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 oflesser 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. 1 Manuscript accepted 20 July 1990. 2 Systematisch-Geobotanisches Institut, Universitat Gottingen, Untere Karspiile 2, D-3400 Gottingen, West Germany. middle of the Pacific Ocean represent a marked exception found perhaps nowhere else on earth. On the islands of Hawaii and Maui alpine ecosystems evolved under the influence of highly oceanic surroundings combined with pronounced daytime-nighttime temperature fluctuations and weak seasonal temperature changes typical for the humid tropics. Comparable ecological conditions could feasibly be expected on Reunion in the Indian Ocean, with its Piton des Neiges (3069 m height), and, with some restrictions, on Tenerife (Pico de Teide: 3718 m [see Walter 1968, Hollermann 1978]). Microclimatological and plant ecological studies in the tropical alpine zone have been conducted in the Andean Paramos (e.g., Troll 1968, Walter and Medina 1969, Vareschi 1970, Lauer 1973, Larcher 1975, Cleef 1978, Sturm 1978, Cuatrecasas 1979, Monasterio 1980, Halloy 1989), the afroalpine region (e.g., Hedberg 1951, 1964, Salt 1954, Coe 1967), and in the Southeast Asian islands (e.g., van Steenis 1964, Lowry et al. 1973, Smith 1977a,b, 1980, Korner et al. 1983). Excepting investigations on subtropical Tenerife 152 TROPICAL ALPINE ECOSYSTEMS HAVE three geographical characteristics: their nearness to the equator, their high elevation above sea level, and their occurrence in a pattern of disjunct or highly isolated small patches in the tropical belt. A review of the field of tropical alpine plant ecology is given by Smith and Young (1987). Most mountain systems of the humid tropics that reach the timberline are situated on the large continents of South and Central America and Africa, as well as on several extended islands of Southeast Asia (e.g., New Guinea and Borneo) that are more or less subject to the "Massenerhebungseffekt" (mass elevation effect) of larger land masses, with its influence on vertical temperature profile, cloud formation, and, consequently, vertical zonation of vegetation (Troll 1959, Grubb 1971). In this context, the high volcanoes of the Hawaiian archipelago in the Tropical Alpine Scrub of Maui-LEuscHNER AND SCHULTE 153 (e.g., H611ermann 1978), no detailed measurements of ecologically significant microclimatological factors have been conducted in the alpine zone of tropical oceanic islands. On the island of Hawaii special interest has been paid to fog ("horizontal") precipitation in the montane and subalpine region (Mordy and Hurdis 1955, Ekern 1964, Juvik and Perreira 1974, Juvik and Ekern 1978). Furthermore, Mueller-Dombois (1967) and Mueller-Dombois and Krajina (1968) investigated the occurrence of nocturnal frost at different elevations of Mauna Loa and Mauna Kea on Hawaii. This paper presents the results of a microclimatological study done in March 1988 in the lower alpine zone of the trade windintercepting slopes of Mt. Haleakala on the island of Maui (Hawaii).