BOREAS RSS-07 LAI, Gap Fraction, and fPAR Data Summary The BOREAS RSS-07 team collected various data sets to develop and validate an algorithm to allow the retrieval of the spatial distribution of LAI from remotely sensed images. Ground measurements of LAI and FPAR absorbed by the plant canopy were made using the LAI-2000 and TRAC optical instruments during focused periods from 09-AUG-1993 to 19-SEP-1994. The measurements were intensive at the NSA and SSA tower sites, but were made just once or twice at auxiliary sites. The final processed LAI and FPAR data set is contained 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-07 LAI, Gap Fraction, and fPAR Data 1.2 Data Set Introduction Ground measurements of Leaf Area Index (LAI) and the Fraction of Photosynthetically Active Radiation (FPAR) absorbed by the plant canopy were made in the BOReal Ecosystem-Atmosphere Study (BOREAS) Northern Study Area (NSA) and Southern Study Area (SSA) using optical instruments. The instruments used were the LI-COR LAI-2000 and the Tracing Radiation and Architecture of Canopies (TRAC), which was developed at the Canadian Centre for Remote Sensing (CCRS). 1.3 Objective/Purpose The objectives of this project were to: 1. Obtain LAI and FPAR for the tower sites in both the BOREAS SSA and NSA. 2. Describe the spatial variability of LAI and FPAR for the sites. 3. Compare methods for indirect measurements of LAI and FPAR. The methods included the LAI-2000 plant canopy analyzer and a sunfleck-LAI instrument, called the TRAC, recently developed at the CCRS. 4. Develop algorithms for retrieving LAI and FPAR from Landsat Thematic Mapper (TM) and Advanced Very High Resolution Radiometer (AVHRR) data. 5. Investigate the effect of plant canopy architecture on indirect measurements of LAI and FPAR. 6. Scale up LAI and FPAR measurements from submeter to km scales National Oceanic and Atmospheric Administration (NOAA) AVHRR pixel scale). 1.4 Summary of Parameters The following parameters were used: 1. LAI-2000: effective LAI 2. TRAC: indirect LAI, foliage clumping index, FPAR, PAR albedo of forest floor 3. Destructive sampling: ratio of needle area to shoot area 1.5 Discussion Measurements of LAI were made using two or three units of the LI-COR LAI-2000 plant canopy analyzer. Data were acquired along three 170-m - 300-m parallel transects separated by 10 m at the tower sites and along 50-m transects at the auxiliary sites. On the same transects, the TRAC was also used to measure LAI, the clumping index, and canopy architectural parameters. The clumping effect within shoots was determined from laboratory analysis on 27- 45 shoot samples for each conifer tower site using a video camera-computer system. 1.6 Related Data Sets BOREAS RSS-04 1994 Southern Study Area Jack Pine LAI and fPAR Data BOREAS TE-23 Hemispherical Photography for Calculating Radiative Transport, LAI BOREAS TE-06 1994 Tower and Carbon Evaluation Site Allometry and Biomass Data 2. Investigator(s) 2.1 Investigator(s) Name and Title Jing M. Chen, Ph.D. Research Scientist Josef Cihlar, Ph.D. Senior Research Scientist Margaret Penner, Ph.D. Research Scientist 2.2 Title of Investigation Retrieval of Boreal Forest Leaf Area Index From Multiple Scale Remotely Sensed Vegetation Indices 2.3 Contact Information Contact 1 --------- Jing M. Chen Ottawa, Ontario (613) 947-1266 Chen@ccrs.emr.ca Contact 2 --------- Josef Cihlar Ottawa, Ontario (613) 947-1265 Josef.Cihlar@geocan.emr.ca Contact 3 --------- Jaime Nickeson Raytheon STX Corporation NASA/GSFC Greenelt, MD (301) 286-3373 Jaime.Nickeson@gsfc.nasa.gov 3. Theory of Measurements Parameter definitions for background: 1) LAI LAI is defined as one half the total green leaf area per unit ground surface area. It has been demonstrated independently by Lang (1991) and Chen and Black (1992a) that this definition is correct for non-flat leaves (including conifer needles) of convex shapes, and it is incorrect to define LAI on the basis of the largest projected area as commonly done before. It must be pointed out that this definition is given based only on radiation interception by leaves. Plant physiologists concerned with gas exchange and stomatal density and distribution may prefer staying with the total leaf area (in case of concave leaves) or the projected area. It is therefore necessary to know the shape factors that convert between these areas. 2) PAI PAI is defined as one half the total area of all above-ground plant materials per unit ground surface area. The plant materials include leaves, branches (live or dead), boles, and attachments to plant parts such as lichen and moss. Without the knowledge of the contribution from each of the components, all optical instruments can measure only PAI rather than LAI. 3) Effective LAI (Le) The term "effective LAI" was used by Chen and Black (1992b) in their earlier papers for the need to provide a measure for the effect of non-randomness of foliage spatial distribution on indirect measurements of LAI. For conifer forest stands, the effective LAI is considerably smaller than LAI (usually 50%) because leaves are grouped together in tree crowns, branches, shoots, and so on. The grouping or clumping of foliage reduces the light interception and hence effectively reduces LAI derived indirectly from the measurements of canopy gap fraction using equipment such as the LI-COR LAI-2000, hemispherical photography, tram, or moving light sensors. The definition of the effective LAI is: Effective LAI = Foliage Clumping Index * LAI (10) The foliage clumping index is smaller than unity for conifer and deciduous stands (usually 0.5). The LI-COR LAI-2000 measures canopy gap fraction and derives LAI from the gap fraction under