Microclimatological Investigations in the Tropical Alpine Scrub of Maui, Hawaii: Evidence for a Drought-induced Alpine Timberline

Micrometeorological measurements were made in the lower alpine zone of Mt. Haleakala on Maui, Hawaiian Islands, in March 1988 to characterize ecologically significant climatological parameters. Daily courses of photosynthetically active and total net radiation; temperatures of air, soil, and plant canopy; wind speed; air humidity; leaf wetness; and precipitation were recorded at an elevation of 2100 m in alpine scrub slightly above the timberline. A battery-powered data-logging system was used, which gave high temporal resolution. Influence of variable cloud cover on microclimate of the study site was evaluated on five selected days with highly differing weather conditions. Based on comparison with data from other high mountains of the humid tropical zone, it is concluded that the alpine timberline on Maui is caused by a complex of factors. Plant water availability is probably the dominating one; temperature seems to be of lesser importance. The possible role of other important factors is discussed. The extraordinary microclimatological conditions of the alpine zone of Maui are examined in the context of the atmospheric circulation system in the region of the Hawaiian archipelago