BOREAS RSS-04 1994 Southern Study Area Jack Pine LAI and FPAR Data Summary The RSS-04 team collected several data sets related to leaf, plant, and stand physical, optical, and chemical properties. This data set contains leaf area indices and FPAR measurements which were taken at the three conifer sites in the BOREAS SSA during August 1993 and at the jack pine tower flux and a subset of auxiliary sites during July and August 1994. The measurements were made with LAI-2000 and Ceptometer instruments. The measurements were taken for the purpose of model parameterization and to test empirical relationships that were hypothesized between biophysical parameters and remotely sensed data. The data are stored in tabular ASCII files. Table of Contents * 1 Data Set Overview * 2 Investigator(s) * 3 Theory of Measurements * 4 Equipment * 5 Data Acquisition Methods * 6 Observations * 7 Data Description * 8 Data Organization * 9 Data Manipulations * 10 Errors * 11 Notes * 12 Application of the Data Set * 13 Future Modifications and Plans * 14 Software * 15 Data Access * 16 Output Products and Availability * 17 References * 18 Glossary of Terms * 19 List of Acronyms * 20 Document Information 1. Data Set Overview 1.1 Data Set Identification BOREAS RSS-04 1994 Southern Study Area Jack Pine LAI and FPAR Data 1.2 Data Set Introduction Estimates of Leaf Area Index (LAI) and fraction of Photosynthetically Active Radiation (FPAR) were made during the Focused Field Campaign in summer 1993 (FFC-93) and the second Intensive Field Campaign (IFC-2) in 1994. In FFC-93, preliminary measurements were made using a LI-COR LAI-2000 and a Decagon Devices Ceptometer at the three conifer tower flux sites Young Jack Pine (YJP), Old Jack Pine (OJP), and Old Black Spruce (OBS) in the BOReal Ecosystem-Atmosphere Study (BOREAS) Southern Study Area (SSA), while in 1994 attention was focused on jack pine tower flux and auxiliary sites in the SSA. LAI was estimated using both instruments; FPAR was obtained using the Ceptometer. Both the Ceptometer and the LAI-2000 rely on the relationships between radiation transmission, canopy gap fraction, and LAI. The assumptions of these approaches are summarized below (see Section 9) and have been assessed for the BOREAS sites in Chen et al. (1997). 1.3 Objective/Purpose The Remote Sensing Science (RSS)-04 investigations were designed to obtain LAI, FPAR, and foliar chemistry data for a complex spatially variable forest canopy in order to: i) Parameterize an ecosystem simulation model. ii) Test empirical relationships hypothesized between biophysical parameters and remotely sensed data. iii) Parameterize a forest reflectance model and compare it with Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) data to deduce whether observed 'relationships' between canopy chemistry and reflectance are a product of canopy structure rather than foliar chemical variations themselves (see references list and biochemistry data set). iv) Drive the ecosystem simulation model with estimates of LAI and chemistry derived from remotely sensed data. 1.4 Summary of Parameters LAI, FPAR 1.5 Discussion The measurements that comprise this data set were collected as a contribution to the determination of the biophysical characteristics of the BOREAS SSA. 1.6 Related Data Sets BOREAS RSS-01 SSA PARABOLA Surface Reflectance and Transmittance Data BOREAS RSS-04 1994 Jack Pine Leaf Biochemistry and Modeled Spectra in the SSA BOREAS RSS-07 LAI, Gap Fraction, and FPAR Data 2. Investigator(s) 2.1 Investigator(s) Name and Title Dr. Stephen Plummer Professor Paul Curran 2.2 Title of Investigation RSS-04: Coupling Remotely Sensed Data to Ecosystem Simulation Models 2.3 Contact Information Contact 1 --------- Dr. Stephen Plummer Role: Data collection, transport, and project supervision Section for Earth Observation Institute of Terrestrial Ecology UK +44 1487 773381 (tel) +44 1487 773277/467 (fax) E-mail: S.Plummer@ite.ac.uk *formerly Remote Sensing Applications Development Unit British National Space Centre Contact 2 --------- Jaime Nickeson Raytheon STX Corporation NASA GSFC Greenbelt, MD (301) 286-3373 (301) 286-0239 (fax) jaime@ltpmail.gsfc.nasa.gov 3. Theory of Measurements For a discussion of the theory behind measurements presented here, users are directed to the review by Welles (1990), Welles and Norman (1991), and Chen et al. (1997). 4. Equipment 4.1 Sensor/Instrument Description i) LAI Two instruments were used to estimate LAI: a LI-COR LAI-2000 leaf area meter and a Decagon Devices Ceptometer. The LAI-2000 uses either one or two sensors to make measurements of the incoming diffuse incident flux above (A) and below (B) the canopy in blue wavelengths (<490 nm). Each reading comprises measurements of the amount of radiation at five zenith angles simultaneously over either the full azimuth or some portion of it, dictated by a viewing mask attached to the optical head. One measurement comprise a minimum of 10 numbers corresponding to 5 values of A (above) and 5 of B (below), at zenith angles of 0, 22.5, 37.5, 52.5, and 67.5 degrees. The Ceptometer is a line quantum sensor consisting of 80 separate photodiodes spaced at 1-cm intervals that are sensitive to radiation in the PAR region (400-700 nm). Measurements above and below the canopy are made to determine canopy transmittance at a number of solar zenith angles. ii) FPAR The Ceptometer was also used to determine the fraction of PAR absorbed by the canopy at a number of solar zenith angles. 