the assumption that the foliage elements are randomly distributed in space. Therefore, it measures only the effective PAI (or loosely effective LAI) rather than LAI when the foliage is not random. It is therefore meaningful to report the popular measurements as effective PAI or LI-COR LAI. Although the effective LAI measured by the LAI-2000 is not LAI, the measurements are still very valuable for estimating the gap fraction and light interception (FPAR and Absorbed Photosynthetically Active Radiation [APAR]) by the canopy, and therefore they should be reported as raw data. 4) Direct LAI and PAI Direct LAI and PAI are measured directly through destructive sampling. In the measurements, relationships may be used between leaf area and leaf fresh or dry weight, between total leaf area in a branch and the branch diameter or branch weight, and between total leaf area in a tree and the tree diameter at breast height (dbh). In other words, some degree of indirectness may still be involved. 5) Allometric LAI and PAI These are obtained using pre-established relationships between leaf area and dbh or sapwood area. Sometimes an additional parameter such as crown width, crown length, or an index related to the tree density is used. 6) Indirect LAI and PAI Indirect LAI and PAI are measured indirectly using optical instruments or by other means. The effective LAI (or more precisely the effective PAI) measured using the LAI-2000 may be corrected to obtain the indirect LAI or PAI. Much research has been done on making such corrections for conifer stands. The corrections include: i) Needle-to-shoot area ratio (gamma): Rationale: Conifer needles are tightly grouped together in shoots (an important scale of foliage clumping), and shoots can be treated as the basic foliage units responsible for light interception. This ratio quantifies how much leaf area there is in an average shoot area (if a shoot projection can be approximated by a sphere, it is half the sphere surface area), which is believed to be the quantity measured by the LAI- 2000. Gower and Norman (1991) successfully applied this type of correction to conifer stands of several species. Further investigation has been made by Fassnacht, et al. (1994). The only underlying assumption for making this correction is that shoots are randomly distributed in space. ii) Shoot clumping index (omega) Rationale: To consider the effect of clumping at scales larger than shoots. For open conifer stands, shoots are obviously grouped into tree crowns; i.e., the assumption of random spatial distribution of shoots is not met. In this case, an additional parameter concerning foliage clumping at scales larger than the shoot scale becomes very important (Chen and Black 1992a). Shoot clumping index accounts for the effect of foliage clumping at scales larger than shoots. It can be obtained from analysis of canopy gap size information (Chen and Black, 1992b). The TRAC, developed by RSS-07 at CCRS, is designed to measure sunfleck sizes along straight transects on the forest floor, from which a distribution of canopy gap size is obtained. Measurements using the TRAC show that the shoot clumping index is about 0.70-0.80 for most conifer stands. iii) Woody-to-total area ratio (alpha): Rationale: Woody and other nonfoliage materials can have an important contribution to light interception in the canopy, and optical instruments can measure only PAI. Alpha is the percentage of total green foliage area to the total area of above-ground materials. For closed stands, it is generally larger than 0.9, but for the open boreal stands, the value would be considerably smaller (0.7-0.9). In conclusion, the following formulae are proposed for calculating the indirect PAI and LAI: PAI = Effective LAI * gamma/omega (11) LAI = (1-alpha)* PAI i.e., LAI = (1-alpha)*effective LAI*gamma/omega (12) Gamma is about 1.3 to 1.5 (Gower and Norman, 1991; Chen and Black, 1992b; Fassnacht, et al., 1994; Deblonde, et al., 1994; and Chen 1996a). It is obtained by directly measuring the total leaf area in a detached shoot and the total shoot area (4 times the average projected shoot area) using a computerized video camera system. From equations 10 and 12, it can be shown that: Foliage clumping index = Omega/gamma (13) 7) FPAR Green FPAR is the fraction of incident PAR that is absorbed by the green leaves in the canopy. It excludes the fraction reflected back to the space and the fraction absorbed by the background (moss, soil, and understory in forest), but it includes the small fraction that is reflected by the background and absorbed by the green leaves on the way back to space. The daily green FPAR is the final result reported here after applying a weighting scheme to the instantaneous green FPAR values (Chen, 1996b). 8) PAR Albedo of Forest Floor This value is calculated as the ratio of the mean values, over a transect, of the transmitted total irradiance and reflected total PAR irradiance from the forest floor using the upward-facing and downward-facing hemispheric PAR sensors of the TRAC. LI-COR LAI-2000: There is a certain probability that a beam of radiation passing through some distance of a vegetative canopy will be intercepted by foliage. The probability of interception is proportional to the path length, foliage density, and foliage orientation. Therefore, if the beam transmittance is known, then it is possible to invert foliage information (Welles and Norman, 1991). Noninterception (i.e.) transmittance [T]) is described by Beer's law: T(zen,azi) = exp (-G(zen,azi) * u * S(zen,azi)) (1) where G(zen,azi) = fraction of foliage projected toward direction (zen,azi) u = foliage density (m2 foliage/ m3 canopy) S(zen,azi) = path