DESERTIFICATION IN CENTRAL ARGENTINA: CHANGES IN ECOSYSTEM CARBON AND NITROGEN FROM IMAGING SPECTROSCOPY

Many arid and semiarid (dryland) regions are subject to desertification from intensive land-use pressures such as cattle ranching. However, the lack of quantitative approaches required to assess desertification has slowed our understanding of how vege- tation and soils are changing in dryland regions. Using airborne high-fidelity imaging spectroscopy and field measurements, we developed the first regional assessment of veg- etation structural and soil biogeochemical properties in the Monte Desert biome, Argentina. We evaluated the long-term impacts of grazing on vegetation cover and soil carbon (C) and nitrogen (N) storage, which are core indicators of biogeochemical status and change in drylands. A comparative analysis was carried out on vegetation and soil properties in four major Monte Desert plant communities, as well as within the U.N. Nacunan Man-and- Biosphere Reserve and adjacent areas subjected to long-term grazing. The four dominant plant communities differed substantially in vegetation cover, leaf area index, foliar N concentration, and soil organic C and N stocks. Imaging spectroscopy with Monte Carlo spectral mixture analysis provided accurate estimates of fractional pho- tosynthetic vegetation (PV), nonphotosynthetic vegetation (NPV), and bare soil cover at ,5-m spatial resolution throughout a 763-km 2 region. Hotspots of grazing management (e.g., ranch centers, water sources) have undergone some woody vegetation encroachment, but the spatial patterns were localized and variable. More clearly, a widespread NPV de- crease and bare soil increase was common outside of the reserve. Soil organic C and N stocks were highly correlated with PV 1 NPV cover fractions. Soil organic C and N storage was 25-80% lower in areas subjected to long-term grazing, as compared to protected ecosystems within the Nacunan Reserve. Long-term grazing has depleted stores of C and N in soils and dramatically altered vegetation structure in the Monte Desert. A persistent lack of NPV due to grazing has likely impaired C and N cycles considered central to the biogeochemical functioning of the region. This study demonstrates a novel approach to remotely measure both surface and soil properties most indicative of progressive land degradation and desertification in dryland regions.