Pacific Science (1992), vol. 46, no. 2: 197-220 © 1992 by University of Hawaii Press. All rights reserved Vegetation of the Wet Windward Slope of Haleakala, Maui, Hawaii1 KANEHIRO KITAYAMA AND DIETER MUELLER-DOMBOIS2 ABSTRACT: The vegetation on the wet windward slope of Haleakala was studied for community organization along a transect between 350 m a.s.l. and the summit (3055 m). The plant communities classified by the Braun-Blanquet synthesis table technique showed a hierarchical arrangement and were correlated with altitude. First, the forest and the treeless vegetation were differentiated by two major species groups. The boundary between the two was coincident with the trade wind inversion (ca. 1900 m a.s.l.) where the wet, low to mid-altitudinal climate changed abruptly upslope to an arid high-altitude one. These two wide-ranging vegetation types were subdivided into three units, corresponding to three broad altitudinal zones: the lowland, the montane, and the high-altitude zones. The three units were further partitioned into seven plant communities, which indicated six altitudinal subzones and one dieback belt. The floristic composition of the communities, the community structures, and their environ­ mental relationships are briefly described with a summarized differential table. The depauperate and disharmonic nature of the Hawaiian flora is reflected in such altitudinal patterns as the low species turnover and the depressed forest line. THE HAWAIIAN ISLANDS, biogeographically the wetter Hawaiian habitats (Mueller­ the world's most isolated archipelago, sup­ Dombois 1987). port high endemism in their flora (956 flower­ The monodominance of M. polymorpha ing plant species with 89% endemic; Wagner seems to be a factor in the recurring phenome­ et al. 1990). The extreme isolation has acted non of canopy dieback (Mueller-Dombois as a sieve allowing only a limited number of 1986, 1987). Metrosideros polymorpha, being species to cross the ocean. As a result, the flora shade-intolerant in the sapling stage (Burton has become disharmonic (Hubbell 1968). For and Mueller-Dombois 1984), depends on instance plants with larger disseminules are canopy openings to maintain its regenera­ not common in the native inland forests in tion. Consequently, episodic stand-level Hawaii. The flora is also depauperate, and the regeneration in response to canopy dieback of number of potential canopy species is low M. polymorpha seems to have worked as a (Mueller-Dombois 1987). mechanism for successive generations. The The taxonomic disharmony and relative process of dieback has been discussed as biotic impoverishment resulted in widespread primarily a demographic phenomenon of monodominance of the native rainforests by cohort senescence, interacting with abiotic Metrosideros polymorpha (Mueller-Dombois stresses (Mueller-Dombois 1988a). 1981a). This is a myrtaceous tree species with We also suspect that the monodominance capsulate fruits, which produce very small is manifested in reduced interspecific competi­ wind-dispersed seeds. The species dominates tion. Therefore, some distributional attributes of plant communities that are released from high species competition may be displayed on 1 This project was supported in part by a grant from the slopes of the Hawaiian high mountains. the National Tropical Botanical Garden. Manuscript accepted for publication 2 May 1991. Investigations have been done to compare 2 Department of Botany, University of Hawaii at species turnover along mountain transects. Manoa, Honolulu, Hawaii 96822. One of the analyses relates to Haleakala, 197 198 PACIFIC SCIENCE, Volume 46, April 1992 Maui, an isolated oceanic island mountain, Conservancy; and the summit area (from 2100 the other to Mt. Kinabalu, Borneo, a species­ to 3055 m) in Haleakala National Park. Cur­ rich continental island mountain. rently, the vegetation is relatively well pro­ In this paper, we present some results ofthe tected. The summit area has been severely Haleakala transect study. The following ques­ influenced by feral ungulates, particularly by tions guided this study: (I) Does a depaupe­ goats (Stone 1985). But control efforts have rate flora become organized into altitudinally supressed their activity. Feral pigs are the definable communities? (2) If definable, are current major disturbance factor in the wetter such altitudinal communities broader and forests (Stone 1985). The vegetation below fewer in number than in floristically richer 350 m a.s.l. has largely been converted to areas? (3) How are the plant distribution plantation forests. patterns related to the altitudinal environ­ Widespread forest dieback was noted in the mental gradient? lower segment of the transect and adjacent areas early in this century by Lyon (1909). Study Area Holt (1988) reassessed the same area. The vegetation of the summit crater was mapped The northeast slope ofHaleakala (3055 m), by Whiteaker (1983). Maui, exposed to the prevailing trade winds, was selected for