length through the canopy (m) zen = zenith angle azi = azimuth angle Because the LAI-2000's optical sensor averages over azimuth, the azimuth angle (azi) is omitted from the following equations with the understanding that the various quantities are azimuthal averages. Rewriting equation 1 yields G(zen) * u = - (ln(T(zen)) / (S(zen)) = K(zen) (2) K(zen) is the contact frequency, and is equivalent to the average number of contacts per unit length of travel that a beam would make passing through a canopy at zenith angle (zen). The foliage density u is defined as p/2 u = 2 ? K(zen) sin(zen) ?zen (3) 0 Foliage density is related to LAI by canopy height (z): LAI = u * z (4) Path length S is related to canopy height (z) by zenith angle (zen): S(zen) = z / cos(zen) (5) Using equations 2, 3, 4, and 5, LAI can be defined in terms of canopy transmittance: p/2 LAI = 2 ? -ln[T(zen)] cos(zen) sin(zen) ?zen (6) 0 Because canopy height cancels out of equation 6, it is numerically identical to equation 3 when S(zen)=1/cos(zen). Thus, equation 3 can be used for either LAI or foliage density: if the distances are 1/cos(zen), then the results should be interpreted as LAI; otherwise, foliage density is computed. Once LAI is estimated from transmittance measurements, five values of G(zen) are determined using equation 2. A straight line is fit to the data, and the slope of that line (?G(zen)/?zen) is used in the following equation to predict mean tilt angle: MTA = 56.82 + 46.85(x) - 64.62(x2) - 158.69(x3) + 522.06(x4) + 1008.15(x5) (7) where x = ?G(zen)/?zen However, it must be realized that the above inversion of LAI is based on the assumption that the foliage is randomly distributed in space. This assumption is not met for forest stands, where leaves are grouped into shoots, branches, and tree crowns. Because foliage of forest stands is generally more clumped than random, the LAI-2000 measures the effective LAI (Chen and Black, 1992b) rather than LAI. TRAC (Tracing Radiation and Architecture of Canopies): The LAI-2000 measures the canopy gap fraction from which to derive LAI. The canopy gap fraction is the percentage of sky seen from underneath the canopy and carries no information on the actual gap size. The TRAC is designed to obtain the canopy gap size information. The instrument measures the transmitted direct light through the canopy at a high spatial density (1 sample/10 mm) on straight transects near the forest floor. The sunfleck size on the forest floor is thus obtained. From the measured sunfleck size, the corresponding canopy gap size can be calculated after considering the penumbral effect (Chen and Cihlar, 1995a). The distribution of the canopy gap size is a description of the canopy architecture and can be used to quantify the effect of nonrandom foliage spatial distribution on the inversion of LAI from gap fractions. The foliage clumping index is a measure of such an effect and serves as a correction to LAI-2000 measurements. Two methods are useful for deriving the foliage clumping index from a canopy gap size distribution. One is developed by Chen and Black (1992b). They derive the foliage (shoot) clumping index based on the assumptions that foliage elements (shoots) are randomly distributed within foliage clumps (tree crowns), and the foliage clumps are randomly distributed in space. In natural stands, these assumptions are generally acceptable, but the method cannot be applied to plantations, where the assumptions are violated. The second method was recently developed by Chen and Cihlar (1995a). They use a gap removal approach to derive the foliage (shoot) clumping index. In their approach, gaps appearing at probabilities larger than those predicted for the same gap size in a random canopy are truncated or partially removed from the total canopy gap fraction. For this project, Chen and Cihlar's method was used for processing the TRAC data. It is believed to be more accurate than Chen and Black's method because it does not require the assumptions made by Chen and Black. 4. Equipment 4.1 Sensor/Instrument Description LI-COR LAI-2000: The LAI-2000 plant canopy analyzer is composed of an LAI-2070 control unit and an LAI-2050 sensor head. The control unit is 21 cm x 11.4 cm x 6.9 cm and has connectors for two sensor heads, two connectors for other LI-COR sensors, and a connector for RS-232 communication. The sensor head projects the image of its nearly hemispheric view onto five detectors arranged in concentric rings (approximately 0-13, 16-28, 32-43, 47-58, 61-74 degrees). Radiation above 490 nm is rejected. Lenses are coated with MgF2 to improve transmission at high oblique angles. For further information, consult the LAI-2000 plant canopy analyzer instruction manual (LI-COR, 1991). TRAC: The TRAC consists of three LI-COR quantum sensors (Model LI-190SB, LI-COR, Lincoln, NE) and a data logger (Campbell Scientific, Logan, UT, Model CR10). Two sensors measure, respectively, the total and diffuse photosynthetic photon flux density (PPFD) transmitted through the forest canopy, and one sensor measures the reflected PPFD from the forest floor. 4.1.1 Collection Environment Most of the LAI-2000 measurements were made near sunset with the solar elevation angle below 15 degrees to minimize the effect of blue light scattering. The measurements were sometimes also made in overcast conditions. The TRAC was used on clear days only. Sometimes it was operated under sparse cloud conditions, but care was taken to ensure no direct cloud effects on the TRAC measurements. 4.1.2 Source/Platform LI-COR LAI-2000: Hand held in a horizontal position at knee height TRAC: Hand-held in a horizontal position at knee height by a person walking on a straight transect on the forest floor. 