Periglacial vegetation-banked terraces: a global survey

Periglacial vegetation-banked terraces (VBTs) are roughly parallel, step-like landforms with bare treads and vegetated risers, where tread length (across-slope) markedly exceeds tread width (downslope). Many follow the contour, but others dip off-contour at angles of up to 40°. Most VBTs with a low off-contour dip occupy lee slopes with gradients of 4°–25°, with dip increasing to windward. They are widespread in marginal periglacial environments characterised by strong winds, abundant precipitation, winter snow cover, shallow ground freezing and frequent freeze–thaw cycles, but also occur in alpine environments and on low-latitude and tropical mountains. They appear to be rare or absent in polar or subpolar permafrost environments. Riser vegetation is often dominated by cushion plants, dwarf shrubs or bunchgrasses. This review (1) outlines the history of research on VBTs; (2) identifies their global distribution and the environmental conditions under which they have developed; (3) summarises data concerning their dimensions and structure, and measurements of current activity; and (4) proposes models of VBT formation. Many VBTs have apparently evolved from prior establishment of alternating bands of bare and vegetated ground across slopes. This may have occurred either through the lateral extension of vegetation bands or through the formation of wind stripes, where alternating bands of vegetated and bare ground have developed normal to the direction of dominant winds through turf exfoliation by needle ice, frost heave and wind scour. Such vegetated bands trap mobile debris to build risers, with consequent lowering of tread gradients. Some terraces are subsequently immobilized by stacking of clasts under risers, but those on frost-susceptible soils may move en masse downslope through solifluction. Other VBTs represent stripping of vegetation cover from the windswept treads of vegetation-covered solifluction terraces, and possibly also selective vegetation colonisation of the sheltered risers of stone-banked solifluction terraces.