Invasibility of tree plantations by native and non-indigenous plant species in Hawaii

Abstract

The Hawaiian archipelago, the most isolated on Earth, has proven to be especially vulnerable to invasions by non-indigenous species. Alien species now outnumber natives and the threat they pose is in part responsible for the fact that approximately 25% of Hawaii's native flora, 90% of which is endemic, has been listed as threatened or endangered. To assess the effect of stand characteristics on the colonization of the plantations by alien and native species, we measured basal area, leaf 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 rainforest 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 times that of the other two plantation species. Nevertheless, when colonizing species were included, total stand basal area and LAI did not differ significantly across the three plantation types. Litter depth ranged from 3.7 cm for Flindersia to 4.7 cm for Eucalyptus, and litter mass ranged from 7.4 Mg/ha for Fraxinus to 10.5 Mg/ha for Eucalyptus. Of 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 species, six of which were unique and Flindersia had 36 species, only one of which was unique. The most abundant understory species under Fraxinus were Cibotium glaucum and Metrosideros polymorpha, two dominant components 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 plot) of both native species and all species combined was significantly greater under Fraxinus than under the other two plantation species, but all plantations contained approximately equal numbers of non-indigenous species. Average density (number of individuals/m 2) was greatest under Flindersia, although the density of native species alone was greatest under Fraxinus. Species richness and plant density were not correlated with litter depth or mass, but total species richness and density of native species did decrease with increasing LAI. In general, abundance of native plants in the understory decreased with increasing abundance of non-indigenous species and increasing basal area of plantation species.