Behavior of native and non-native plants in two tropical rain forests on Oahu, Hawaiian Islands
Gerrish, Grant; Mueller-Dombois, Dieter
Factors governing the establishment and distribution of non-native (exotic) plant species in the Metrosideros spp. rain forest of the Koolau Mountains (island of Oahu, Hawaii) were investigated in two climatically similar areas. One study area was located on Mt. Tantalus, immediately adjacent to the city of Honolulu; the other was located at Pupukea, about 24 miles northwest of Tantalus. All vascular plants within 400 m2 relevés (16 at Pupukea, 18 at Tantalus) were recorded with estimates of their biomass cover. Floristic data from the two study areas were compared by multivariate analysis techniques. It was found that while some exotic species are common to both study areas, many are exclusive to one or the other area; and the exotic species are of greater importance in the Tantalus study area in terms of numbers of species and biomass cover. The hypothesis that the greater importance of exotic species in the vegetation of Tantalus may be due to the proximity to Honolulu was supported by the observation that many of the exotics there are ornamental or garden plants that have escaped from homes and gardens in the city. However, the finding that some exotic plant species occur in the Pupukea study area, but not on Tantalus, indicates that factors other than proximity to Honolulu are involved. This suggested as a second hypothesis that species distributions within this rain forest are controlled by soil characteristics. Analyses showed the younger soils of Tantalus to be significantly more fertile than the highly weathered soils of Pupukea. At Pupukea, the very low concentrations of bases, the near absence of available phosphorus, and the low soil pH make Pupukea a high stress environment. The greater abundance of native plants at Pupukea suggests that these native species are better adapted to these extreme soil conditions and are not easily displaced by exotics. The observed distribution patterns of most of the exotic species indicate that they are plants of early succession and, as such, do not threaten native species. Detrimental effects associated with the presence of these exotic species can be more directly attributed to the man-induced disturbances which allow these species to become established. Distribution patterns of several exotic species indicate that they may be altering the vegetation structure over large areas in a manner that may reduce the quality of the habitat for endemic plants.