RESUMO
Selected morphological features were measured in five populations of the giant rosette plant Espeletia schultzii occurring along an elevation gradient from 2600 to 4200 m in the Venezuelan Andes. Pith volume per amount of leaf area increases with elevation resulting in significantly larger water storage capacity at higher elevations. Thickness of leaf pubescence and, therefore, leaf boundary layer resistance, also increases with elevation resulting in both potentially higher leaf temperatures relative to air temperature and higher leaf to air vapor pressure gradients. The net effect on transpiration rate would depend on ratios of stomatal to boundary layer resistance and leaf energy balance. At higher elevations the central rosette leaves are more vertically oriented and the leaf bases show a pronounced curvature as the intersection with the main axis is approached. This gives these rosettes a distinctly paraboloid appearance and probably enhances capture and retention of incident long and shortwave radiation by the apical bud and expanding leaves. Features which result in enhanced water storage capacity and higher plant temperatures relative to air temperature without greatly increasing water loss are adaptive in high altitude paramo habitats where water availability and growth are limited by year round low temperatures (mean 2-3° C).
RESUMO
Despite the extrame aridity of the coastal Atacama Desert in northern Chile, sparse communities of leaf succulent shrubs and small cacti are regularly present. While most shrub species have small succulent leaves and accumulate high concentrations of salts in their tissues, the variable rooting patterns and mixed dominance of CAM and C3 species indicates a significant divergence in adaptive strategies. All dominant shrubs are readily surviving extended drought, but some species are much better able than others to maintain active growth and flowering. Regular flowering may not be a prerequisite for shrub population maintenance since large piles of viable seeds are present under the canopies of many species.
RESUMO
In the Atacama Desert of northern Chile, cacti of the genus Copiapoa occur in extensive, relatively dense, monospecific stands. The spatial distribution patterns within several stands of Copiapoa cinerea v. columna-alba were analyzed for evidence of competitive interactions among individuals. There was no indication that competition was affecting stand density. Mean annual precipitation is only 25 mm/yr, but estimates of maximum possible water use within a stand were much lower. It was shown that the time between rainstorms is long compared to the water storage capacity of the plant stems, especially in the case of small individuals. We conclude that population densities in Copiapoa are limited by the difficulty of establishment, not by competition.