Karl Fred Huemmrich

Remote sensing of photosynthetic-light-use efficiency of boreal forest

Using a helicopter-mounted portable spectroradiometer and continuous eddy covariance data we were able to evaluate the photochemical reflectance index (PRI) as an indicator of canopy photosynthetic light-use efficiency (LUE) in four boreal forest species during the Boreal Ecosystem Atmosphere experiment (BOREAS). PRI was calculated from narrow waveband reflectance data and correlated with LUE calculated from eddy covariance data. Significant linear correlations were found between PRI and LUE when the four species were grouped together and when divided into functional type: coniferous and deciduous. Data from the helicopter-mounted spectroradiometer were then averaged to represent data generated by the Airborne Visible Infrared Imaging Spectrometer (AVIRIS). We calculated PRI from these data and relationships with canopy LUE were investigated. The relationship between PRI and LUE was weakened for deciduous species but strengthened for the coniferous species. The robust nature of this relationship suggests that relative photosynthetic rates may be derived from remotely-sensed reflectance measurements. ©2000 Elsevier Science B.V. All rights reserved.

Use of narrow-band spectra to estimate the fraction of absorbed photosynthetically active radiation

Abstract We propose a novel approach for using high-spectral resolution imagers to estimate the fraction of photosynthetically active radiation adsorbed, f apar , by vegetated land surfaces. In comparison to approaches using broad-band vegetation indices, the proposed method appears to be relatively insensitive to the reflectance of nonphotosynthetically active material beneath the canopy, such as leaf litter or soil. The method is based on a relationship between the second derivative of the reflectance vs wavelength function for terrestrial vegetation and f apar . The relationship can be defined by the second derivatives in either of two windows, one in the visible region centered at 0.69 μm, another in the near-infrared region centered at 0.74 μm.

Vegetation canopy PAR absorptance and NDVI: an assessment for ten tree species with the SAIL model

Abstract The relation between the normalized difference vegetation index (NDVI) and the fraction of absorved photosynthetically active radiation (f APAR ) was examined for ten different forest types by using the scattering-from-arbitrarily-inclined-leaves (SAIL) radiative transfer model. Leaf reflectance and transmittance, twigreflectance, and background reflectance data were collected as part of field experiments whose sites contain species whose ranges cover a significant part of western and northern North America. This provides a sense of various that occur at continental scales. Actual backgrounds of forests include litter and mosses; these materials did not fall along a soil line in red-near infrared reflectance space. the simulations indicated that, at low values of the leaf area index (LAI), the background reflectance had a significant effect on the canopy reflectance, although little effect on photosynthetically active radiation (PAR) absorption. At higher values of LAI, leaf optical properties were the factors that dominated canopy reflectance and NDVI. Variations in canopy reflectance due to leaf optical properties were large, but most species had similar reflectance patterns. Green leaf optical properties, among the species studied, had little effect on f APAR . The presence of twigs in the canopy had a noticeable effect on canopy reflectance and absorption of PAR, but these effects were secondary to the effects of background and leaf optical properties.

Effects of shadows on vegetation indices

An important methodology in the remote sensing of biophysical variables is the use of Spectral Vegetation Indices (SVI). An SVI is an algebraic combination of reflectance values from different wavelength bands to produce a single value. Two SVI are evaluated for their usefulness in determining the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). Evaluating an SVI requires a comparison to a perfect metric for the desired variable. For remote sensing that means the SVI being tested should be sensitive to the variable to be measured, and should provide consistent results for different vegetation types, minimizing the effect of varying canopy characteristics, such as soil background reflectance. The SVI evaluated in this study are the Normalized Difference Vegetation Index (NDVI) and the Soil Adjusted Vegetation Index (SAVI). Most work on the remote sensing of biophysical variables has been performed on crop and grassland canopies. This study broadens the landscape types to include forest and woodland canopies. Forest canopy spectral reflectance is simulated using the GeoSail model. The GeoSail model combines a geometric model, which calculates the amount of shadows, with the Scattering from Arbitrarily Inclined Leaves (SAIL) model, which calculates the reflectance of the trees. Scene reflectance is modeled by calculating an area weighted average of three landscape components: illuminated canopy, illuminated background, and shadowed background. The reflectance of these components are obtained from the SAIL model.

BOREAS TF-8 NSA-OJP Tower Flux, Meteorological, and Soil Temperature Data

The BOReal Ecosystem-Atmosphere Study Tower Flux (BOREAS TF-3) team collected tower flux, surface meteorological, and soil temperature data at the BOREAS Northern Study Area-Old Black Spruce (NSA-OBS) site continuously from the March 1994 through October 1996. The data are available in tabular ASCII files.