4.1.3 Source/Platform Mission Objectives The objectives were to measure and analyze LAI and FPAR taken at various BOREAS sites using the LI-COR LAI-2000 and TRAC instruments. 4.1.4 Key Variables From the LI-COR LAI-2000: Effective LAI. From the TRAC: Shoot clumping index. From shoot sample analysis: needle-to-shoot area ratio. From destructive tree sampling: foliage-to-total area ratio 4.1.5 Principles of Operation LI-COR LAI-2000: The amount of foliage in a vegetative canopy can be deduced from measurements of radiation attenuation as it passes through the canopy at several angles from the zenith. Foliage orientation information can also be obtained. The LAI-2000 measures the attenuation of diffuse sky radiation at five zenith angles simultaneously. A 90-degree mask was used all the time to prevent interference caused by the operator's presence. The same mask was used for the reference sensor to reduce the influence of the Sun. For further information, consult the LAI-2000 plant canopy analyzer instruction manual (LI-COR, 1991). TRAC: To obtain the canopy gap size distribution, the direct light transmitted through the canopy is measured at a high spatial density (one sample/10 mm) along straight transects. From the canopy gap size distribution, the shoot clumping index is calculated, which is used as a correction factor for the LAI-2000 measurements, in addition to the correction factor of shoot projection ratio. 4.1.6 Sensor/Instrument Measurement Geometry LI-COR LAI-2000: The LAI-2000 was hand-held in a horizontal position at knee height. The LAI- 2050 (sensor head) has a near-hemispherical field-of-view (FOV). The effective view area is: A = p * H2 (8) where A = area p = 3.1416 H = canopy height The potential view area is larger than the effective because the effective range of view is reduced by foliage. The potential area viewed is: A = p * (3*H)2 (9) TRAC: The TRAC was hand-held in a horizontal position. Two sensors measure, respectively, the total and diffuse PPFD transmitted through the forest canopy, and one sensor measures the reflected PPFD from the forest floor. The view area of the TRAC is hemispherical for diffuse radiation. 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 TRAC: 3rd Wave Engineering P.O. Box 13460 Kanata, Ontario Canada K2K 1X6 Contact: Mr. Mike Kwong (613) 828-2195 (613) 828-9498 (fax) email: mikek@3wce.com 4.2 Calibration 4.2.1 Specifications LAI-2000: The front lens of the LAI-2050 sensor head should be kept clean and dry for comparable readings. Recalibration of the sensor head may never be necessary, as long as the optics within the sensor remain in place. The detectors may have long term electrical drift, but this would not affect LAI determinations. TRAC: The LI-COR quantum sensor measures PPFD in the spectral range from 0.4 to 0.7 micrometers. A filter having the transmittance linearly increasing with wavelength is used for this purpose. The filter has sharp cutoffs at both ends of the spectrum. The sensor has a time constant of 10 microseconds. 4.2.1.1 Tolerance LAI-2000: It is very difficult (if not impossible) to determine the precision and accuracy of LAI measurements. LAI is estimated by the LAI-2000 from all the light not intercepted by any object in the sensor's FOV (thus, foliage area index would be a more appropriate term). Some assumptions must be met for accurate estimates of foliage amount and orientation when using the LAI-2000. The degree to which these assumptions are violated will affect the degree to which the calculations can be trusted. The major assumptions are: 1) The foliage is black. It is assumed that the below-canopy readings do not include any radiation that has been reflected or transmitted by foliage. There is an optical filter in the LAI-2050 (sensor head) that rejects radiation above 490 nm. In this blue portion of the spectrum, foliage typically reflects and transmits relatively little radiation. 2) The foliage is randomly distributed within certain foliage-containing envelopes. These envelopes might be parallel tubes (a row crop), a single ellipsoid (an isolated bush), an infinite box (turf grass), or an infinite box with holes (deciduous forest with gaps). 3) The foliage elements are small compared to the view area of each ring. An approximate guideline is this: the distance from the sensor to the nearest leaf over it should be at least four times the leaf width. 4) The foliage is azimuthally randomly oriented. That is, it does not matter how the foliage is inclined, as long as all the leaves are not facing the same compass direction. 5) The sky brightness is azimuthally uniform. 6) Blockage of the sensor's FOV is the same amount for both incoming and transmitted readings. No canopy conforms exactly to the first four assumptions. Foliage is never random, but is clumped along stems and branches, and is certainly not black. It was felt that in open boreal forests, the effect of light scattering caused by greyness of leaves is small, and that the LAI-2000 measures the canopy gap fraction or the effective LAI accurately. TRAC: The sensor after recalibration is accurate to 98%. The major error arose from the difficulty of maintaining the sensor at a constant horizontal position during measurements. A level is mounted on the holding arm close to the sensor. While walking, the level was watched and the holding arm was adjusted instantly to keep the sensor in a level position. However, because the whole system moves at a walking pace, a slight deviation from the horizontal position is inevitable. The deviation was generally kept within 2 degrees. The TRAC measures the transmittance of the direct light by subtracting the diffuse fraction from the total irradiance in post-measurement data processing. The diffuse fraction was identified as a steady baseline appearing in the total irradiance plot for each 10-m section of the transect. The transmittance can be determined with a 97-98% accuracy. The inversion from the apparent sunfleck size to the corresponding canopy gap size was accurate for large gaps but less accurate for small gaps because of the penumbral effect. A procedure is being developed to minimize this effect (Chen and Cihlar, 1995a). 