The influence of vegetation on frost dynamics, infiltration rate and surface stability in Icelandic Andisolic rangelands

Abstract

Degradation of Icelandic rangelands characterized by Andisols has been extensive. The goal of this research was to assess the effect of contrasting land cover types on ecological factors believed to be important for the resistance and resilience of Icelandic rangelands. Specific objectives were to characterize and quantify soil temperature and frost dynamics, water infiltration, and surface stability in contrasting land cover types. The modulating influence of vegetation on soil temperature was evaluated with soil thermometers at 5, 15 and 30 cm soil depths. Temperature differences were pronounced during snow free periods, and varied with vegetation cover and type. Soil temperature fluctuations were most extreme where vegetation cover was sparse. When snow cover was present soil temperature fluctuations were negligible. Incidence of frost heaving of wooden pegs in sparsely vegetated plant community was 3x greater than that observed in well-vegetated communities. Heaving of willow seedlings only occurred where vegetation cover was sparse. Seasonal changes in infiltration rates, measured with double-ring infiltrometers, varied with soil frost depth and type, as indicated by the depth of visible ice crystals and size and number of ice crystals. Infiltration rates were low to nil when soil frost was deep and of the dense 'concrete' type. Deep soil frost formed where snow depth was shallow, and plant ground cover and biomass low. Formation of infiltration-retarding soil frost types coincided with deep soil frost in sandier soils, and formed at shallower depths in finer textured and organic rich soils. Terminal infiltration rates were high in unfrozen soils (102 - 369 mm h⁻¹ in the sandier soils, versus 28 - 94 mm h⁻¹ in the finer soils, for the whole dataset), and were influenced by both vegetation and soil properties. Infiltration rates were significantly highest in birch woodlands (326 - 369 mm h⁻¹ in the sandier soils; 75 - 89 mm h⁻¹ in the finer soils) compared to the other land cover types (119 - 216 mm h⁻¹ in the sandier soils; 28 - 31 mm h⁻¹ in the finer soils) on more than half of the growing season measurement dates. Infiltration rates in a grassland community increased threefold after only one year of grazing exclusion. The results are discussed in relation to processes leading to loss of vegetation and soils, and management plans to restore ecosystem function.