4.1.1 Collection Environment Measurements were collected during ambient surface atmospheric conditions at the SSA during August 1993 (FFC-93) and July to August 1994 (IFC-2). The LAI-2000 measurements were made at dusk and dawn, Ceptometer data were collected on sunny days. 4.1.2 Source/Platform The instruments were held by the human investigators. 4.1.3 Source/Platform Mission Objectives The objective was to collect LAI and FPAR measurements at various locations. 4.1.4 Key Variables LAI, FPAR, Mean Tip Angle 4.1.5 Principles of Operation The LAI-2000 and Ceptometer are instruments that make measurements of the incoming radiation below a plant canopy. In the case of the LAI-2000, measurements are made in the blue (<490 nm) region, while the Ceptometer records incoming radiation in the PAR (400-700 nm) region. These data can be used to estimate LAI indirectly by making assumptions about the canopy architecture (leaf distribution). LAI is derived by inverting the radiation transmission (or gap fraction) information. The gap fraction is the fraction of view in any direction from below a plant canopy that is not blocked (by leaves). If it is assumed that leaves are the only elements that block radiation, that they are nontransmitting, and that they are randomly distributed in space, then the transmission of a beam of radiation can be related to the mean foliage density (see Section 9 for equations). If the canopy is horizontally homogenous, then the mean foliage density can be related to LAI. The LAI-2000 can be considered to be analogous to hemispherical photography in that it uses a fish eye light sensor that measures diffuse radiation in five distinct angular bands about zenith. The incoming radiation is projected onto photodiodes arranged as concentric rings. To derive gap fraction information, light conditions are assumed diffuse and uniform (dusk, dawn, or overcast conditions), and measurements are made of the sky above the canopy followed by readings below the canopy. The ratio of above-to-below canopy readings is used to obtain gap fraction. Because foliage is not normally randomly distributed and there is a proportion of nonfoliage matter that blocks radiation (dead leaves, branches, stems, or trunks), the derived measurement is normally referred to as ëeffectiveí LAI. Correction of these measurements can be made by derivation of factors that represent the woody-to-total area ratio and the clumping factors (needle-to-shoot area ratio and foliage clumping at greater than shoot level). The Ceptometer measures PAR radiation using 80 photodiodes spaced at each centimeter along an 80-cm rod. These are used to provide an average value of PAR over the 80-cm length and for the specific solar zenith angle and relative azimuth. To account for azimuthal variation in foliage distribution, it is advisable to use the average over several azimuth directions (in this case, eight). Because the instrument is primarily used to measure the fraction of absorbed PAR (FPAR), measurements are taken in direct sun conditions, with the proviso that the solar illumination is consistent over the period of measurement. In the same way as with the LAI-2000, measurements are obtained first outside the canopy and subsequently below the canopy. In line with gap fraction measurements, these can be converted into an estimate of LAI using the approach of Norman (1988) (see Section 9). This also requires measurement of the ratio of diffuse-direct incident radiation. The diffuse component can be obtained by shading the Ceptometer from direct sun using an appropriately sized object such that the entire length of instrument is covered for the width of the solar disk (this is an acquired skill). More recent instruments (Delta-T SunScanô) come with a separate quantum sensor and shading ring for use with a line PAR sensor. Chen et al. (1997) indicate that the Ceptometer provides a measurement of plant area index, although the finite averaging does account for error caused by clumping at scales larger than the averaging length. 4.1.6 Sensor/Instrument Measurement Geometry LAI-2000 The LAI-2000 is simple to operate and acquires data relatively fast. Operation can be through single-head, dual-head, or remote-head modes. In single-head mode, as used in this experiment, both A (above) and B (below) measurements are made with the same optical head. Because the calculation is made by comparison of A and B measurements, the instrument does not require either absolute or relative calibration. However, there is a need for ready access to a clear sky view near to the measurement site. In BOREAS operation, this caused difficulties, especially at mature pine sites with tree heights of approximately 15 m. Measurements had to be tailored to site conditions, and given the need to cover as many sites as possible, operation often occurred in nonoptimal conditions. Sites were measured at dawn or dusk, although the distance between sites limited coverage on any given day. For short tree sites, e.g., SSA-YJP, it was possible to get above the canopy (using the back of a standard Jeep or by climbing the flux tower in periods of nonoperation (1993) to a height of approx. 