Cryptogamic soil buds in the equatorial Andes of Venezuela

Abstract

Soil buds, formed by particle heaving and sorting by needle ice, were studied in a high Andean paramo. Soils were finer in buds than in adjacent trough areas, where pebbles had been concentrated by lateral sorting. Some buds were composed only of mineral grains, others were covered by a cryptogamic crust of liverworts (Marsupella spp.) or mosses (Grimmia longirostris). Particle sorting was studied by comparing the grain-size distributions of buds and troughs, and by using a numerical sorting index. Degree of sorting varied widely among bare bud sites, where it was dependent on the presence of gravel in the substrate. Sorting was pronounced in sites with cryptogamic plants, where fines (<0.05 mm) were concentrated within buds, which also contained higher amounts of organic matter than the adjacent troughs. Microscopic examination showed that cryptogamic buds were layered in three concentric bands. This resulted in a gradual increase of fines and organic matter toward the bud top, as well as in a lower pH and darker colour. The pronounced rise in silt and clay near bud surfaces is due to interception of aeolian material by plants, which can trap fines and then grow above the deposited dust, thus forming a cumulic sequence of thin crustal laminae. Vughs and vesicles formed by frost activity were common in buds, particularly in their basal layer. Field experiments showed that cryptogamic crusts increase the infiltration rates and compressive strength of soil buds. This results in a lower chance of runoff generation and in an increased resistance to detachment by various geomorphic agents. An idealized developmental sequence of cryptogamic soil buds is discussed in detail.