Desert-scrub as the cause of reduced reflectances in protected versus impacted sandy arid areas

Abstract

High contrast, commonly in the 1-3— 1-5 range, was observed from satellite multi-spectral radiometers between overgrazed sandy terrain and adjacent protected (fenced-off) areas. Approximately the same contrast was reported in the visible and near-infrared spectral bands. These observations were first conducted at the border of the bright overgrazed Sinai and the darker Negev (in israel). Extensive ground observations were carried out at this border and in the 6 km x 6 km exclosure in the northern Sinai, in which plant growth spontaneously recovered after the area was fenced off in 1974. Karnieli and Tsoar (1995) rejected the predominant role of plants in producing such contrast, concluding that the well-known contrast between Sinai and the Negev, that has drawn the attention of many scientists, is not a direct result of vegetation cover but is caused by an almost complete cover of biogenic crust', This conclusion does not account for these factors: (I) the contrast between dune sand and biogenic crust measured by Karnieli and Tsoar is at most 1-25, and therefore cannot create contrasts of 1-5 observed from satellites; (ii) the dune sand to crust contrast is appreciable only above 0-6 ixm, and the satellite sensor contrast measurements include the 0-5-0-6μ m band; and (iii) ground visual observations and published photographs show individual desert-scrub plants much darker than the soil interstices close to the observer, but at elevated view directions the plants merge, producing a nearly uniform darkening of the terrain. This limb-darkening, observed on the Israeli side along the Sinai/ Negev border, is a clear indication of the strong optical effects of the plants on ihe terrain reflectances.