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 investigated along catenas in the semi-arid climatic zone of Greece. These catenas are located in hilly areas with rolling topography and soils formed on Tertiary and Quaternary deposits of marl, conglomerates and shale-sandstones. Total above ground biomass production was measured on specific hillslope components (shoulder, backslope and footslope) in three successive growing periods and was related to the soil properties, landscape position and climatic data. Crop-water use, calculated according to a broadly used simplified methodology, was logarithmically 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 mulching and preventing large areas from desertification. Stony soils along slope catenas of conglomerates and shale-sandstones, despite their normally low productivity, may supply appreciable amounts of previously stored water to the stressed plants and to secure a not negligible biomass production even in extremely dry years. Soils formed on marl are free of coarse fragments and despite their considerable depth and high productivity in normal and wet years, they are very susceptible to desertification, being unable to support any vegetation in particularly dry years due to adverse soil physical properties and the absence of gravel and stone mulching.