In northern Australia, concern has been raised about the impacts
of introduced grass species, in particular Andropogon
gayanus (gamba grass). Gamba grass is a perennial species of
African grass that was introduced in the Northern Territory as a
replacement for native pastures, however gamba grass is now
invading savanna ecosystems throughout the Top End (Howard 2002).
This invasion could have significant consequences for native
communities, as well as ecosystem function and stability,
comparable to the dramatic effects of invasion documented in other
ecosystems (D'Antonio & Vitousek 1992, Vitousek et al. 1996).
This is of particular concern as gamba grass invasion has the
potential to alter all three determinants of savanna functioning:
nutrient and water availability and fire regimes. However, at
present these consequences are largely unknown, as there is little
information on the ecosystem effects following the replacement of
native grasses with gamba.
The objective of this study is to quantify the changes in
ecosystem processes that occur following the establishment of gamba
grass. This project will use a comparative approach to determine
how the invading grass may alter:
One of the most significant changes in resource dynamics caused
by invading grasses is the alteration of the nitrogen cycle (Evans
& Belnap 1999). Exotic grass invasion can lead to significant
decreases in soil nitrogen (N) availability (Ibarra-Flores et al.
1999, Evans et al. 2001). In turn, alteration in the availability
of nitrogen may drive changes in species composition (Tilman 1982),
creating the potential for a feedback cycle to emerge, where an
invader may alter species composition and modify the ecosystem
controlling community dynamics.
Nitrogen availability is a major constraint for plant growth in
savannas (Medina & Silva 1990, Solbriget al. 1996). Plants
generally respond by producing biomass with lower nutrient
concentration. However, exotic grasses tend to preferentially
allocate nitrogen to the production of leaves (Baruch 1996). It has
been suggested that exotic grasses in Australia’s savannas
require more nutrients than native grasses in order to achieve
their higher growth rates and that they seem to use nitrogen more
efficiently (Baruch et al. 1985, Bilbao & Medina 1990). The
higher biomass in communities invaded by exotic grasses indicates
that the amounts and rates involved in nutrient cycling should be
higher than in native savannas. It is possible that the high
nitrogen requirements of the exotic grasses may deplete the soil
nitrogen reserves in the savannas. Exotic grasses may also affect
savanna nitrogen cycling by influencing the chemistry and quantity
of litter entering the soil organic matter (SOM) pool, and the
microclimate conditions in which decomposition occurs (Mack et al.
2001). The alteration in N availability may be further compounded
by the effect of exotic grass on fire regimes and the potential for
significant nitrogen loss via volatilisation, due to frequent,
intense fires.
Exotic grass also has the potential to drastically alter fire
regimes (D'Antonio & Vitousek 1992). Exotic grass invasion can
initiate a positive feedback cycle with fire, causing a decrease in
tree cover and the conversion of forests and woodlands to
grasslands. D'Antonio and Vitousek (1992) termed this process the
grass–fire cycle. It has been suggested that exotic grass
invasion could be creating a grass–fire cycle in the savannas
of northern Australia (Bowman 1999, Russell-Smithet al. in review).
Particular concern has been expressed that invasion by gamba grass
could be altering the fire regime characteristics of the savannas,
promoting intense, late dry season fires (Williams et al. 1997,
Bowman 1999, Russell-Smith et al. in review). Gamba grass is a
highly productive grass, which significantly increases the fuel
load in savannas, producing up to 10 times the biomass of native
grasses (Barrow 1995, Bowman 1999, Howard 2001, 2002). It grows in
excess of 4.5 m high, and unlike native grasses, it remains upright
throughout the fire season, creating a tall standing fuel load.
Furthermore, anecdotal evidence suggests that gamba grass dries
later in the dry season than native grasses. In areas invaded by
gamba, this leads to a high fuel load in the mid to late dry
season, which has the potential to produce intense, late-dry season
fires. Others have speculated that the high fuel loads of gamba
grass can build up in a single growing season, supporting fires
every year, and sometimes twice a year (Cook 1991).
Such changes in fire regimes have the potential to initiate a
grass–fire cycle on a broad ecological scale in the savannas
of northern Australia. Fire intensity in the early dry season in
savannas invaded by gamba grass is up to 12 times higher than that
in native grass savannas (Rossiter et al. 2003). However, this
study was based on only limited observations, and there is an
urgent need for a more comprehensive study of the effect of the
effect of exotic grass invasion on the prevailing fire regimes in
the savannas of northern Australia.
An understanding of savanna ecosystem processes such as fire
regimes, and nutrient, cycling is fundamental to understanding
savanna function. Such information is vital to ensure the
sustainable management of Australia’s tropical savannas.
Native grasses have been identified as important elements of the
savanna ecosystems, as they are significant sources of carbon
(Eamus et al. 2001) and water (Hutley et al. 2000) flux in savanna
ecosystems and they contribute largely to the existing fire regimes
in the savannas. Furthermore, nutrient and water availability and
fire regimes have been identified as the three key determinants of
savanna function (Solbrig et al. 1996) and any alteration to these
ecosystem processes due to invasion is likely to have significant
long-term ecological consequences.
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