4.2.2 Frequency of Calibration LAI-2000: Two LAI-2000 units were used, one in the forest stand and the other on the ground in an opening to be used as a reference for the above-the-forest-canopy radiation. These two units were calibrated against each other under overcast conditions prior to the field measurements in each Intensive field Campaign (IFC). The calibration procedures are given in the LAI-2000 Plant Canopy Analyzer Instruction Manual, Chapter 4-1 (LI-COR, 1991). TRAC: The sensing units, the LI-COR quantum sensors, were calibrated by the manufacturer. Before the 1994 experiment, the sensors were recalibrated against a new LI-COR quantum sensor maintained as a laboratory standard. 4.2.3 Other Calibration Information Not applicable. 5. Data Acquisition Methods 1993: In August, LAI-2000 measurements were made in the Old Black Spruce (OBS), Old Jack Pine (OJP), and Young Jack Pine (YJP) tower sites in both the SSA and NSA. In each site, measurements were made on three transects parallel to one another and separated by 10-m. The transects are from 170 m to 350 m long, running the flux tower at 135° (SE) and/or 215° (NE). The measurements were made at 10-m intervals along the transects. Two LAI-2000 units were used, one in the forest stands and the other in a nearby opening as the reference. The reference unit was set in a remote logging mode at a sampling frequency of one sample per 15 seconds. The measurements were made in the evening, shortly before and after sunset. The TRAC was also used in the OJP tower site in the NSA and OBS, OJP, and YJP tower sites in the SSA. The measurements were made along the middle transect among the three parallel transects for each site on clear days. The measurements were repeated several times in a day for the same site to obtain the canopy gap size distributions at different solar zenith angles. 1994: Similar LAI-2000 and TRAC measurements were made in IFC-1, -2, and -3 for the tower flux sites. Transect measurements were also made at the SSA-Old Aspen (OA) in a southwest direction from the tower for 300 m. In IFC-2 and -3, both instruments were used at 12 auxiliary sites on two transects oriented S-N and E- W and orthogonal. The LAI-2000 was used on overcast days or near sunset to minimize the effect of blue light scattering on LAI measurements. The TRAC was used on clear days or on sunny days when the duration of the full sunshine was long enough to permit complete transect measurements. 6. Observations 6.1 Data Notes None. 6.2 Field Notes Field notes recorded detailed weather conditions that might affect LAI-2000 and TRAC measurements. These notes are not useful in using the data sets because the results presented here are final, with environmental effects minimized. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage Tower and auxiliary sites in the NSA and SSA were visited, although more measurements were concentrated at the forested tower sites. The maximum length of the transects is about 300 m, which is long enough to obtain the mean value for the stand as long as the stand is homogeneous at large scales. The variation along the transect characterizes the spatial variability. At the auxiliary sites, two 50-m transects were usually used for the TRAC and LAI-2000 that were perpendicular to each other and crossed in the middle to form a “+” shape. The locations of the transects at each tower site are reported below relative to the micrometeorological flux tower. All coordinates reported in this section are based on the North American Datum 1983 (NAD83). Tower Site Coordinates Transect Site grid ID Latitude Longitude Zone ------------- ------ --------- --------- --------- ---- T-300SE SSA-OA C3B7T 53.62937 106.19819 13 150NW-T-150SE SSA-YJP F8L6T 53.87581 104.64529 13 T-200SE SSA-OJP G2L3T 53.91634 104.69203 13 T-300SE SSA-OBS G8I4T 53.98717 105.11779 13 T-300SE NSA-OBS T3R8T 55.88007 98.48139 14 170NW-T-260SE NSA-OJP T7Q8T 55.92842 98.62396 14 170NW-T-170SE NSA-YJP T8S9T 55.89575 98.28706 14 ------------------------------------------------------------- T-300SE SSA-OA C3B7T 53.62937 106.19819 13 150NW-T-150SE SSA-YJP F8L6T 53.87581 104.64529 13 T-200SE SSA-OJP G2L3T 53.91634 104.69203 13 T-300SE SSA-OBS G8I4T 53.98717 105.11779 13 T-300SE NSA-OBS T3R8T 55.87758 98.47765 14- 170NW-T-260SE NSA-OJP T7Q8T 55.92842 98.62396 14 170NW-T-170SE NSA-YJP T8S9T 55.89575 98.28706 14 Notes: "150NW-T-150SE" means that the measurements were made on transects from 150 m northwest of the flux tower to 150 m southeast of the tower. T-300SE means that the measurements were made on transects from the tower to 300 m southeast of the tower. There were always three transects parallel to one another and separated by 10-m. LAI-2000 measurements were made at 10-m intervals, while TRAC measurements were made continuously (1-cm interval for the transmitted PAR) along the transects. The auxiliary sites listed below were visited: SSA: BORIS West North UTM UTM UTM Grid notes Longitude Latitude Easting Northing Zone ----- ------ ---------- -------- -------- --------- ---- F7J0P JMH-5 105.05115 53.88336 496667 5970323.3 13 F7J1P JMH-A2 105.03226 53.88211 497879.4 5970405.6 13 