CSIRO PUBLISHING - Australian Journal of Botany

Studies on Leaf Characteristics of a Cline of Eucalyptus urnigera From Mount Wellington, Tasmania. I. Water Repellency and the Freezing of Leaves

DA Thomas and HN Barber

Abstract

A cline in Eucalyptus urnigera is described in which waxy glaucousness of the surfaces of leaves increases with altitude up Mount Wellington. Other leaf characteristics, such as adult shape, antho- cyanin content of young leaves, and age of transition to adult leaf form, may also vary.

If the leaves of E. urnigera with a dry surface are cooled, they freeze at a temperature of -8 to -10°C. If the leaves are wet, they freeze at -2 to -4°. Such freezing always kills the leaves, whilst leaves which have supercooled below the true freezing point (-2 to -3°) escape damage. It is suggested that the ability of dry leaves to supercool may explain part of the selective advantage possessed by the glaucous water-repellent leaves of E. urnigera at high altitudes as compared with the green wettable leaves of plants at low altitudes. This selective advantage of water repellency is probably of ecological significance in protecting leaves from freezing when there are rapid falls in the ambient temperature below c. -2 to -3°, either as radiation frosts or as short periods of snow after plant growth has started in the spring. Introduction