Abstract
Remote sensing is often used to assess rangeland condition and biophysical parameters across large areas. In particular, the relationship between the Normalized Difference Vegetation Index (NDVI) and above-ground biomass can be used to assess rangeland primary productivity (seasonal carbon gain or above-ground biomass “yield”). We evaluated the NDVI–yield relationship for a southern Alberta prairie rangeland, using seasonal trends in NDVI and biomass during the 2009 and 2010 growing seasons, two years with contrasting rainfall regimes. The study compared harvested biomass and NDVI from field spectrometry to NDVI from three satellite platforms: the Aqua and Terra Moderate Resolution Imaging Spectroradiometer (MODIS) and Système Pour l’Observation de la Terre (SPOT 4 and 5). Correlations between ground spectrometry and harvested biomass were also examined for each growing season. The contrasting precipitation patterns were easily captured with satellite NDVI, field NDVI and green biomass measurements. NDVI provided a proxy measure for green plant biomass, and was linearly related to the log of standing green biomass. NDVI phenology clearly detected the green biomass increase at the beginning of each growing season and the subsequent decrease in green biomass at the end of each growing season due to senescence. NDVI–biomass regressions evolved over each growing season due to end-of-season senescence and carryover of dead biomass to the following year. Consequently, mid-summer measurements yielded the strongest correlation (R2 = 0.97) between NDVI and green biomass, particularly when the data were spatially aggregated to better match the satellite sampling scale. Of the three satellite platforms (MODIS Aqua, MODIS Terra, and SPOT), Terra yielded the best agreement with ground-measured NDVI, and SPOT yielded the weakest relationship. When used properly, NDVI from satellite remote sensing can accurately estimate peak-season productivity and detect interannual variation in standing green biomass, and field spectrometry can provide useful validation for satellite data in a biomass monitoring program in this prairie ecosystem. Together, these methods can be used to identify the effects of year-to-year precipitation variability on above-ground biomass in a dry mixed-grass prairie. These findings have clear applications in monitoring yield and productivity, and could be used to support a rangeland carbon monitoring program.
Highlights
Rangelands comprise between 26 to 36 percent of the terrestrial land surface [1,2]
Since accuracy is crucial for carbon markets and insurance programs, we considered the relative performance of two satellite sensors, Système Pour l’Observation de la Terre (SPOT) and Moderate Resolution Imaging Spectroradiometer (MODIS) (Aqua and Terra), using ground Normalized Difference Vegetation Index (NDVI)
Our results demonstrate accurate NDVI-based estimates of biomass production and phenology in a mixed-grass prairie, seasonal shifts in the NDVI–biomass relationships emerged from temporal patterns of carryover and senescence
Summary
Rangelands comprise between 26 to 36 percent of the terrestrial land surface [1,2]. They consist of many ecosystems, including shrublands, savannas, tundra, deserts, alpine communities, coastal marshes, meadows, and grasslands [3]. Grasslands covered 300 million ha in the United. States and 50 million ha in Canada [4]. The mixed prairie, one grassland type, is located in the brown and dark brown soil regions of southern Alberta and Saskatchewan, Canada, and extends southward into northern Texas [5]. 7 percent of the area of Alberta is covered by the dry mixed-grass prairie subregion, of which 43 percent remains [6]. Climate change is expected to increase the amount and duration of growing season drought, as well as alter the frequency and intensity of growing season precipitation [1,11]
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