Needle-Ice Activity and the Distribution of Stem-Rosette Species in a Venezuelan Paramo

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Abstract
The spatial patterns of Andean stem-rosettes and needle-ice activity were studied in the Paramo de Piedras Blancas, Venezuela, at 4300 m altitude. Downslope movement of painted soil particles was significantly higher in the lower-slope positions and in northwest aspects; wooden pegs were also heaved more in northwest slopes. Needle ice formed more often and densely in such locations. The variability of soil properties might have affected needle-ice growth: soils with a northwest aspect were deeper, had a more fully developed profile, higher organic matter and fine particle content, and a larger water content at field capacity than soils with a southeast aspect. Two stem-rosette species dominated the study plots; Coespeletia lutescens was found in southeast and northwest aspects, while Espeletia semiglobulata was restricted to the latter, where it grew preferentially in the footslopes. Espeletia semiglobulata, unlike the first species, has a prostrate growth habit and facultative vegetative reproduction, which enable it to colonize areas heavily affected by needle ice. Seedlings of C. lutescens and other stem-rosette species became established near rocks, cushions, and other ground features providing protection from needle-ice heave. Stone edafids were the most important refuge against ground disturbance, and stem-rosette roots, both of seedlings and mature plants, were frequently associated with them. The size-class distributions of all stem-rosette populations showed a reduced frequency in the lower classes, which may indicate either episodic reproduction or recently intensified browsing pressure by cattle.