Some Consequences of Leaf Pubescence in the Andean Giant Rosette Plant Espeletia Timotensis

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Abstract
The effects of leaf pubescence on leaf energy exchange and transpiration were evaluated for Espeletia timotensis, a caulescent giant rosette plant that occurs in the paramo zone of the Venezuelan Andes at elevations up to 4500 m. Computer simulations using standard energy balance equations and field and laboratory measurements showed that the thick leaf pubescence (up to 3 mm) modifies energy balance mainly by increasing boundary layer thickness and resistance to convective and latent heat transfer. Effects of the pubescence on solar radiation absorption are minor compared to its influence on boundary layer resistance. The degree of coupling between leaf temperature and incident solar radiation is thus greater in pubescent than in nonpubescent leaves, despite the greater absorptance to solar radiation of the later. Vertically oriented expanding leaves, which have the thickest pubescence, also have higher stomatal resistance than the other leaves in the rosette. The higher stomatal resistance enhances coupling between leaf temperature and incident radiation. Pubescence—induced increases in leaf temperature of 5°C or more can be attained without significant increases in transpiration if stomatal resistance is maintained at the proper levels. These features may be advantageous in the native habitat of E. timotensis where incoming radiation can reach a high level but mean air temperature is °3°C and maximum air temperatures are usually below 10°C. Process of assimilate translocation and leaf growth, which normally would be severely inhibited at prevailing environmental temperatures, may be favored by the higher leaf temperatures. The ability to increase leaf temperature while at the same time avoiding increases in transpiration is also of adaptive importance, since water availability in the habitat of E. timotensis is often limited by low temperatures.