3-4 m). The LAI-2000 head requires leveling, but if contains a bubble level to make leveling easier. The best practice is to maintain instrument orientation for all measurements (A and B). This is particularly important where the availability of a clear area is restricted to a logging road. In such cases, the A reading was made from the back of a Jeep with the optical head masked downward such that only a proportion of the view was used (see Section 5). In all cases, the optical head was masked for the observer using a 45-degree mask unless more was required. A measurements were made as regularly as possible (before and after single B runs) to minimize the effect of variation in sky conditions and linear interpolation used to provide A values for each B value. Repetitions of each measurement were made to increase the representativeness, and in cases where the B value exceeded a corresponding A, the data point was ignored. This usually applied to measurements from the 5th ring. Removal of the 5th ring measurement in software meant that the data could be reintroduced. In all cases, the measurements were made at the same height, 1 m above the ground, except for reference measurements. The latter were made 1 m above the canopy (F8L6T), from the top of a large flux tower (G2L3T), or from a Jeep + 1 m height. Ceptometer As with the LAI-2000, there are a number of issues that affect the representativeness of measurements made with the Ceptometer. In all cases, reference measurements were made at the same locations as for the LAI-2000, and all measurement runs started and finished with a reference measurement. Since the same set of measurements was used for FPAR and LAI, these measurements were all made under direct sun conditions and included diffuse incident, canopy, and understory measurements. Each measurement comprised eight azimuth directions with the instrument level and out of any shadow cast by the operator. The eight measurements were made at the cardinal and subcardinal directions. Measurements were made at approximately 1 m where significant understory existed or 30 cm above ground where the understory was moss/lichen. Experience dictated whether a particular set of measurements was acceptable, especially for diffuse measurements, which required shading of the 80 photodiodes. It should be noted that because each individual measurement (1 through 8) was averaged on the fly one unshaded diffuse measurement could bias the data. When this occurred, measurements were repeated. A measurement sequence took considerably longer than the equivalent LAI-2000, and the processing was more laborious because no software was written for this purpose. This is now available with the Delta-T Sunscan and may be available with the Decagon Accupar. 4.1.7 Manufacturer of Sensor/Instrument LI-COR LAI-2000: LI-COR, Inc. 4421 Superior Street P.O. Box 4425 Lincoln, NE 68504 (402) 467-3576 http://env.licor.com/products/LAI2000/2000.htm Ceptometer: Decagon Devices, Inc. 950 NE Nelson Ct. P.O. Box 835 Pullman, WA 99163 (509) 332-2756 http://www.decagon.com/ 4.2 Calibration i) LAI-2000 Sensor calibration consists of five multipliers for the outputs of the five rings of the detector. These values are supplied on a calibration sheet and relate one ring to another. The values do not convert outputs to absolute values of energy. In one-sensor mode this is unimportant because the same sensor makes both A and B measurements. In two-sensor mode the two sensors are compared under the same diffuse light conditions to adjust the sensitivity of the two sensors. The instrument is calibrated using an integrating sphere supplied, by the UK representatives of LI-COR, Inc., Glen Spectra Ltd. ii) Ceptometer For LAI and FPAR determination, as long as the relative calibration of the 80 sensors is consistent, the same method applies as above. However, for determination of PAR, APAR, or incident PAR (PAR0), calibration must be applied to convert from output to SI units (µ mol m-2 s-1). Calibration is checked regularly by the owners of the instrument, University of Wales, Swansea, by comparison against an accurate PAR sensor under a standard daylight lamp. The accurate PAR quantum sensor is traceable to UK national standards. 4.2.1 Specifications Ceptometer ñ The ideal spectral response for a quantum sensor is a top hat function with 100% dropoff at 400 nm and 700 nm. All quantum sensors approximate this; however, errors caused by imperfect spectral response are small for daylight conditions. The system cosine response matches the ideal accurately (>90% below 70 degrees). Individual Ceptometer measurement time is nominally 120 ms over a range of 2500 µ mol m-2 s-1 with a resolution of 0.3 µ mol m-2 s-1. Accuracy of the instrument is ±10%. Maximum recorded values were approximately 1,500 µ mol m-2 s-1. The LAI-2000 measurements were made in one-sensor mode because calculation is based on the ratio of ‘A’ to ‘B’ measurements. Issues of calibration, resolution, and accuracy are not a problem provided that the relative calibration between individual view rings is accurate and consistent. 