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Improving estimates of vegetation structure and biomass in global drylands with ICESat-2

05-26-2020 -

The tropical and temperate drought-seasonal savannas, shrublands and grasslands of the world are under-studied, but critical, biomes found on all continents, except Antarctica. These biomes, collectively referred to in this proposal as "savannas and drylands", are under increasing pressures of land use change for small-grain agriculture driven by population, economic and globalization trends. The drought seasonal savannas and drylands are often overlooked in Earth Observation and remote sensing studies, relative to the taller woodland and forest zones with high carbon-density (but relatively smaller area). However, the savanna-drylands also support large quantities of carbon in above- and below-ground woody biomass and soil organic matter. This tendency of Earth Observing systems to ignore the savannas and drylands holds true for the Ecosystem Structure and Biomass Estimation research foci of both the ICESat-1 and ICESat-2 missions, where the terrestrial vegetation emphasis has been on taller stature and higher carbon density regions of the World. In particular, the ICESat-2 Land-Vegetation Along-Track Product (ATL08) effectively ignores most of the World's savannas and drylands. We propose activities that will fill the gaps we perceive in ICESat-2 processing in savannas and drylands and realize opportunities to estimate not only canopy heights but also canopy cover and above ground woody biomass in these regions. Our specific objectives, described in more detail in later sections, include: Goal 1: Expansion of ICESat-2 along-track processing for global savannas and drylands, most of which are currently omitted from standard ATL08 processing. Goal 2: Interpolation of ICESat-2 along-track canopy heights using Sentinel-1 and -2 data to create new ~100 m gridded canopy height products for global savanna-dryland systems. Goal 3: Application of a Google Earth Engine mapping tool (developed with earlier NASA support) to create ~100 m gridded canopy cover products for global savanna-dryland systems. Goal 4: Collation of existing and new field and virtual field data for improved biomass allometries. Goal 5: Creation of ~100 m gridded above ground wood biomass estimates for global savannas and drylands using allometric data, canopy height and cover datasets. Goal 6: Accuracy and uncertainty assessment of gridded products via uncertainty propagation and error analysis using field, virtual field, LTER, NEON and GEDI data. The products from this research will allow us, and the wider research community, to better quantify and understand the spatial and temporal variability in woody vegetation structure and biomass in global savannas and drylands. The gridded canopy heights, canopy cover and woody biomass products we propose will transform our understanding of global savanna and dryland vegetation structure, opening the door to numerous applications in terrestrial ecology, rangeland science, carbon cycle science and land surface studies.