the study. Haleakala is a Climate shield-shaped volcano of early Pleistocene origin (0.8 million yr) and now quiescent. Itis There are great changes in climate over the third highest mountain in Hawaii (after short distances along the transect, largely due Mauna Kea, 4205 m, and Mauna Loa, 4169 to two factors: the altitudinal reduction ofair m). The summit, located at 20° 45' Nand 156° temperature and the midslope increase in 15' W, has a huge caldera (12 km long, 4 km cloudiness. The climate of the windward low­ wide) with cinder cones on its floor and land, classified as Af in Koppen's system exposed pyroclastic materials on the outer (Koppen 1936), is warm-tropical and per­ walls. The northeast slope is covered with soils humid year-round. The summit climate, derived from recent (late Pleistocene) volcanic which may be classified as Cs in Koppen's rocks of the Kula volcanic series. There are system, is cool-tropical with a dry summer also still younger (Holocene) rocks from the season. Hana volcanic series. The study area is located The mean annual air temperature at the on the Kula volcanic series. The parent rock Kailua meteorological station (213 m a.s.l.), is largely from alkalic basalt (Stearns 1985). which is near the low end of the transect, is The topography of the study area consists 21SC (Figure 2). The mean monthly tem­ of undulating gentle slopes (generally less perature is 22.9°C in the warmest month than 8°), dissected by numerous streams run­ (August) and 20.l oC in the coldest month ning parallel to one another downslope. The (February). This indicates a maritime temper­ lateral dissections become steeper and wider ature regime, characterized by a small annual near the coast, where they form deeply sliced, change (2.8°C). However, the diurnal fluctua­ V-shaped valleys. tion is nearly 10°C. Temperatures generally The study area is a belt transect, I km wide. decrease upslope in accordance with the lapse It starts at 350 m a.s.l. near Kailua and rate of 0.55°C on Mauna Loa, Hawaii (Blu­ extends upward on the interfluves of either menstock 1961). However, Haleakala's up­ side ofWaikamoi Stream to the summit (Fig­ slope temperatures may divert from this lapse ure I). The belt transect traverses three pro­ rate because ofdifferences in cloudiness. Tem­ tected areas: a watershed forest (from 350 to perature increases sharply upslope at the trade 1600 m), managed by the East Maui Irrigation wind inversion because ofdescending dry air. Company; the Waikamoi Preserve (from 1600 Results of a short-term measurement in June to 2100 m), managed by the Hawaii Nature 1988 (Figure 2) indicate the presence of the Vegetation of Windward Ha1eaka1a-KITAYAMA AND MUELLER-DoMBOIS 199 o o 160 W 155 W •Kauai Haleakala Transect Oahu ... / ,~ 41 Maui Hawaii HAWAII N o 5km '---' 1 FIGURE 1. Location of the transect established on the windward slope of Haleakala, Maui, Hawaii. inversion at 1900 m with a sharp temperature the mean monthly air temperature is ca. 6°C increase by 5°C. The inversion fluctuates, in the coldest month, and slightly exceeds but most frequently appears between 1900 lOoC in the warmest month. and 2000 m (Mendonca and Iwaoka 1969, Noguchi et al. (1987) estimated the occur­ Noguchi et al. 1987). At the summit (3055 m), rence of ground frost to be 187 days per year 200 PACIFIC SCIENCE, Volume 46, April 1992 Air Temperature (OC) Annual Rainfall (x 1,000 mm) 30,----------------------------------, 7.5 ................................................................... 1"".25 25 20 5.0 temperature maxima June 1988 15 .75 temperature lapse rate 0.550 C/100m 10 / 2.5 1.25 5 mean annual rainfall o o +------r---------,-----,-------,r------,--------~---,-J o 500 1000 1500 2000 2500 3000 Altitude (m) KAILUA 21.5·C SUMMIT 2131R 2.985l1t11 3.055. 1.000.11I 5 •• 300 3•• '0. t•• I. I. I. I. o. 2. o. 2. •• 2• J J 0 J J o FIGURE 2. Climate along the transect. The climate diagram of Haleakala's summit is shown with monthly mean daily maximum and minimum air temperatures. The air-temperature lapse rate, 0.55°CjI00 m, is a theoretical value. The air temperature maxima measured in June 1988 indicate an inversion at 1900 m. at the summit. They also found evidence of The moisture regime ofthe northeast slope ground frost as low as ca. 2700 m a.s.l. This of Haleakala is largely controlled by the elevation coincides closely with the ground­ northeast trade winds and the trade wind frost line of Mauna Loa found by Mueller­ inversion (Lyons 1979). The orographic uplift Dombois (1967). of the trade winds results in high rainfall Vegetation ofWindward Haleakala-KITAYAMA AND MUELLER-DoMBOIS 201 below the inversion. Rainfall rapidly increases RESULTS upslope, from 4000 mm at 350 m a.s.l. to a maximum mean annual amount of 6500 mm General Description ofthe Soils at ca. 1000 m a.s.l. (Giambelluca et al. 1986; Figure 2). A dry area occurs above the inver­ The soils below the inversion are wet and sion because clouds are prevented from mov­ histic.