Ra. Harrington et Jj. Ewel, INVASIBILITY OF TREE PLANTATIONS BY NATIVE AND NON-INDIGENOUS PLANT-SPECIES IN HAWAII, Forest ecology and management, 99(1-2), 1997, pp. 153-162
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.