G2I4S BMH 105.13964 53.93021 490831.4 5975766.3 13 G9I4S BDL-20 105.11805 53.99877 492291.2 5983169.1 13 BMM-1 BMM-1aa 105.28275 53.65383 481312.5 5945044.9 13 BMM-2 BMM-1ab 105.28705 53.65411 481028.4 5945077.2 13 BMM-3 BMM-1ac 105.28807 53.65393 480960.9 5945057.4 13 G1K9P JMM-6 104.74812 53.9088 516546.7 5973404.5 13 Note: BMM-1, BMM-2, and BMM-3 are Remote Sensing Science (RSS-07) sites (the RSS-07 team did not find the BOREAS auxiliary site BMM-1 (D0H6S), and therefore set up its own sites. Darcy Snell later surveyed them with the global Positioning System (GPS) and reported them as BMM-1aa, BMM-1ab, and BMM-1ac. NSA: BORIS West North UTM UTM UTM Grid notes Longitude Latitude Easting Northing Zone ----- --------- --------- -------- -------- --------- ---- T2Q6A TE Carbon 98.67479 55.88691 520342 6193540.7 14 T6R5S BIH-9 98.51865 55.90802 530092 6195947 14 T7R9S BDH-3 98.44877 55.91506 534454.5 6196763.6 14 T8Q9P JIH-2 98.6105 55.93219 524334.5 6198601.4 14 T6T6S BIL-2 98.18658 55.87968 550887.9 6192987.9 14 T8S4S 98.37111 55.91689 539306.4 6197008.6 14 T8T1P JDM-1 98.26269 55.90539 546096.3 6195795.3 14 T9Q8P JIL-1 98.59568 55.93737 525257.1 6199183.2 14 T7T3S BML-21 98.22621 55.89358 548391.8 6194505.6 14 7.1.2 Spatial Coverage Map Not available. 7.1.3 Spatial Resolution Not applicable for LAI-2000. The spatial resolution of LAI measured by the TRAC is 100 mm, which is 10 times the measurement interval of 10 mm. The height of TRAC measurements was lowered to 10-20 cm from the ground in SSA-YJP and NSA- YJP, where trees are short (1-5 m), to include all foliage above ground. The sensor height is less critical in mature stands, where trees are about 10 m and higher. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage Measurements with the LAI-2000 began at about 20 minutes before sunset or under overcast sky conditions. Measurements at a location typically took half a minute and about 30-45 minutes for a whole stand with about 90 measurement locations. Measurements with the TRAC were made under clear sky conditions. In 1994, the LAI measurements were made for the same sites in IFC-1, -2, and -3 to show the seasonal variation. 7.2.2 Temporal Coverage Map Dates and Instruments Used at the Various Sites dd/mm/yy Site LAI-2000 TRAC -------- ----- -------- ---- 15-Aug-93 NOJP x x 16-Aug-93 NYJP x 17-Aug-93 NOBS x 19-Aug-93 SOJP x x 20-Aug-93 SYJP x 21-Aug-93 SYJP x 22-Aug-93 SYJP x 23-Aug-93 SOBS x 24-Aug-93 SOBS x 25-Aug-93 SOBS x 25-Aug-93 SOJP x 27-Aug-93 SOA x x 26-May-94 SOJP x 27-May-94 SOJP x 30-May-94 SOJP x 31-May-94 SOBS x 01-Jun-94 SOA x 02-Jun-94 SOBS x 02-Jun-94 SOA x 03-Jun-94 SYJP x 04-Jun-94 SOBS x 06-Jun-94 SYJP x 07-Jun-94 NOJP x 08-Jun-94 NOA (aux.) x 09-Jun-94 NYJP x 10-Jun-94 NOBS x x 11-Jun-94 NOA x 11-Jun-94 NOJP x 13-Jun-94 NYJP x x 28-Jun-94 W0Y5A x 29-Jun-94 T4U9S x 01-Jul-94 T8T1P x 01-Jul-94 T8S4S x 01-Jul-94 T6T6S x 23-Jul-94 SOBS x 24-Jul-94 SOJP x 26-Jul-94 SYJP x 27-Jul-94 SYJP x 27-Jul-94 Sfen x x 28-Jul-94 SOA x 29-Jul-94 SOBS x 29-Jul-94 SOJP x 31-Jul-94 Nfen x 02-Aug-94 NYJP x x 03-Aug-94 NOA x 03-Aug-94 NOJP x 04-Aug-94 NOBS x x 30-Aug-94 NOJP x 31-Aug-94 NOJP x 31-Aug-94 NYJP x 01-Sep-94 t7t3s,t7r9s x 02-Sep-94 NOBS x x 03-Sep-94 Nfen, t6r5s x 05-Sep-94 NYJP x 06-Sep-94 Nfen x 06-Sep-94 NOJP, t8q8p x 07-Sep-94 NOJP x x 08-Sep-94 NOA x 08-Sep-94 Aux Sites x (t3u9s, t6r8s, t7r9s, t7t3s, t8q9p) 10-Sep-94 SOJP x 10-Sep-94 SYJP x 10-Sep-94 Sfen x 11-Sep-94 SOBS x 12-Sep-94 Sfen x 12-Sep-94 SYJP x 12-Sep-94 SOJP x 13-Sep-94 SOBS x 16-Sep-94 Aux. Sites x (g1k9p, g2l7s) 17-Sep-94 SOA x 18-Sep-94 Aux. Sites x x (g2i4s, g3i4m, g9i4s, f7j0p, f7j1p) In addition to these optical measurements, 27-45 conifer shoot samples were taken from each stand in each IFC 1994 for laboratory analysis of the needle-to- shoot area ratio and other shoot properties. In IFC-3 1994, destructive sampling of LAI was made in NSA-OJP, SSA-OJP, and SSA-OBS. All the above measurements were reported in Chen (1996a). Shoot angle measurements were made in IFC-3 1994 in SSA-YJP, SSA-OBS, and NSA-YJP. These measurements are reported in Chen (1996b). 7.2.3 Temporal Resolution Measurements of the LAI-2000 began at about 20 minutes before sunset or under overcast sky conditions. Measurements at a location typically took half a minute and about 30-45 minutes for a whole stand with about 90 measurement locations. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (r07elaid.def). 7.4 Sample Data Format Sample data format shown in the companion data definition file (r07elaid.def). 8. Data Organization 8.1 Data Granularity The LAI, Gap Fraction, and fPAR Data are contained in one dataset and effective LAI data are contained in a separate 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 (r07elaid.def). 9. Data Manipulations 9.1 Formulae See Section 7.3.2. 9.1.1 Derivation Techniques and Algorithms Not yet available. 9.2 Data Processing Sequence 9.2.1 Processing Steps LAI-2000: In-stand files from one or two LAI-2000 units were merged with an above/outside-stand reference file from another LAI-2000 unit within the program “c2000.com” to calculate the effective LAI. TRAC: The element (leaf for aspen and shoot for conifers) clumping index was calculated for each transect measurement. A canopy gap size distribution was first calculated from the stream of transmitted PAR data, and a gap removal approach (Chen and Cihlar 1995a) was used to calculate the clumping index. TRAC: Measurements from the upward and downward-facing PAR sensors were used to calculate the mean transmitted PAR and the PAR albedo of the forest floor. The above-canopy incident PAR was taken as the PAR measurements outside the stand or in large canopy gaps. The stand PAR albedo was obtained on the top of the flux tower with the TRAC or stationary PAR sensors (Chen, 1996b). 