Search

Abstract: Biological soil crusts (BSCs) abound in the Hallamish dune field in the western Negev Desert, Israel. While their abundance may imply high adaptability to environmental change, such as fluctuations between wet and dry conditions following winter rains, summer rains, although rare, may also occur in the Hallamish dune field. The aim of the present paper is to examine crust responses to winter and summer rains, focusing particularly on its biomass components, chlorophyll and carbohydrate. In addition, species composition, during summer and winter was examined. Analysis took place during an exceptionally wet winter (1994/95 with 172mm) and a summer rainstorm (12.5.93 with 9.7mm). The data showed a 2–3 fold increase in chlorophyll a and total carbohydrates and a much richer species composition following the heavy winter rains of 1994/95. Yet, the data also showed ~15–30% decrease in the chlorophyll content of the crust (with no concomitant significant decrease in total carbohydrates) following the summer rainstorm. Intense weathering by the summer rain coupled with cell mortality may explain the decrease in the chlorophyll content following the summer rainstorm, suggesting possible changes in the BSC following a potential change in the precipitation regime due to global climate change. [Copyright &y& Elsevier]

Abstract: Drylands are regarded as highly sensitive to climatic change. The putative positive relationship between average annual rainfall and runoff, assumed for areas between 100 and 300mm ignores the fact that climatic change in drylands is not limited to climatic factors alone, but is often accompanied by a parallel change in surface properties. Data on rainfall, runoff and soil moisture regime were collected at five monitoring sites in a sandy area, along a rainfall gradient from 86 to 160mm. Despite the uniform sandy substratum the frequency and magnitude of runoff declined with increasing annual rainfall. Under wetter conditions a thick topsoil biological crust develops. This crust is able to absorb and retain large rain amounts, limiting the depth to which water can penetrate, and therefore water availability for the perennial vegetation. In the drier area, the thin crust can absorb only limited rain amounts, resulting in surface runoff and deeper water infiltration at run-on areas. Our findings demonstrate the important role played by different types of biological soil crusts along the rainfall gradient considered, and question the generally held belief that higher rainfall necessarily leads to deeper water infiltration in sandy arid areas; and higher water availability for the perennial vegetation. [Copyright &y& Elsevier]