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NASA Land Cover Land Use Change Program
Quantifying Grassland-to-Woodland Transitions and the Implications for Carbon and Nitrogen Dynamics in the Southwest
United States
Principal Investigators
Carol Wessman, University of Colorado
Steven R. Archer, Texas A&M Research Foundation
Gregory P. Asner, University of Colorado
C. Ann Bateson, University of Colorado
Project Summary
Environmental impacts associated with slashing and burning of tropical rain forest have received considerable attention
and are the focus of much current research. However, reductions in biomass associated with deforestation are in stark
contrast to vegetation changes occurring on many arid/semi-arid rangelands, where grasslands and savannas are being
replaced by shrublands and woodlands. Though not well quantified on a global scale, this vegetation change has been widely
reported in tropical, temperate and high-latitude rangelands worldwide. These changes in vegetation structure are relevant
to global change as (a) they may reflect historical shifts in climate and land use and (b) they may influence biodiversity,
productivity, above- and belowground carbon and nitrogen sequestration and biophysical aspects of land surface-atmosphere
interactions.
Our overall goal is to investigate how changes in the relative abundance of herbaceous and woody vegetation affect
carbon and nitrogen dynamics across topoedaphically diverse landscapes. Grassland-to-woodland transitions occur at small
scales relative to the spatial resolutions of current and planned sensors having regional- to continental-scale monitoring
capabilities (e.g. AVHRR, MODIS). The functional properties of heterogeneous savannas and shrub/woodlands will likely go
undetected by single indices such as the NDVI as structural and background factors confound the remotely sensed signal. We
propose to refine and apply a sub-pixel analytical technique (spectral mixture analysis) to determine grass-woody
fractional cover. By linking actual land-cover composition with a process-based ecosystem model, Century, we will generate
explicit predictions of the C and N storage in plants and soils resulting from changes in vegetation structure. We will
base our studies on a grassland-to-woodland chronosequence in Texas which represents the range in vegetation structure
expected in the southwestern United States and having functional counterparts in Central and South America, Africa, Asia
and Australia. Our specific objectives will be to (1) continue development and test applications of spectral mixture
analysis across grassland-to-woodland transitions; (2) quantify temporal changes in plant and soil C and N storage and
turnover for remote sensing and process model parameterization and verification; and (3) couple landscape fraction maps
to Century to observe biogeochemical dynamics under changing landscape structure and climatological forcings. We will
utilize high spectral and spatial resolution data (AVIRIS, TM, and Lewis) to establish landscape-level predictions and then
extrapolate to the southwestern United States using MODIS and readily available climate data.
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