The factors controlling cuticular transpiration of timberline trees during winter were examined to assess their relative influence on the desiccation of foliage above winter snowpack. Transpiration was calculated as a function of cuticular resistance, wind speed, absorbed radiation, air temperature, and relative humidity, which were varied among values common to temperate timberlines during winter. Transpiration rate was strongly influenced by cuticular resistance; this supports the hypothesis that excessive winter transpiration is due in part to inadequate cuticle maturation at treeline. The daytime environment in late winter also greatly promoted water loss. It is hypothesized that wind influences the physiognomy of timberline trees by mechanisms other than its direct effect on transpiration.