Edaphic interslope and valley bottom differences at "Evolution Canyon", lower Nahal Oren, Mount Carmel, Israel

The opposite slopes of lower Nahal Oren, Mount Carmel, Israel, designated " Evolution Canyon", display physical and biotic contrasts, although both are cut in Upper Cenomanian calcareous limestone. The three-fold greater solar radiation makes the south-facing slope warmer, drier and more variable tha n the north-facing slope and valley bottom. Consequently, biodiversity is g reater. Microclimate (mesic-xeric) is a major force driving adaptive evolut ion, and causing soil divergence. The soils on both slopes are Terra Rossas in the Israeli classification system (Rhodoxeralfs in US taxonomy). Howeve r, selected properties (pH, humus. content, NH4-N content, cation exchange capacity and others) display greater interslope than intraslope differences . The north-facing Terra Rossas have a dark-coloured Al humic horizon, whic h is better developed than in south-facing Terra Rossas. Differences in soi l morphology, moisture regime and microfabric between the north- and south- facing soils suggest that they belong to different taxonomic classes. These differences probably result in part from the forested ecosystem of the nor th-facing slope versus the savanna-like ecosystem of the opposite slope. (C ) 1998 Elsevier Science B.V. All rights reserved.