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The structure and dynamics of vegetation in a Chilean arid thorn-scrub community within Parque Nacional Fray Jorge (30-degrees 38’s, 71-degrees 40’W) were analyzed for a period of three years (1989-1991). These years differed markedly in annual precipitation; 1989 was a year of average rainfall (89 mm), 1990 was an extremely dry year (32 mm), and 1991 was a comparatively wet year (233 mm). Rainfall had a noticeable effect on the cover of annual herbs, especially species that are restricted to areas under bushes, but it had little effect on the cover of perennial shrubs and herbs, except for the summer drought-deciduous species. Shrub cover in the study site has remained about the same (60%) for almost 50 years.