INVASIBILITY OF TREE PLANTATIONS BY NATIVE AND NON-INDIGENOUS PLANT-SPECIES IN HAWAII

The Hawaiian archipelago, the most isolated on Earth, has proven to be especially vulnerable to invasions by non indigenous species. Alien s pecies now outnumber natives and the threat they pose is in part respo nsible for the fact that approximately 25% of Hawaii's native flora, 9 0% of which is endemic, has been listed as threatened or endangered. T o assess the effect of stand characteristics on the colonization of th e plantations by alien and native species, we measured basal area, lea f area index (LAI), litter depth and standing litter mass in 26-to 32- year-old plantations of Eucalyptus saligna, Flindersia brayleyana and Fraxinus uhdei. The plantations are surrounded by native Hawaiian rain forest which has been invaded by numerous non-indigenous species. The basal area of the planted Flindersia was 45.2 m(2)/ha, three to four t imes that of the other two plantation species. Nevertheless, when colo nizing species were included, total stand basal area and LAI did not d iffer significantly across the three plantation types. Litter depth ra nged from 3.7 cm for Flindersia to 4.7 cm for Eucalyptus, and litter m ass ranged from 7.4 Mg/ha for Fraxinus to 10.5 Mg/ha for Eucalyptus. O f 51 species identified in the plantation understories, 23 (45%) were common to all three plantation species. Eucalyptus plantations had 42 species, seven of which were uniquely found there, Fraxinus had 39 spe cies, six of which were unique and Flindersia had 36 species, only one of which was unique. The most abundant understory species under Fraxi nus were Cibotium glaucum and Metrosideros polymorpha, two dominant co mponents of native Hawaiian rainforest. In contrast, the most abundant understory species in the Eucalyptus plantations was the alien tree, Psidium cattleianum and the understory of Flindersia was dominated by its own offspring. Average species richness (number of species per plo t) of both native species and all species combined was significantly g reater under Fraxinus than under the other two plantation species, but all plantations contained approximately equal numbers of non-indigeno us species. Average density (number of individuals/m(2)) was greatest under Flindersia, although the density of native species alone was gre atest under Fraxinus. Species richness and plant density were not corr elated with litter depth or mass, but total species richness and densi ty of native species did decrease with increasing LAI. In general, abu ndance of native plants in the understory decreased with increasing ab undance of non-indigenous species and increasing basal area of plantat ion species. (C) 1997 Elsevier Science B.V.