4.2.1.1 Tolerance Ceptometer - Accuracy of the instrument is ±10%. Maximum recorded values were approximately 1,500 µ mol m-2 s-1, well within the stated instrument range. Comparison of Ceptometer measurements against the Tracing Radiation and Architecture of Canopies (TRAC) instrument indicated a discrepancy of approximately 200 counts, but this was consistent over a range of light conditions. Absolute values of PAR were therefore not reported. Error analysis for the FPAR measurements could not be conducted against those from other Principal Investigators (PIs) (Chen, RSS-07) because measurements were not coincident. Within canopy variability (n=10) indicates that for FPAR values greater than 0.7, an error of ±10% should be expected, but because of clumping this can rise to 20-30% for FPAR values of 0.5. The LAI estimates from the Ceptometer, LAI-2000, TRAC, and allometry were assessed by Chen et al. (1997). All the methods have limitations, and an absolute accuracy could not be attributed to any one method. The error range for LAI measurement method is on the order of 15-30%. 4.2.2 Frequency of Calibration See Section 4 comments above ñ all measurements are ratio values, and the only issue is sensitivity of the instrument across the measurement range. This has not been checked but is assumed to be acceptable. Intercalibration between instruments was not performed for the LAI-2000 beause it was used in one-sensor mode. Comparison of Ceptometer measurements against the TRAC instrument indicated a discrepancy of approximately 200 counts, but this was consistent over a range of light conditions. Absolute values of PAR are not provided in this data set. 4.2.3 Other Calibration Information None given. 5. Data Acquisition Methods 1993 Procedure: i) LAI using the LAI-2000 At each of the three conifer tower flux sites, 100-m transects were established with access to a nonforest area for determination of radiation flux. At the SSA-YJP site, four transects were laid out on the cardinal points centered on the road next to the site hut (Figure 1). Data acquisition comprised an A measurement followed by 10 B measurements for each transect and finished with additional A measurement. A samples were taken at the flux tower and B samples at 10-m intervals along each transect. Data were collected on 14- Aug-1993 at 08:00 local time (15:00 Greenwich Mean Time (GMT). All the A measurements were acquired above the canopy at 3-4 m using the steps on the flux tower. In all cases, a 45-degree view cap was used to eliminate the observer. Figure 1. Transect locations at the SSA Young Jack Pine Site. At the SSA-OJP site, suitable access to a clear area at the flux tower site to make irradiance measurements was not available. Therefore, a similar stand adjacent to the site access road near a small lake (Figure 2) was selected. Irradiance measurements were taken at the lake edge using a 180-degree view cap. Four transects were established; however, the restricted size of the stand required a reorientation of these transects at some point along the 100-m length. Data were acquired on 16-Aug-1993 at 05:20 (12:20 GMT) and 06:30 (13:30 GMT). Figure 2. Transect locations at the SSA Old Jack Pine Site. Similarly, in the SSA-OBS, reasonable access to the flux tower site in 1993 was unavailable and there was no access above the canopy. Transects were therefore established nearer the road, and the road served as the irradiance site with a 315-degree view cap (Figure 3). A transect was established in each of the two main strata types, tall, high LAI black spruce, and short, low LAI black spruce. Data were acquired every 5 meters to give a sample size of 20 for each transect. Data were recorded on 25-Aug-1993 at 13:30 (20:30 GMT). Figure 3. Transect locations for the SSA Old Black Spruce Site. LAI was calculated using options available in the LAI-2000 processing software. Four different methods were used: standard, slope/intercept (Lang, 1987), ellipsoidal leaf distribution (Campbell, 1986), and constrained least squares (Norman and Campbell, 1989). ii) LAI using the Ceptometer At the same sites as in i), measurements were made several times per day. Each measurement on a transect was the average of eight azimuthal measurements of PAR flux. Total and diffuse incident flux were measured before and after each transect using a shade to block the sun. Similarly, at the first and last transect point, diffuse within-canopy flux and PAR reflected by the understory were measured. LAI was determined empirically using the method described by Norman (1988) with the beam fraction determined by interpolation for each transect point (see Section 9). iii) FPAR The measurements described above for the Ceptometer were also used to calculate the variation in FPAR for each transect using measurements of canopy transmission and interpolated values of incident flux and soil reflected flux. The canopy reflected flux was calculated using an average of eight measurements of a short roadside tree. FPAR was calculated for three solar zenith angles and for two transects at each site. 1994 Procedure: i) LAI with the LAI-2000 The data acquisition in 1994 concentrated on the jack pine tower and auxiliary sites in the SSA and covered the same transects/plots as those laid out by RSS- 07 and Terrestrial Ecology (TE)-23 for allometric measurements. For each auxiliary site, this comprised two perpendicular 50-m transects oriented in due S-N