChlF) parameters to express the differences in foliar properties associated with varying light environment. In the summer of 2007 a tall crane was used to acquire foliar reflectance measurements and collect samples from sunlit and shaded tree crowns, from the upper and lower canopy of a mature tulip poplar (Lirodendron Tulipifera L.) forest. Leaf-level photosynthesis, R, ChlF spectra and kinetics and bio-physical parameters were measured on excised samples. The differences in the light environment greatly affected the spectral and biophysical data, especially the photosynthetic parameters (Amax, LUE, photochemical quenching, Qp, Qn, Qp/Qn). For sunlit foliage, ChlF kinetics (Fo, Fm, Fs, and Fv/Fm) were significantly lower than shaded foliage. Our analysis indicated that some of the tested spectral bio-indicators (e.g. PRI1, RE2, G035, Dmax) were strongly associated with LUE and differed significantly depending on the light environment. ChlF as compared to R indices followed more closely the trends in LUE, which emphasizes the application of ChlF for the timely detection of changes in vegetation physiology and carbon dynamics.

Comparisons between in Situ Anisotropic Reflectance Measurements and Simulations for Vegetation Canopies: Validation and Sensitivity Analysis

In situ directional reflectance observations were taken on trees of three species and were used to validate the Four-Scale Linear Model for AnIsotropic Reflectance (FLAIR). FLAIR-simulated spectra were satisfactory in general, especially for the shaded and shaded/sunlit mixed portions of the canopy but were species dependent. Good agreements with field observations were found when utilizing these simulations to derive band ratio indexes, Normalized Difference Vegetation Index (NDVI) and Photochemical Reflectance Index (PRI). Sensitivity analysis were performed to investigate the importance of three canopy biophysical parameters, and the leaf to shoot ratio showed the most impact on simulation results. The results in this study showed the potential to simulate directional reflectances with FLAIR on canopies with complicated structure.

Relating a Spectral Index from MODIS and Tower-Based Measurements to Ecosystem Light Use Efficiency for a Fluxnet-Canada Coniferous Forest

Hyperspectral reflectance data collected diurnally from an instrumented tower were examined in conjunction with the eddy correlation fluxes and meteorological measurements made throughout a growing season at a mature Douglas fir forest in British Columbia, Canada (DF49). Here we present 2006 in situ results relating the Photochemical Reflectance Index (PRI551) to photosynthetic light use efficiency (LUE). Canopy structure information was used to partition the forest canopy into sunlit and shaded fractions. At each observation period, the PRI551 was examined for the sunlit, shaded, and mixed sunlit/shaded canopy segments as defined by their instantaneous position relative to the solar principal plane (SPP). We show that the PRI551 clearly captures the differences in leaf groups on sunny days. We also examined PRI551 from MODIS ocean band imagery acquired over DF49 during a five year period (2001-2006) from both Terra (late morning) and Aqua (early afternoon) platforms. The MODIS observations from Terra and Aqua were acquired in different viewing planes above the landscape over a range of view zenith angles, and sampled the backscatter (sunlit) and forward scatter (shaded) sectors of the forests bidirectional reflectance distribution function. When tower-based bulk canopy LUE from 2006 was recalculated to estimate foliage-based values for the three foliage groups under their incident light environments, a strong linear relationship with PRI551 was demonstrated (r2~0.80). A similar relationship between the MODIS PRI551 and tower-based bulk LUE was obtained from satellite observations (r2~0.62), but only for the backscatter observations obtained at high light levels (APAR, >1500 mumol m-2 s-1). The MODIS observations, while not ideal in terms of spatial resolution (> 1 km2) or optimal viewing configuration, nevertheless provided a means to monitor forest under stress using narrow spectral band indices and off-nadir observations.