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INTRODUCTION
Until recently, the Advanced Very High Resolution Radiometer (AVHRR) was the only broad scale,
globally applicable satellite that provided direct spectral data suited for continual monitoring
of vegetation. As such, many studies have successfully used AVHRR normalized difference vegetation
index (NDVI) to infer photosynthetic monitor growing season phenology and estimate vegetation.
On 18 December, 1999 the first Moderate Resolution Imaging Spectroradiometer (MODIS) was launched
on the Terra platform of the Earth Observing System (EOS). The MODIS instrument provides new and
improved capability for terrestrial remote sensing intended for global change research including a
suite of standard products designed to remove the burden of most data processing requirements. To
determine the practical usefulness of MODIS products, we sought to establish a relationship between
MODIS leaf area index (LAI), enhanced vegetation index (EVI), and NDVI and the above-ground herbaceous
green biomass in a semi-arid grassland ecosystem. This study focuses on relating the MODIS eight - day
composite LAI, and 16 - day composite EVI and NDVI, to direct measures of above - ground biomass generated
within the growing season during four time periods in two steps: 1) establish a methodology for converting
plot level biomass measurements to a regional scale; and 2)
characterize the relationship between selected MODIS land products and spatially scaled - field
observations of grassland vegetation productivity.
STUDY AREA
Field data were collected in the Little Missouri National Grasslands (LMNG) of western North Dakota (Figure 1). .
This 809,380 ha area is managed primarily by the USDA Forest Service for cattle grazing, oil and gas
leasing, wildlife habitat, and recreational uses (Jensen et al. 2001). Due to their large geographic
expanse and the dominance of federal ownership, the LMNG provided an excellent opportunity for collecting
field data and therefore relating MODIS - derived land products to grassland vegetation.
METHODS
Biomass observations were made during the 2001 growing season at
2,200 plots (473 transects) across four time periods, each five days in length,
in the LMNG. All herbaceous biomass within a 0.5 m2 quadrat was clipped at ground
level every fifty meters along each transect and percentage of living vegetation
was recorded. Herbaceous biomass was subsequently dried at 65 C° for at least 48
hours and weighed. Final estimates of above - ground net primary productivity within
each clipped plot were determined by multiplying the percentage of living vegetation
by the weight of dried biomass. For scaling, biomass measures were subsequently
modeled within the spatial extent of each Thiesson polygon using a multiple
regression formula combining ETM + NDVI, accumulated growing degree days (thermal time),
and summation of precipitation, of the form:
biomass = NDVI (65.0112)+ (pptsum(0.9) - gddsum^2(0.0013))
where biomass is the estimated biomass within each Thiesson polygon, NDVI is the
average NDVI for a given polygon, pptsum is the summation of precipitation from 1
January 2001 to the date of ground sampling, and gddsum is the summation of thermal
time (TAVGdaily - 0) from 1 January, 2001 to the date of ground sampling where
TAVGdaily is the daily average temperature.
Scaled biomass measurements were compared with MODIS LAI, NDVI and EVI.
RESULTS
MODIS LAI (Figure 2) , EVI (Figure 3)
and NDVI (Figure 3) were all closely
related to observed biomass. The results of this study present a framework
for linking small-scale field observations to MODIS LAI, EVI and NDVI while
simultaneously providing much needed insight to the relationship between
MODIS land products and vegetation productivity. The high correlation between
MODIS land products and observed above - ground green biomass proved that MODIS
land products are suitable for monitoring grassland vegetation dynamics
in northern mixed grass prairie and appear to offer improved capabilities
compared with the AVHRR NDVI relationships presented in previous work.
Contact
University of Montana
School of Forestry/NTSG
Missoula, MT
59812
Phone: (406) 243-6228 email: reeves@ntsg.umt.edu