9.2.2 Processing Changes None. 9.3 Calculations 9.3.1 Special Corrections/Adjustments A key correction to the final LAI results is the 15% increase in the mean value of the effective LAI from the LAI-2000, according to the finding of Chen (1996b), who found that the LAI-2000 underestimates the effective LAI by about 15% in comparison with TRAC measurements, which are free from the light- scattering effect. This correction was not made in the raw transect LAI-2000 measurements but was made in the final LAI files. 9.3.2 Calculated Variables See Section 3.0. 9.4 Graphs and Plots None. 10. Errors 10.1 Sources of Error LAI is estimated with the LAI-2000 from all light not intercepted by objects in the sensor's FOV so foliage area index would be a more appropriate term. Some assumptions must be met for accurate estimates of foliage amount and orientation when using the LAI-2000. The degree to which these assumptions are violated will affect the degree to which the calculations can be trusted. The major assumptions are: 1) The foliage is black. It is assumed that the below-canopy readings do not include any radiation that has been reflected or transmitted by foliage. There is an optical filter in the LAI-2050 (sensor head) that rejects radiation above 490 nm. In this blue portion of the spectrum, foliage typically reflects and transmits relatively little radiation. 2) The foliage is randomly distributed within certain foliage-containing envelopes. These envelopes might be parallel tubes (a row crop), a single ellipsoid (an isolated bush), an infinite box (turf grass), or an infinite box with holes (deciduous forest with gaps). 3) The foliage elements are small compared to the view area of each ring. An approximate guideline is this: the distance from the sensor to the nearest leaf over it should be at least four times the leaf width. 4) The foliage is azimuthally randomly oriented. That is, it does not matter how the foliage is inclined, as long as all the leaves are not facing the same compass direction. 5) The sky brightness is azimuthally uniform. 6) Blockage of the sensor's FOV is the same amount for both incoming and transmitted readings. No canopy conforms exactly to the first four assumptions. Foliage is never random, but is clumped along stems and branches, and is certainly not black. Many species exhibit some degree of heliotropism, which violates the azimuthal randomness assumption. However, the practical compromises that must be made are often not serious. Many canopies can be considered to be random, and living foliage does have relatively low transmittance and reflectance below 490 nm. Offsetting errors may be common, such as when leaves are grouped along stems (increasing light transmittance), but arranged to minimize overlap (decreasing transmittance). View restrictors on the LAI-2050 (sensor head) can reduce errors associated with assumptions 5 and 6 but at the potential cost of reduction of sample size. The sensor after recalibration is 98% accurate. The major error arose from the difficulty of maintaining the sensor at a constant horizontal position during measurements. A level is mounted on the holding arm close to the sensor. While walking, the level was watched and the holding arm was adjusted instantly to keep the sensor in a level position. However, because the whole system moves at a walking pace, a slight deviation from the horizontal position is inevitable. The deviation was generally kept within 2 degrees. 10.2 Quality Assessment 10.2.1 Data Validation by Source LAI values were compared with values obtained from different methods and values from other investigators, and the LAI results presented here have been validated with direct LAI measurements through destructive sampling (Chen et al., 1997b). 10.2.2 Confidence Level/Accuracy Judgment LAI-2000 measurements alone considerably underestimate LAI because of the foliage clumping; i.e., the effective LAI is usually about 50% of the true LAI for conifers and 75% of the true LAI for deciduous. Hemispherical photography can provide only the effective LAI. The clumping index obtained from the TRAC and shoot sample analysis provides a critical improvement to bring optical LAI into agreement with destructive sampling results obtained by Terrestrial Ecology (TE-06). 10.2.3 Measurement Error for Parameters The measurement error for the final LAI is assessed to be within 25% of the reported value and daily green FPAR within 5%. 10.2.4 Additional Quality Assessments Not available. 10.2.5 Data Verification by Data Center BOREAS Information System (BORIS) staff has viewed the data and performed some basic checks before loading. 11. Notes 11.1 Limitations of the Data The number of sites included in this investigation is too small for accurate assessment of the mean LAI and FPAR values for boreal forest. Remote sensing methods are used to extend these sites to the BOREAS region (see the document for RSS-07 AVHRR LAI and FPAR data sets). The needle-to-shoot area ratio appears to have little variation between the stands and can be taken as the representative value for boreal forests, while the element clumping index varies greatly and is largely affected by stand density. This clumping index has to be assessed for individual sites using instruments like the TRAC. 11.2 Known Problems with the Data See Section 10.1. 