Microenvironment and the Distribution of Two Species of Draba (Brassicaceae) in a Venezuelan Paramo

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Abstract
The microenvironment of the habitats of two species of Draba in Venezuelan paramo were characterized. The rosette species D. chionophila occurs in extensive areas of frost-heaved soil with very low plant cover and species richness; D. arbuscula and other shrub species occur preferentially on rock outcrops, areas with higher plant cover and species richness. The thermal mass of the rock outcrops results in air temperatures near the soil surface and soil temperatures that are 2 to 4.degree. C higher than in the frost-heaved soil habitat throughout the day and night. Minimum soil temperatures in the habitat of D. chionophila frequently are low enough after drawn to inhibit the uptake of water (0 to 3.degree. C); the rock soils are not. The thin soils on rock outcrops because quite dry during extended periods without precipitation, while the frost-heaved soils of D. chionophila habitat did not. Consequently, the microenvironmental limitations imposed in the habitat of D. chionophila are primarily due to the pronounced diurnal temperature fluctuations and low soil temperatures. The warmer soils of the rock outcrop habitat of D. arbuscula reduce the potential for a diurnal temperature limitation, but are subject to seasonal drought. The occurrence of rock outcrops contributes to the diversity of microenvironments on the paramo landscape, through both diurnal and seasonal effects, and this heterogeneity is reflected in the distribution of plant species on the landscape.