Biology Reference
In-Depth Information
of the Great Basin, however, native communities have been replaced with
B. tectorum
grasslands. In these areas management is focusing on restoration of native commu-
nities that will be resistant to further invasion by the introduced grasses.
7.2
Case Study I: Hawaiian Dry and Submontane
Seasonal Environments
7.2.1 Study System
The Hawaiian Islands are characterized by large environmental gradients driven by
the prevailing trade winds and the volcanic shield topography. Dry forests and
shrublands exist on the leeward side of the high islands while wet forests occur on
the windward sides of the high islands. In contrast to the strongly varying microcli-
mates, soil chemistry across the islands is relatively constant with all soils ultimately
deriving from basaltic lava or ash. The soils are typically nitrogen limited when
young, colimited by nitrogen (N) and phosphorus (P) at intermediate ages (10,000-
100,000 years), and P limited on the older surfaces (Vitousek 2004).
Grasses from other regions of the world have invaded virtually all of the many
microclimates in Hawaii from dry coastal terraces receiving <20-cm precipitation
to sites with >3 m of rainfall annually. A detailed list of grass invaders is not availa-
ble but most species are listed in the flora of Hawaii (Wagner et al. 1999).
7.2.2 Grass Invasion Impacts
While introduced perennial grasses are widespread throughout the Hawaiian
Islands, those that have invaded dry shrubland and open woodland habitats appear
to be causing the most dramatic changes because they change the continuity and
density of fuels (Mueller-Dombois 1981; Smith 1985; Smith and Tunison 1992).
The most ecologically significant grass invaders on Hawai'i Island include species
from both the new and old world. Three of these are (1)
Pennisetum setaceum
(fountain grass), a perennial bunchgrass from Africa, (2)
Melinis minutiflora
(molasses grass), a mat-forming grass from Africa, and (3)
Schizachyrium conden-
satum
(bushy beardgrass), a perennial bunchgrass from Central America.
P. seta-
ceum
has invaded the leeward side of Hawaii Island from sea level up to 3,000 m.
It competes with native vegetation (Cabin et al. 2000, 2002) and regrows after fire.
In mid-elevation environments that have experienced fire, native species richness
has declined due to the lack of ability of native species to compete with
P. setaceum
and their relatively slow regrowth after fire (Shaw et al. 1997). Although
P. seta-
ceum
is an ecologically important invader and a large new project has been initiated
to assess ways to reduce its impact (S. Cordell, Institute of Pacific Island Forestry,
