Agriculture Reference
In-Depth Information
more frequent fires because of their high flammability (Baird
1977
). A similar
pattern of sensitivity of native resprouting Proteaceae to frequent fires has been
documented in mediterranean-type vegetation, including both shrublands and
woodlands, of New South Wales (Bradstock & Myerscough
1988
).
The ability of invasive perennial grasses and herbs to alter the structure and
function of
Banksia
woodlands has been well documented in a series of studies
focusing on South African
Ehrharta calycina
and
Pelargonium capitatum
as
examples of invaders (Fisher
et al.
2009a
,
2009b
). These two plants represent
growth forms unlike those typically present in the understory of
Banksia
woodlands. Once established from propagules in sites with high fire frequency
these resprouting species change the understory dominance from native reseeding
species to alien resprouting species. The extensive root system of
E. calycina
provides this species with advantages over native species in competing for limited
water and nutrient resources. Studies have shown that
E. calycina
is very competi-
tive postfire but not a strong competitor in intact bushland (Baird
1977
). Rapidly
growing herbaceous species, with
Pelargonium capitatum
as a well-studied
example, have been shown to interfere with the success of slow-growing native
seedlings (Hobbs & Atkins
1991
). Once established, flammable grasses such as
Ehrharta
provide a positive feedback on increasing fire frequency, which results,
with short fire intervals, in eliminating native species that do not have adequate
fire-free intervals to reproduce and replenish canopy- and soil-stored seedbanks,
and sustain underground storage organs.
Studies in fragmented mallee shrublands have shown that alien grass invasion is
limited by both the lack of propagules and low nutrient levels, and that fire and/or
fragmentation disrupts these limits (C. Gosper
et al.
2010 unpublished manu-
script). Along the perimeter edges of mallee stands, the biomass of invasive grasses
is increased with propagule availability and elevated nutrient levels, with fire
having a neutral to negative effect. Nutrient-enriched edges are susceptible to
invasion with or without fire. Away from perimeter edges, neither fire nor frag-
mentation by interior roads enhanced invasive plant abundance or biomass.
Overall, fire does not promote alien grass invasion in mallee and is thus available
as a viable disturbance strategy for biodiversity conservation in large native
vegetation stands.
Conclusions
Invasive species problems are of far less importance in the Mediterranean Basin
than in other MTC regions. The Mediterranean Basin, however, has been a rich
source of invasive species that have spread widely around the world. In California
fire has played a major role in driving annual grass and forb invasion of many
shrub communities, the dynamics of which are captured in a variation of the
classic grass fire cycle. Widespread loss of native shrublands and type conversion
to alien annual grasslands are due to the lack of resilience among most woody