and E-W directions. Deviation from such an arrangement is explained in Section 6. A total of nine jack pine sites were sampled with the LAI-2000. In all cases, the data were acquired either before sunrise or after sunset and with a large view cap (315 or 330 degrees) because the only irradiance site in most cases was the access road. Tree edge effects were reduced by measuring irradiance from the top of a Jeep. ii) LAI with the Ceptometer Ceptometer data were acquired at the same sites as the LAI-2000 using the method described above for 1994 site locations. Ten sites were covered, with runs recorded at three solar zenith angles for seven of the sites and two solar zenith angles for the other three sites. In addition, a series of comparative analyses was conducted with Jing Chen (RSS-07) at the following sites: SSA-Young Aspen (YA) (D6H4T), Black Spruce (D0H6S) and Mixed (D9I1M) and with Don Deering (RSS-01) at the SSA-OJP. iii) Fraction of Absorbed Photosynthetically Active Radiation (FPAR) Measurements of FPAR for each of the sites above were also made using the method described in 1993. At sites with significant understory vegetation, additional measurements were made above and below the understory (D6H4T, D0H6S). iv) Canopy Chemistry Samples for determination of nitrogen, chlorophyll, water, etc., were acquired for the nine sites covered by the LAI-2000 using clippers for the short sites and with a shotgun for the sites with tall canopies. Branchlets were subdivided into current year and 1 year or greater, and the needles were then stripped off and frozen with CO2 (see related data sets Section 1.6). 6. Observations 6.1 Data Notes None given. 6.2 Field Notes 27-Jul-1994 Site D9I1M Run 1: Transect of Pink Flags E/W from Tower - TE-06? Run 2: Transect of Red Stakes N/S from Tower - RSS-07. 27-Jul-1994 Site D4H6T Aspen: Understory data acquired with Ceptometer. 27-Jul-1994 Site D0H6S Black Spruce: Cirrus Cloud. Problem finding site. Used RSS-07 Transect. 28-Jul-1994 Site G9L0P: Cirrus Cloud. 28-Jul-1994 Site F7J1P: Cumulus Cloud. 28-Jul-1994 Comparison of Ceptometer with TRAC. 30-Jul-1994 Site G8L6P: No Markers present - Markers inserted N/E/S/W. 2-Aug-1994 Site F5I6P: Site not found so established at 150 m in from Route 913 on a bearing of 59 degrees. 2-Aug-1994 Site F7J1P: 60% Jack Pine, 20% Spruce, 20% Aspen. 2-Aug-1994 Site F7J0P: 34% Jack Pine, 33% Spruce, 33% Aspen. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage Data were collected at treed SSA tower sites and SSA jack pine auxiliary sites. The following table lists the North Amercan Datum 1983 (NAD83) coordinates of the site locations that were sampled : West North UTM UTM UTM Name Notes Longitude Latitude Easting Northing Zone ----- ----------- ---------- -------- -------- --------- ---- F8L6T SSA YJP 104.64527 53.87581 523350.7 5969540.0 13 G2L3T SSA OJP 104.69203 53.91634 520257.0 5974035.0 13 G8I4T SSA OBS 105.11779 53.98718 492306.1 5981879.0 13 F5I6P JIH-4 Pine 105.11174 53.86608 492681.9 5968405.0 13 F7J0P JMH-5 Pine 105.05116 53.88334 496666.7 5970320.0 13 F7J1P JMH-A2 Pine 105.03226 53.88211 497909.2 5970183.0 13 G1K9P JMM-6 Pine 104.74810 53.90881 516576.8 5973183.0 13 G4K8P JMM-5 Pine 104.76399 53.91884 515529.6 5974295.0 13 G7K8P JMM-8A Pine 104.77147 53.95882 515023.9 5978742.0 13 G8L6P JDM-8 Pine 104.63755 53.96558 523807.6 5979530.0 13 G9L0P JMH-10 Pine 104.73778 53.97576 517227.6 5980634.0 13 D0H6S BMM-1 Spruce 105.29534 53.64878 480506.0 5944267.0 13 D0H4T SSA YA 105.32313 53.65602 478675.2 5945077.0 13 D9I1M AIH-3 Mixed 105.20643 53.72540 486410.2 5952768.0 13 7.1.2 Spatial Coverage Map Not available. 7.1.3 Spatial Resolution The measurements provided by the LAI-2000 should be regarded as representative of the sample site, while those by the Ceptometer (LAI and FPAR) are provided as individual samples for each sample point. It is recommended that these also be used at the sample site level rather than individually since the variation from point to point can be large because of clumping; this is particularly important in the case of FPAR. Where there was significant understory, the Ceptometer data are provided at two levels, above and below the understory layer. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage FFC-93: 07-Aug to 30-Aug-1993 IFC-94: 25-Jul to 05-Aug-1994 7.2.2 Temporal Coverage Map 1993 LAI using the LAI-2000 SSA-YJP - Data were collected on 14-Aug at 08:00 local time (15:00 GMT). SSA-OJP - Data were acquired on 16-Aug at 05:20 (12:20 GMT) and 06:30 (13:30 GMT) SSA-OBS - Data were recorded on 25-Aug at 13:30 (20:30 GMT). 