XVI International Botanical Congess
Winter desiccation of needles is common at alpine timberline and because stomata are assumed to be closed, water loss is thought to be through cuticular transpiration. Picea engelmannii Parry ex Engelm. and Pinus aristata Engelm. shoots were collected during winter at timberline in Colorado, and the cuticular conductance of needles was measured in vitro. For both species the cuticular conductance was significantly reduced by cold (temperatures as low as -24°C), dry (relative humidities as low as 3%) conditions. These results suggest that current models of winter desiccation that use values of cuticular conductance measured for excised needles in ambient laboratory conditions may overestimate cuticular water loss.