11.3 Usage Guidance There can be considerable variability in LAI values, especially in natural ecosystems. Grazing of trees can enhance the natural variability. Extreme water stress causing leaf rolling could change the effective LAI. 11.4 Other Relevant Information The LAI-2000 is definitely a field-proven instrument. It measures the canopy gap fraction accurately, from which the effective LAI is derived accurately. No artificial influence can be easily introduced into the measurements. It is the state of the art for measuring the canopy gap fraction. The TRAC is a recent development for measuring the canopy gap size distribution from which the effect of nonrandom foliage spatial distribution on LAI-2000 measurements of LAI can be quantified. The error in the foliage (shoot) clumping index derived using the TRAC is within 5%. 12. Application of the Data Set To establish relationships between LAI, FPAR, and remotely sensed radiances and their transformations. 13. Future Modifications and Plans 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-07 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 Tabular ASCII files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation LI-COR LAI-2000 plant canopy analyzer instruction manual. LI-COR, Inc., Lincoln, NE (1991). LI-COR LAI-2000 plant canopy analyzer technical information. LI-COR, Inc., Lincoln, NE (1989). 17.2 Journal Articles and Study Reports Chen, J.M. and T.A. Black. 1992a. Defining leaf area index for non-flat leaves. Plant, Cell and Environment. 15:121-129. Chen, J.M. and T.A. Black. 1992b. Foliage area and architecture of plant canopies from sunfleck size distributions. Agric. For. Meteorol. 60:249-266. Chen, J.M. and J. Cihlar. 1995a. Plant canopy gap size analysis theory for improving optical measurements of leaf area index. Applied Optics. 34:6211-6222. Chen, J.M. and J. Cihlar. 1995b. Quantifying the effect of canopy architecture on optical measurements of leaf area index using two gap size analysis methods. IEEE Trans. Geosci. Remote Sens. 33:777-787. Chen, J.M. and J. Cihlar. 1996. Retrieving leaf area index for boreal conifer forests using Landsat TM images. Remote Sensing of Environment. 55:153-162. Chen, J.M. 1996a. Optically-based methods for measuring seasonal variation in leaf area index of boreal conifer forests. Agricultural and Forest Meteorology 80:135-163. Chen, J.M. 1996b. Canopy architecture and remote sensing of the fraction of photosynthetically active radiation in boreal conifer stands. IEEE Transactions on Geoscience and Remote Sensing. 34:1353-1368. Chen, J.M. 1996c. Evaluation of vegetation indices and a simple ratio for boreal applications. Canadian Journal of Remote Sensing 22: 229-242. Chen, J.M., P. Blanken, T.A. Black., M. Guilbeault, and S. Chen. 1997a. Radiation regime and canopy architecture of a boreal aspen forest. Agricultural and Forest Meteorology 86:107-125. Chen, J. M., P. M. Rich, S. T. Gower, J. M. Norman, S. Plummer, 1997b, “Leaf Area Index of Boreal Forests: Theory, techniques, and measurements,” Journal of Geophysical Research, BOREAS Special Issue, 102, 29429-29443. Deblonde, G., M. Penner, and A. Royer. 1994. Measuring leaf area index with the LI-COR LAI-2000 in pine stands. Ecology 75:1507-1511. Fassnacht, K.S., S.T. Gower, J.M. Norman, and R.E. McMurtie. 1994. A comparison of optical and direct methods for estimating foliage surface area index in forests. Agric. For. Meteorol. 71:183-207. Gower, S.T. and J.M. Norman. 1991. Rapid estimation of leaf area index for forests using LI-COR LAI-2000. Ecology. 72:1896-1900. Lang, A.R.G.. 1991. Application of some of Cauthy’s theorems to estimation of surface areas of leaves, needles and branches of plants, and light transmittance. Agric. For. Meteorol. 55:191-121. 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. and F. Hall. 1997. BOREAS Overview Paper. JGR Special Issue, 102. 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. 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 18. Glossary of Terms None. 19. List of Acronyms APAR - Absorbed Photosynthetically Active Radiation ASCII - American Standard Code for Information Interchange AVHRR - Advanced Very High Resolution Radiometer BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System CCRS - Canada Centre for Remote Sensing DAAC - Distributed Active Archive Center dbh - diameter at breast height EOS - Earth Observing System EOSDIS - EOS Data and Information System FOV - Field of View FPAR - Fraction of Photosyntheically Active Radiation GMT - Greenwich Mean Time GPS - Global Positioning System GSFC - Goddard Space Flight Center IFC - Intensive Field Campaign IPAR - intercepted photosynthetically active radiation IRT - infrared thermometer LAI - Leaf Area Index NAD83 - North American Datum 1983 NASA - National Aeronautics and Space Administration NOAA - National Oceanic and Atmospheric Administration NSA - Northern Study Area OA - Old Aspen OBS - Old Black Spruce OJP - Old Jack Pine ORNL - Oak Ridge National Laboratory PAI - Plant Area Index PANP - Prince Albert National Park PBFD - Photosynthetic Photon Flux Density RSS - Remote Sensing Science SSA - Southern Study Area TE - Terrestrial Ecology TM - Thematic Mapper TRAC - Tracing Radiation and Architecture of Canopies URL - Uniform Resource Locator UTM - Universal Transverse Mercator 20. Document Information 20.1 Document Revision Date Written: 29-Dec-1995. Last Updated: 01-Jul-1998 20.2 Document Review Dates BORIS Review: 10-Sep-1997 Science Review: 24-Nov-1997 20.3 Document ID 20.4 Citation Please contact the PI for more information when publishing this data and acknowledge the efforts of J.M. Chen, J. Cihlar, and M. Penner. Please also make references to appropriate publications in section 17. 20.5 Document Curator 20.6 Document URL KEYWORDS: LAI FPAR LAI-2000 Effective LAI Plant Area Index Clumping Index Vegetation Indices Canopy Gap Fraction RSS07_Ground_LAI_FPAR.doc 07/07/98