1993 LAI using the Ceptometer SSA-YJP - Data were collected several times a day on 14th August SSA-OJP - Data were acquired several times a day on 16th August SSA-OBS - Data were recorded several times a day on 25th August 1994 LAI using the LAI-2000 F5I6P - 04AUG94 F7J0P - 03AUG94 F7J0P - 31JUL94 F7J1P - 03AUG94 F7J1P - 31JUL94 G1K9P - 31JUL94 G2L3T - 05AUG94 G4K8P - 31JUL94 G7K8P - 31JUL94 G8L6P - 05AUG94 G9L0P - 03AUG94 G9L0P - 31JUL94 1994 LAI using the Ceptometer D0H6S - 28JUL94 D6H4A - 27JUL94 D9I1M - 27JUL94 F5I6P - 03AUG94 F5I6P - 04AUG94 F7J0P - 04AUG94 F7J0P - 28JUL94 F7J1P - 03AUG94 F7J1P - 28JUL94 F8L6T - 02AUG94 F8L6T - 30JUL94 G1K9P - 26JUL94 G2L3T - 30JUL94 G4K8P - 26JUL94 G4K8P - 27JUL94 G7K8P - 03AUG94 G7K8P - 28JUL94 G8L6P - 30JUL94 G9L0P - 26JUL94 G9L0P - 27JUL94 G9L0P - 28JUL94 7.2.3 Temporal Resolution Measurements were made during FFC-93 and IFC-2 in 1994. LAI changes relatively slowly for conifer species; hence, the values can be considered to be representative of the field campaign specified. FPAR varies continuously, for a given location, as a function of solar geometry because penetration is dependent on the interception of incident PAR by individual crowns clumped in a nonrandom fashion. Three sets of measurements for each sample site are given along with time and date to provide an indication of FPAR variability. Individual runs of measurements took approximately 10 minutes from first point to last plus the time taken to gain access to the site from the nearest clear sky location (varied from 1-10 minutes with most sites less than 5 minutes). 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (r04laifd.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (r04laifd.def). 8. Data Organization 8.1 Data Granularity All of the BOREAS RSS-04 1994 Southern Study Area Jack Pine LAI and FPAR Data are contained in one dataset. 8.2 Data Format(s) The data files contain a series of numerical and character fields of varying length separated by commas. The character fields are enclosed within single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition file (r04laifd.def). 9. Data Manipulations 9.1 Formulae LAI-2000 method: In calculation of LAI with the LAI-2000, the foliage density is derived by measurement of the contact frequency over the azimuthal averaged zenith bin represented by each viewing ring. Foliage density is calculated as: µ = 2?-ln(T(?))sin?/S(?) d? where T(?) is the probability of noninterception, ? is zenith angle, and S(?) is the path length. If there is a homogeneous cover, foliage density µ is related to LAI by canopy height and path length S is dependent on canopy height and zenith angle: LAI = µz and S(?) = z/cos(?) Thus, LAI can be calculated by summation of the azimuthal averaged transmission over the five zenith bins: LAI = 2? -ln(T(?))sin?cos? For boreal forest canopies where the cover is not homogeneous (i.e., the foliage distribution is not random, and a proportion of the intercepting elements are not foliage), the measurement is of effective LAI (see Chen et al., 1997). To derive LAI, it is necessary to adjust the effective LAI for the woody-to-total area ratio and the foliage clumping both within conifer shoots and at large scales. A discussion of the derivation of such correction factors is given in Chen et al. (1997), with values provided for the tower flux sites. Ceptometer Method: The derivation of LAI from Ceptometer measurements uses the formula derived by Norman (1988) to convert from PAR transmission to LAI. This approach was derived from simulation results from a radiative transfer model under assumptions of a spherical leaf angle distribution and random leaf positioning. It does not involve gap fraction analysis and can be used with any beam fraction from clear sky to overcast. Although it does not require a series of solar angles, averaging across a range of solar zenith angles is recommended. The empirical fit of LAI to PAR transmission is: LAI = [{fb(1-cos??s?????ln(Ei/Ea)]/(0.72-0.337fb) fb = (Ea- Ead)/Ea where fb is the beam fraction, EI is incident PAR under the canopy, Ea is incident PAR above the canopy, Ead is the diffuse incident PAR, and ?s is solar zenith angle. Descriptions of these and other methods of LAI estimation can be found in the cited literature (Section 17.2) and a review by Welles (1990). FPAR is calculated using the following formula: FPAR = (PAR0 - PARr - (PARt - PARs ))/PAR0 The subscripts 0, r, t, and s refer to the incoming PAR, PAR reflected by the canopy, PAR transmitted through the canopy, and PAR reflected by the substrate, respectively. The measurements in all cases are the average of eight azimuthal measurements to eliminate azimuthal bias. 9.1.1 Derivation Techniques and Algorithms None given. 9.2 Data Processing Sequence 9.2.1 Processing Steps See Section 9.1. 9.2.2 Processing Changes None. 9.3 Calculations i) LAI from LAI-2000 LAI was calculated using the LAI-2000 operating in one-sensor mode with one run constituting 10 B measurements separated by A measurements at the start and end of each run. Linear interpolation between the A values was used to calculate A values for each B measurement. B values were rejected if the B:A ratio was greater than 1 in any of the five rings. ii) LAI from Ceptometer Determined using the method of Norman (1988), a typical Ceptometer run comprised: Total incoming Diffuse incoming Canopy Measurement 1 Substrate at Measurement 1 Diffuse at Measurement 1 Measurement 2 Measurement 3.... Measurement 10 Total incoming Diffuse incoming Linear interpolation based on time (decimal) for Total and Diffuse incoming was used to calculate incoming for each measurement. Solar zenith angle was calculated based on the time and site latitude/longitude obtained from the BOREAS Auxiliary Site Guide. iii) FPAR from Ceptometer FPAR calculations used the same data set and methods as ii) to determine the incoming. The canopy and substrate components were as a percentage of incoming at each sample point. 9.3.1 Special Corrections/Adjustments None given. 