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tropospheric ozone from biogenic emissions of volatile organic compounds naturally released into
the atmosphere (Geron et al., 1994). The NDVI has commonly been used as an indicator of biomass
(Eidenshink and Haas, 1992) and vegetation vigor (Carlson and Ripley, 1997). NDVI has been
applied in monitoring seasonal and interannual vegetation growth cycles, land-cover (LC) mapping,
and change detection. Indirectly, it has been used as a precursor to calculate LAI, biomass, the
fraction of absorbed photosynthetically active radiation (fAPAR), and the areal extent of green
vegetation cover (Chen, 1996).
Direct estimates of LAI can be made using destructive sampling and leaf litter collection
methods (Neumann et al., 1989). Direct destructive sampling is regarded as the most accurate
approach, yielding the closest approximation of “true” LAI. However, destructive sampling is time-
consuming and labor-intensive, motivating development of more rapid, indirect field optical meth-
ods. A subset of field optical techniques include hemispherical photography, LiCOR Plant Canopy
Analyzer (PCA) (Deblonde et al., 1994), and the Tracing Radiation and Architecture of Canopies
(TRAC) sunfleck profiling instrument (Leblanc et al., 2002).
forest measurements serve as
both reference data for satellite product validation and as baseline measurements of seasonal
vegetation dynamics, particularly the seasonal expansion and contraction of leaf biomass.
The development of appropriate ground-based sampling strategies is critical to the accurate
specification of uncertainties in LAI products (Tian et al., 2002). Other methods that have been
implemented to assess the MODIS LAI product have included a spatial cluster design and a patch-
based design (Burrows et al., 2002). Privette et al. (2002) used multiple parallel 750-m TRAC
sampling transects to assess LAI and other canopy properties at scales approaching that of a single
MODIS pixel. Also, a stratified random sampling (SRS) design element provided sample intensi-
fication for less frequently occurring LC types (Lunetta et al., 2001).
In situ
4.1.1
Study Area
The study area is the Albemarle-Pamlico Basin (APB) of North Carolina and Virginia (Figure
4.1). The APB has a drainage area of 738,735 km
and includes three physiographic provinces:
mountain, piedmont, and coastal plain, ranging in elevation from 1280 m to sea level. The APB
subbasins include the Albemarle-Chowan, Roanoke, Pamlico, and Neuse River basins. The Albe-
marle-Pamlico Sounds compose the second-largest estuarine system within the continental U.S.
The 1992 LC in the APB consisted primarily of forests (50%), agriculture (27%), and wetlands
(17%). The forest component is distributed as follows: deciduous (48%), conifer (33%), and mixed
(19%) (Vogelmann et al., 1998).
2
4.2 BACKGROUND
4.2.1
TRAC Measurements
The TRAC sunfleck profiling instrument consists of three quantum PAR sensors (LI-COR,
Lincoln, NE, Model LI-190SB) mounted on a wand with a built-in data logger (Leblanc et al.,
2002) (Figure 4.2). The instrument is hand-carried along a linear transect at a constant speed,
measuring the downwelling solar photosynthetic photon flux density (PPFD) in units of micromoles
per square meter per second. The data record light-dark transitions as the direct solar beam is
alternately transmitted and eclipsed by canopy elements (Figure 4.3). This record of sunflecks and
shadows is processed to yield a canopy gap size distribution and other canopy architectural param-
eters, including LAI and a foliage element clumping index.
From the downwelling solar flux recorded along a transect, the TRACWin software (Leblanc
et al., 2002) computes the following derived parameters describing forest canopy architecture: (1)
