Published estimates of leaf area index (LAI) covering the period 1932-2000 have been compiled at the ORNL DAAC into a single data set to support model development and EOS MODIS product validation. Like net primary productivity (NPP), leaf area index (LAI) is a key parameter for global and regional models of biosphere/atmosphere exchange of carbon dioxide, water vapor, etc. This data set provides a benchmark of typical values and ranges of LAI for a variety of biomes and land cover types, in support of model development and validation of satellite-derived remote-sensing estimates of LAI and other vegetation parameters. For example, maximum values for point measurements are unlikely to be approached or exceeded for area-weighted LAI, which is what satellites and spatial models are measuring or modelling.
Approximately 1000 published estimates of leaf area index (LAI) are available, from nearly 400 unique field sites, together with associated variables such as latitude/longitude, elevation, stand age, above-ground NPP, etc. The LAI data are linked to a bibliography of over 300 original-source references.
Other relevant NASA-associated activities on comparing ground-based LAI measurements to remotely sensed LAI include the following:
Summary of Data CollectedGlobal coverage, point data: About 1000 LAI values from 0.1-0.18 (minimum; desert and tundra) to 47.0 (maximum; a peculiarity of one allometric method for estimating all-sided LAI in coniferous tree stands). Units are m2/m2 or dimensionless. However, only 14% of the records have LAI greater than 8.0 (a more typical maximum value for one-sided or projected LAI, unlikely to be exceeded except with peculiar conditions or methodology.)
Associated variables: sitename, country, latitude, longitude, time of measurement, methodology, biome/land cover, dominant species, leaf type/ leaf angle, author, year, year of measurement, ANPP (above-ground NPP), BNPP (below-ground NPP), TNPP (total [above+below-ground] NPP), elevation, stand age, remarks [N.B.: Not all these variables are available for all points.]
Spatial coverage: 54.5 S to 71.3 N, 156.67 W to 172.75 E
Temporal coverage: 1932-2000.
DiscussionThis data compilation attained its target - it was originally estimated that about 1000 distinct LAI measurements might be available worldwide from the scientific literature - and a possible further 500-1000 data points have been identified. In order to develop the data set, it was necessary to (1) review the literature, (2) compile existing and additional estimates of LAI and ancillary variables (including identification of measurement methods for each value and acquisition of site coordinates), (3) conduct data quality assessment, and (4) write and review documentation.
These historic LAI data are mostly from natural and seminatural (managed) ecosystems, although some agricultural estimates are also included. Although methodologies for determining LAI have changed over the decades, it is useful to represent the inconsistencies (e.g., in maximum value reported for a particular biome) that are actually found in the scientific literature. Needleleaf (coniferous) forests are by far the most commonly measured biome/land cover types in this compilation, with 22% of the measurements from temperate evergreen needleleaf forests, and boreal evergreen needleleaf forests and crops the next most common (about 9% each). About 40% of the records in the data set were published in the past 10 years (1991-2000), with a further 20% collected between 1981 and 1990.
Mean LAI (± standard deviation), distributed between 15 biome/land cover classes, ranged from 1.31 ± 0.85 for deserts to 8.72 ± 4.32 for tree plantations, with evergreen forests (needleleaf and broadleaf) displaying the highest LAI among the natural terrestrial vegetation classes.
Additional ancillary variables have been suggested by a number of sources, including time course (phenology) of LAI, fPAR (fraction of photosynthetically active radiation intercepted by the canopy), and leaf area density distribution within the canopy. Strictly speaking, LAI alone is a rather broad descriptor of a study site and assumes random distribution of foliage within the canopy - this may not be adequate for modeling of photosynthesis/radiation absorption at the stand level, although a general LAI is more useful for regional/global scale modeling. At a future date, it may be possible to compile a sub-set of the LAI data reported here for those more intensively measured and reported study sites, where additional detailed metadata and ancillary variables (fPAR, leaf area density, etc.) are available.
Data Products and DocumentationThe available data consist of a spreadsheet table (in several formats), a bibliography of more than 300 original-source references and several companion files. A summary table and histogram of LAI mean values and ranges by biome is included, and a world map is presented. showing the distribution of field sites. Further details are available in the accompanying Technical Memorandum (Scurlock et al., 2001), which includes a brief definition of leaf area index and methods for its measurement (ranging from leaf weight/litterfall to optical instruments such as the LI-COR 2000). It also discusses why the older literature tends to contain higher values of LAI than later references (methodologies are not strictly comparable, and there is a noticeable decline in reported values over time). High values have been reported previously for double-sided or all-sided LAI in wetlands and coniferous forest (as opposed to one-sided LAI commonly reported for broad-leaved forests, crops, etc.), and the high numbers reported for needle-leaf stands such as ponderosa pine are probably more suitable for modeling gas exchange across all leaf surfaces than for comparing canopy light scattering between needle-leaved and broad-leaved "planophile" canopies.
See these resources:
Data CitationCite this data set as follows:
Scurlock, J. M. O., G. P. Asner, and S. T. Gower. 2001. Global Leaf Area Index Data from Field Measurements, 1932-2000. Data set. Available on-line [http://www.daac.ornl.gov] from the Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge, Tennessee, U.S.A.
ReferencesScurlock, J. M. O., G. P. Asner, and S. T. Gower. 2001. Worldwide Historical Estimates and Bibliography of Leaf Area Index, 1932-2000. ORNL Technical Memorandum TM-2001/268, Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A.
Contact InformationContact: Dr. Bob Cook
Environmental Sciences Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831
Tel. +1 (865) 574-7319