9.3.2 Calculated Variables LAI, FPAR. 9.4 Graphs and Plots None. 10. Errors 10.1 Sources of Error None given. 10.2 Quality Assessment 10.2.1 Data Validation by Source Checks were made prior to storage to make sure anomalous values were not stored. In the case of LAI-2000, measurements were acquired only before sunrise and after sunset and in the absence of significant amounts of cloud, or where the cloud was fast moving. Measurements were redone if the number of invalid data points (indicated by an instrument beep) was higher than 2 on a standard run. These data sets were discarded. Similarly, where a single Ceptometer measurement contributing to the average was unexpectedly high or low, for example in the case of diffuse measurements, where the shadowing device was not completely over the instrument, the average was recalculated using an additional eight samples. 10.2.2 Confidence Level/Accuracy Judgment It is difficult to draw conclusions on the subjective quality of the data; however, it is clear that such estimates are only as good as the assumptions made in derivation. The Chen et al. (1997) paper discusses these assumptions for LAI and compares a range of instrument measurements. The cause and effects of error on the representativeness of measurements varies with instrument and tree species, and particularly with clumping at a range of scales. Nevertheless, the estimates are consistent with alternative approaches. 10.2.3 Measurement Error for Parameters Error in LAI from LAI-2000 and Ceptometer from Chen et al. (1997) varies with tree species, but is in the range of 15-30%. This error is comparable but not necessarily the same as for allometry. Error (n=10) in FPAR varies with the degree of clumping in a canopy but is typically less than 10% for FPAR of greater than 0.7. The error increases and may reach 30% where the canopy is highly variable. There is no consistent pattern in error variation with solar geometry. 10.2.4 Additional Quality Assessments None given. 10.2.5 Data Verification by Data Center BORIS Staff reviewed the data during the data base loading process for clarity and consistency. 11. Notes 11.1 Limitations of the Data None given. 11.2 Known Problems with the Data i) LAI from LAI-2000 The data set is exclusively of an incomplete conifer canopy. The assumptions in translating from transmission to LAI may not strictly apply. In particular, the assumption of random arrangement of needles in space is not true, which leads to underestimation of LAI. An empirical correction to convert from shoot projected area to needle projected area has been suggested to adjust for this effect (Gower and Norman, 1990). These empirical corrections, however, have not been applied to the data. Representative correction values can be found in Chen et al. (1997), along with a discussion of their use. The irradiance measurement was constrained by access to an open area. In most cases, this was the road. Edge effects were reduced by limiting the view azimuth and standing on a raised platform; however, it is anticipated that there may still be some influence caused by the reduction in representativeness of the azimuth average. ii) LAI from Ceptometer The azimuth averages of incoming PAR could potentially be biased by the presence of tree trunks or gaps near the sample point. Variations in averages were noted by repeated sampling near the central grid point. Absolute values of PAR are not given because cross comparison with the TRAC instrument (Chen et al. 1997)(RSS- 7) revealed a consistent discrepancy of approximately 230 counts. iii) FPAR from Ceptometer All values were calculated as an average of eight azimuth measurements. In the case of canopy and substrate measurements, low incoming values resulted in null returns as an average. In these cases, the percentage reflectance was derived from repeat measurements of the site. The canopy measurement was not derived under ideal conditions because it was not possible to obtain PAR reflectance from the real canopy. Measurements of short trees at the roadside were substituted. The average values were not azimuth independent because of problems with the bias introduced from direct irradiance incident on the Ceptometer in the antisolar direction; therefore,.canopy values may not be truly representative. The average values were computed and stored by the instrument, so no record of the eight values that generated the average is available. 11.3 Usage Guidance None given. 11.4 Other Relevant Information None. 12. Application of the Data Set This data set can be used for model parameterization and to test empirical relationships hypothesized between biophysical parameters and remotely sensed data. 13. Future Modifications and Plans None. 14. Software 14.1 Software Description None given. 14.2 Software Access None given. 15. Data Access 15.1 Contact Information Ms. Beth Nelson BOREAS Data Manager NASA GSFC Greenbelt, MD (301) 286-4005 (301) 286-0239 (fax) Elizabeth.Nelson@.gsfc.nasa.gov 15.2 Data Center Identification See Section 15.1. 