DESERT-SCRUB AS THE CAUSE OF REDUCED REFLECTANCES IN PROTECTED VERSUSIMPACTED SANDY ARID AREAS

High contrast, commonly in the 1.3-1.5 range, was observed from satell ite multi-spectral radiometers between overgrazed sandy terrain and ad jacent protected (fenced-off) areas. Approximately the same contrast w as reported in the visible and near-infrared spectral bands. These obs ervations 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 th e northern Sinai, in which plant growth spontaneously recovered after the area was fenced off in 1974. Karnieli and Tsoar (1995) rejected th e predominant role of plants in producing such contrast, concluding 't hat the well-known contrast between Sinai and the Negev, that has draw n the attention of many scientists, is not a direct result of vegetati on 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 f rom satellites; (ii) the dune sand to crust contrast is appreciable on ly above 0.6 mu m, and the satellite sensor contrast measurements incl ude the 0.5-0.6 mu m band; and (iii) ground visual observations and pu blished photographs show individual desert-scrub plants much darker th an the soil interstices close to the observer, but at elevated view di rections 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 effect s of the plants on the terrain reflectances.