Cover Beds As Evidence of a Humped Soil Production Function Associated with Forest Succession.

Observations of laterally traceable colluvial and loess deposits in elevated terrain across central Europe led to extensive development of the cover bed concept. Cover beds appear to be relicts of periglacial processes operating at the end of the Pleistocene and sporadically through the Holocene. Viewed as climatic facies, cover beds have been used to reconstruct past environmental conditions both in Europe and in the Colorado Rockies. However, in semi-arid forests of central Oregon, slope deposits which appear similar in gross morphology to those described as cover beds occur in volcanic ejecta erupted during the Holocene and are unlikely to have a periglacial origin. Soil geomorphic observations suggest that these colluvial slope deposits preserve a brief but intense period of regolith mobilization and stripping. We suggest that observed soils record the transgression of a forest facies onto formerly treeless landscape positions due to edaphically driven colonization.  The associated bioturbation processes present in forests, especially tree throw, embody the instability predicted by the humped soil production function.