Morphological changes along an altitude gradient and their consequences for an andean giant rosette plant.

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).