— From the book Remains of a Rainbow by David Liittschwager and Susan Middleton
Biological Communities of the Leeward Haleakala Watershed
Over 90% of terrestrial and freshwater species and 24% of marine species in Hawaiʻi are found nowhere else in the world. This uniqueness, however, means that once a species is lost in Hawaiʻi, it is probably gone forever. Hawaiʻi has approximately 25% of endangered species in the United States. Numerous rare native plant species occur or did occur on leeward Haleakalā. For some of these, the watershed forests are their sole natural habitat and hence the only realistic hope for their survival in the wild. Efforts to restore these forests have been identified as priority tasks in the U.S. Fish and Wildlife Service Recovery Plan for Hawaiian Forest Birds, and specifically as a top priority for the recovery of the highly endangered Maui parrotbill. Although some these native communities have disappeared, there is slight glimpse into the past in what remains which inspires us to continue our restoration efforts.
Alpine Rockland Subzone
The Alpine Rockland subzone above about 8,000 feet on Haleakalā has very sparse vegetation due to the combination of harsh climatic and soil properties. This zone corresponds to Whiteaker’s High-Altitude Desert Communities of Haleakalā. The boundary between subalpine and alpine in Hawaii as elsewhere is generally agreed to be the timberline (Fosberg 1959). Sophora chrysophylla (mamane) occur as high as 9,000 ft. on Haleakalā’s west slope. Above 8,000 ft., few trees on the south slope surpass 3 m in height. This subzone has probably been less modified by browsing ungulates than others, partially due to inherently scarcity of vegetation, and introduced plant species are less apparent here than elsewhere.
Subalpine Shrubland Subzone
The Subalpine Shrubland Subzone (6,000-8,000 ft) of Haleakalā generally lies above the temperature inversion layer. The lower portion of the zone is often immersed in clouds and is substantially more moist. Santalum haleakalae and Metrosideros are sporadic. Native shrubs of this zone have been resilient to long-term browsing pressure, possibly due to the leathery foliage of certain species and the ability of many of them to reproduce vegetatively. Ungulates, specifically goats and feral pigs have disrupted the native grass cover.
The Montane Shrubland subzone occurs between 4,000 and 6,000 ft on relatively poorly weathered Hana lavas and is dominated by Dodonaea viscosa (aʻaliʻi), Stypelia tameiameiae (pūkiawe), with scattered individuals of Metrosideros polymorpha (ohia). Feral goats have apparently greatly altered the vegetation. There may be little justification for separating this subzone from Subalpine Shrubland since most species are common to both. The abundance of Osteomeles anthyllidifolia (ulei) is it’s best distinguishing features.
The Koa forest (4,000-6,000 ft) is dominated by Metrosideros polymorpha (ohia) and Acacia koa (koa) and occurs on relatively well-weathered Kula series lavas. Median annual rainfall is approximately 1,000-1,200 mm (40-50 in., State of Hawaii, 1982). Acacia tends to occur on ridges adjacent to the gulches. Metrosideros dominates near the upper forest line. Acacia seems to have been particularly hard hit in browsing because of the tendency of browsing animals to move along ridgetops. Neither Acacia nor Metrosideros is reestablishing significantly. Although Acacia koa reproduces both vegetatively and by seedlings, goats and cattle eliminate new growth. Where forest is open, introduced grasses, especially Sporobolus africanus (kikuyu grass), dominate the understory. Gulch ecosystems have not felt the full impact of browsing and introduced grasses until recently and comprise some of the most interesting native south slope vegetation.
Dry Forest Shrubland Zone
Dry forests of the south slope extend from just above sea level up to 4,800 ft., rarely higher. This area is particularly rich in native tree species, some of which are narrowly distributed, while others are broadly distributed (e.g., Chamaesyce celestroides (akoko), Myoporum sandwicense (naio). The native vegetation of this area has been nearly completely replaced by introduced species-predominantly Prosopis pallida (kiawe) and range grasses – except in gulches and in rocky areas where occasional relicts of native communities persist. Flowering plant species characteristic of this subzone include: Charpentiera obovata (papala), Chamaesyce celestroides, Myrsine lanaiensis (kolea), Nestegis sandwicensis (olopua), Ochrosia haleakalae (holei), Pouteria sandwicensis (ala’a), Pleomele auwahiensis (Halapepe), Santalum freycinetianum auwahiense (‘iliahi), Streblus pendulinus (aʻiaʻi), Tetraplasandra oahuensis (‘ohe mauka), Xylosma hawaiiense (maua), Zanthoxylum kauaense (aʻe), and Zanthoxylum hawaiiense.