Ecology of the silversword, Argyroxiphium sandwicense DC. (Compositae), Haleakala Crater, Hawaii

Abstract

Argyroxiphium sandwicense DC. is an endemic composite of the alpine zone in Hawaii. Western man and his herbivores have restricted its present distribution mostly to the cinder cones and lava flows within Haleakala Crater, Maui. Here the silversword is able to sustain a high regeneration rate under dynamic substrate conditions that eliminate all but a few exotic and endemic species. Long-term field measurements of environmental parameters, and growth cabinet experiments on germination and seedling survival established the following optimal conditions. Germination and seedling survival depend on a temperature not exceeding about 30°C, and the relatively high moisture content of a stable sandy substratum completely covered by cinder fragments no thicker than about 5 cm. The maintenance of the thin complete cover over an area larger than a hectare for a period of a few decades, is best met by a shelf of agglutinates supplying tabular fragments which slide over a sloped sandy germination layer. The slope angle is about 35 degrees; low enough to stabilize the sandy layer, yet steep enough to be slightly unstable for tabular fragments. A thin cover is maintained by removal of fragments at the foot of the slope by wind and water during winter storms. Vandalism and browsing are probably not important under the protection of the Park Service, but root breakage by trampling may become a problem with further increase in visitors. Seed damage by insects may not be important as previously reported, but confirmation awaits further investigation during a good flowering season.