Applications of the BIOPHYS Algorithm for Physically-Based Retrieval of Biophysical, Structural and Forest Disturbance Information

Canopy reflectance model inversion using look-up table approaches provides powerful and flexible options for deriving improved forest biophysical structural information (BSI) compared with traditional statistical empirical methods. The BIOPHYS algorithm is an improved, physically-based inversion approach for deriving BSI for independent use and validation and for monitoring, inventory and quantifying forest disturbance as well as input to ecosystem, climate and carbon models. Based on the multiple-forward mode (MFM) inversion approach, BIOPHYS results were summarised from different studies (Minnesota/NASA COVER; Virginia/LEDAPS; Saskatchewan/BOREAS), sensors (airborne MMR; Landsat; MODIS) and models (GeoSail; GOMS). Applications output included forest density, height, crown dimension, branch and green leaf area, canopy cover, disturbance estimates based on multi-temporal chronosequences, and structural change following recovery from forest fires over the last century. Good correspondences with validation field data were obtained. Integrated analyses of multiple solar and view angle imagery further improved retrievals compared with single pass data. Quantifying ecosystem dynamics such as the area and percent of forest disturbance, early regrowth and succession provide essential inputs to process-driven models of carbon flux. BIOPHYS is well suited for large-area, multi-temporal applications involving multiple image sets and mosaics for assessing vegetation disturbance and quantifying biophysical structural dynamics and change. It is also suitable for integration with forest inventory, monitoring, updating, and other programs.

Managing and supporting large integrated and interdisciplinary field studies: The BOREAS example

Large integrated and interdisciplinary field studies, such as the Boreal Ecosystem-Atmosphere Study (BOREAS), are conducted to refine our understanding of the interactions between the land surface and the atmosphere. Viewed as a case study, the BOREAS research objectives and final data set exemplify the complex nature and requirements of earth systems science research. The management and data system activities required to execute the study also echo this complexity. Rather than several research teams providing the needed management and data support, BOREAS management used a dedicated project staff to handle these functions. As the study progressed, the project staff transitioned from support of logistics and study management to information system operation and data publication, drawing upon the background knowledge gained from the earlier stages of the project. Data publication involves the creation and distribution of quality-checked and documented data with all ancillary information required to make it useful to someone unfamiliar with the study. We assert that the success of large integrated and interdisciplinary field studies depends upon having a dedicated staff. This staff focuses on the overall goals of the study throughout all phases of the effort: contributing to project planning, logistics, management, and data collection efforts; distributing, quality checking, and integrating the diverse data sets; working with the science teams to develop standardized data set documentation; integrating the diverse data and documentation for archiving; and publishing the data for long-term use by the larger scientific community. In this paper, the different phases of BOREAS are discussed, and the contributions that the dedicated staff made are examined. The value of spending resources on a centralized staff for project support and data publication activities is also examined.

BOREAS TF-10 NSA-Fen Tower Flux and Meteorological Data

The BOREAS TF-10 team collected tower flux and meteorological data at two sites, a fen and a young jack pine forest, near Thompson, Manitoba, Canada, as part of BOREAS. A preliminary data set was assembled in August 1993 while field testing the instrument packages, and at both sites data were collected from 15-Aug to 31-Aug. The main experimental period was in 1994, when continuous data were collected from 08-Apr to 23-Sep at the fen site. A very limited experiment was run in the spring/ summer of 1995, when the fen site tower was operated from 08-Apr to 14-Jun in support of a hydrology experiment in an adjoining feeder basin. Upon examination of the 1994 data set, it became clear that the behavior of the heat, water, and carbon dioxide fluxes throughout the whole growing season was an important scientific question, and that the 1994 data record was not sufficiently long to capture the character of the seasonal behavior of the fluxes. Thus, the fen site was operated in 1996 in order to collect data from spring melt to autumn freeze-up. Data were collected from 29-Apr to 05-Nov at the fen site. All variables are presented as 30-minute averages. The data are stored in tabular ASCII files.

BOREAS TF-10 NSA-YJP Tower Flux, Meteorological, and Porometry Data

The BOREAS TF-10 team collected tower flux and meteorological data at two sites, a fen and a young jack pine forest, near Thompson, Manitoba, Canada, as part of BOREAS. A preliminary data set was assembled in August 1993 while field testing the instrument packages, and at both sites data were collected from 15-Aug to 31-Aug. The main experimental period was in 1994, when continuous data were collected from the young jack pine site from 23-May to 20-Sep. Upon examination of the 1994 data set, it became clear that the behavior of the heat, water, and carbon dioxide fluxes throughout the whole growing season was an important scientific question, and that the 1994 data record was not sufficiently long to capture the character of the seasonal behavior of the fluxes. Thus, the young jack pine site was operated from 08-May to 07-Nov in 1996 in order to collect data from spring melt to autumn freeze-up. All variables are presented as 30-minute averages. Supporting data were also collected to describe the surface#s state and to provide the information, in association with the flux data, to build SVAT models. For the young jack pine site, these supporting data included stomatal conductance measurements. The data are stored in tabular ASCII files.

BOREAS TF-02 SSA-OA Tower Flux, Meteorological, and Precipitation Data

The BOReal Ecosystem-Atmosphere Study Tower Flux-2 (BOREAS TF-2) team collected energy, carbon dioxide, water vapor, and momentum flux data above the canopy and in profiles through the canopy, along with meteorological data at the BOREAS Southern Study Area-Old Aspen (SSA-OA) site. Above-canopy measurements began in early February and ran through mid-September of 1994. Measurements were collected over a longer period of 1994 than most BOREAS flux sites. Daily precipitation data from several gauges were also collected. The data are available in tabular ASCII files.