THE IMPACTS OF PARENT MATERIAL AND LANDSCAPE POSITION ON DROUGHT AND BIOMASS PRODUCTION OF WHEAT UNDER SEMIARID CONDITIONS

The effect of drought on the biomass production of rainfed wheat was i nvestigated along catenas in the semi-arid climatic zone of Greece. Th ese catenas are located in hilly areas with rolling topography and soi ls formed on Tertiary and Quaternary deposits of marl, conglomerates a nd shale-sandstones. Total above ground biomass production was measure d on specific hillslope components (shoulder, backslope and footslope) in three successive growing periods and was related to the soil prope rties, landscape position and climatic data. Crop-water use, calculate d according to a broadly used simplified methodology, was logarithmica lly correlated with biomass production. Gravel and stones on the soil surface appeared to be extremely important in dry years by conserving appreciable amounts of soil water from evaporation through surface mul ching and preventing large areas from desertification. Stony soils alo ng slope catenas of conglomerates and shale-sandstones, despite their normally low productivity, may supply appreciable amounts of previousl y stored water to the stressed plants and to secure a not negligible b iomass production even in extremely dry years. Soils formed on marl ar e free of coarse fragments and despite their considerable depth and hi gh productivity in normal and wet years, they are very susceptible to desertification, being unable to support any vegetation in particularl y dry years due to adverse soil physical properties and the absence of gravel and stone mulching.