15.3 Procedures for Obtaining Data Users may place requests by telephone, electronic mail, or fax. 15.4 Data Center Status/Plans The RSS-04 LAI and FPAR data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The BOREAS contact at ORNL is: ORNL DAAC User Services Oak Ridge National Laboratory Oak Ridge, TN (423) 241-3952 ornldaac@ornl.gov ornl@eos.nasa.gov 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None. 16.3 Other Products The data are available as tabular American Standard Code for Information Interchange (ASCII) files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation None. 17.2 Journal Articles and Study Reports Campbell, G.S. 1986. Extinction coefficients for radiation in plant canopies calculated using an ellipsoidal inclination angle distribution. Agric For Meteorol 36, 317-321. Chen, J.M., P.M. Rich, S.T. Gower, J.M. Norman, and S.E. Plummer. 1997. Leaf area index of boreal forests: Theory, techniques and measurements. JGR, 102 (D24), 29,429-29,443. Gower, S.T. and J.M. Norman. 1990. Rapid estimation of leaf area index in conifer and broad-leaf plantations. Ecology, 72, 1896-1900 Lang, A.R.G. 1987. Simplified estimate of leaf area index from transmittance of the sun's beam. Agric. For. Meteorology, 41, 179-186. Norman, J.M. 1988. Crop canopy photosynthesis and conductance from leaf measurements. Workshop prepared for LI-COR, Inc., Lincoln, NE. Norman, J. M. and G.S. Campbell. 1989. Canopy structure. In: Plant Physiological Ecology: Field methods and instrumentation. (eds. R. W. Pearcy, J. Ehleringer, H. A. Mooney, and P. W. Rundel). Chapman and Hall, London and New York. 301-325. North, P.R. and S.E. Plummer. 1994. Estimation of conifer bi-directional reflectance using a Monte Carlo method. IGARSS'94, IEEE, Piscataway, NJ, Vol. I, 114-116. North, P.R. 1995. A three-dimensional forest light interaction model using a Monte-Carlo method. IEEE Trans. Geosci. and Rem. Sens., 34, 946-956. Plummer, S.E. and N. Lucas. 1993. Report of the BOREAS Intensive Field Campaign 1993. Remote Sensing Applications Development Unit, Report No. 93/5. Sellers, P. and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan.Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). Sellers, P.and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1996-2.0, NASA BOREAS Report(EXPLAN 96). Sellers, P., F. Hall, and K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94). Sellers, P., F. Hall, and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPS DOC 96). Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The boreal ecosystem-atmosphere study (BOREAS): an overview and early results from the 1994 field year. Bulletin of the American Meteorological Society. 76(9):1549-1577. Sellers, P.J., F.G. Hall, R.D. Kelly, A. Black, D. Baldocchi, J. Berry, M. Ryan, K.J. Ranson, P.M. Crill, D.P. Lettenmaier, H. Margolis, J. Cihlar, J. Newcomer, D. Fitzjarrald, P.G. Jarvis, S.T. Gower, D. Halliwell, D. Williams, B. Goodison, D.E. Wickland, and F.E. Guertin. (1997). "BOREAS in 1997: Experiment Overview, Scientific Results and Future Directions", Journal of Geophysical Research (JGR), BOREAS Special Issue, 102(D24), Dec. 1997, pp. 28731-28770. Walker, G.K., R.E. Blackshaw, and J. Dekker. 1988. Leaf area and competition for light between plant species using direct transmission. Weed Technology, 2, 159- 165. Welles, J.M. 1990. Some indirect methods of estimating canopy structure. In: Instrumentation for Studying Vegetation Canopies for Remote Sensing in Optical and Thermal Infrared Regions. Remote Sensing Reviews 5(1) (eds. N. S. Goel and J. M. Norman). Harwood Academic Publishers, London and New York. Welles, J.M. and J.M. Norman. 1991. Instrument for indirect measurement of canopy architecture. Agronomy Journal, 83: 818-825. 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms None. 19. List of Acronyms APAR - Absorbed Photosynthetically Active Radiation ASCII - American Standard Code for Information Interchange AVIRIS - Airborne Visible and Infrared Imaging Spectrometer BNSC - British National Space Centre BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System DAAC - Distributed Active Archive Center EOS - Earth Observing System EOSDIS - EOS Data and Information System FFC - Focused Field Campaign FPAR - Fraction of absorbed Photosynthetically Active Radiation GMT - Greenwich Mean Time GSFC - Goddard Space Flight Center IFC - Intensive Field Campaign IPAR - Incident PAR LAI - Leaf Area Index NAD83 - North American Datum of 1983 NASA - National Aeronautics and Space Administration NSA - Northern Study Area OBS - Old Black Spruce OJP - Old Jack Pine ORNL - Oak Ridge National Laboratory PANP - Prince Albert National Park PAR - Photosynthetically Active Radiation PI - Principal Investigator RSADU - Remote Sensing Applications Development Unit RSS - Remote Sensing Science SSA - Southern Study Area TE - Terrestrial Ecology TRAC - Tracing Radiation and Architectre of Canopies URL - Uniform Resource Locator UTM - Universal Transverse Mercator YA - Young Aspen YJP - Young Jack Pine 20. Document Information 20.1 Document Revision Dates Written: 02-Aug-1995 Last Updated: 05-Aug-1998 20.2 Document Review Dates BORIS Review: 00-Aug-1998 Science Review: 03-Aug-1998 20.3 Document ID 20.4 Citation LAI-2000 and Ceptometer data were gathered by Dr. Stephen Plummer (British National Space Centre/Remote Sensing Applications Development Unit) and Mr. Neil Lucas (University College of Swansea). 20.5 Document Curator 20.6 Document URL Keywords: LAI FPAR Ceptometer Jack Pine RSS04_LAI_FPAR.doc 08/20/98