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A vegetation map of the Crater District of Haleakala National Park was produced at a scale o f 1:24,000 which can be used as an overlay of the United States Geological Survey (USGS) topographic quadrangle maps. Fifty-three structural-floristic communities were mapped which were grouped into four structural vegetation-types (forest, scrub, grassland, and high altitude desert). Areas were calculated for each community using an electronic planimeter. The total area mapped was 7544.8 ha (18,643 acres). Topographic vegetation profiles were constructed which show changes in vegetation-types in relation to climatic gradients. Also, matching correlations were observed between certain substrates and community-types. Phytosociological analyses of releve (vegetation sample) data by the synthesis table technique and the dendrograph technique resulted in ecologically meaningful groupings. Both analyses produced similar groupings though detailed comparison of the results of the two methods revealed interesting minor variations. Some releves were left ungrouped. These were interpreted as unique community types within the sampling area. The hypothesis that the community-types that are characteristic of other tropical alpine and subalpine ecosystems occur in the Crater District of Haleakala National Park was partially supported by the map units and phytosociological analyses of the releve data. Ericaceous (pukiawe-type) scrub, tussock grassland, and high a1titude desert occur as mappable vegetation units. Only arborescent and rosette life-forms did not occur as mappable units. However, a rosette life-form, Argyroxiphium sandwicense DC. (silversword), does occur in the study area and may have had a wider and more abundant distribution in the past. The hypothesis that the vegetation map of the Crater District of Haleakala National Park has similar vegetation units and vegetation-types as those mapped by Mueller-Dombois and Fosberg (1974) for the tropical alpine and subalpine ecosystem on the slopes of Mauna Loa in Hawaii Volcanoes National Park was supported by a comparison of the two maps. Variation of the vegetation within the study area was associated with variations in climate, substrate, mechanical influences, and the effects of exotic plant species. Climate diagrams confirmed the tropical high mountain character of the study area, while they also illustrated the considerable variation of climate within the Crater as well as its seasonal variation. It was concluded that the diurnal (daily) frost boundary, as indicated by the vegetation, should be used to define the lower limit of the alpine zone in tropical high mountain areas. This conclusion implies that the subalpine zone be defined as those are as below the diurnal frost boundary and above the montane forests and grasslands.