Agriculture Reference
In-Depth Information
have been shown for
Teline
(
Genista
)
monspessulana
(French broom) in areas
dominated by pine plantations in south-central Chile, with fire favoring the
presence and dominance of this species (Pauchard
et al.
2008
).
South Africa
The fynbos shrublands of the Cape region of South Africa have been strongly
influenced over broad areas by invasive woody species. Alien species of
Hakea
from Australia and
Pinus
from California and the Mediterranean Basin have
become widely established in fynbos, as have Australian
Acacia
species in riparian
habitats of the Cape region. These invasions have had major ecosystem impacts
including the promotion of fires with higher intensity, with potentially negative
impacts on native seedbanks (Holmes
2002
).
Once alien trees have established, they typically grow faster and taller than
indigenous species, and after one or two fire cycles form closed stands with
reduced light penetration and altered nutrient cycling patterns, litterfall, and fuel
properties (Richardson
et al.
2000a
). Such stands may replace fynbos vegetation
and their impacts intensify with time elapsed since invasion (Holmes & Cowling
1997b
). Invasions by shrubs and trees have been the subject of extensive studies
because of the ecological impacts that these invaders have had on competition
with native species, hydrologic flow, nutrient enrichment and increased fire hazards
(Holmes & Cowling
1997a
,
1997b
; Le Maitre
et al.
2002
; Richardson
et al.
2004
;
Yelenik
et al.
2004
).
Although fynbos vegetation is well adapted to moderately frequent fire return
intervals (van Wilgen
et al.
1992b
), natural fires promote the spread of invasive
shrubs and trees. One impact is that fires in areas heavily infested with dense stands
of alien woody species are less easily ignited and spread more slowly than in
pristine fynbos, where there is an abundance of fine material in the herbaceous
layers (van Wilgen & Richardson
1985
; van Wilgen
2009
). However, under extreme
weather conditions, the high fuel loads of invaded stands promote fires that burn
with very high intensity. Such high fire intensities are difficult to contain and are
potentially more damaging to ecosystem structure and processes than natural fires
in fynbos vegetation. While fire may exacerbate invasions, there is some evidence
that it may provide a valuable control on these invasives as well (
Box 12.2
).
Compared with studies of the ecosystem impacts and flammability issues related
to invasive woody species in the Cape region of South Africa, there has been
relatively little concern placed on environmental problems posed by the presence
of alien grasses in the Cape region (Milton
2004
; Musil
et al.
2005
). Much of
the apparent inability of annual grasses from the Mediterranean Basin to become
widely established and ecologically significant in the region relates to the extreme
oligotrophic conditions presented by soils formed from the widespread
Table Mountain sandstone. Nevertheless, there are reports that alien grasses have
increased in abundance, especially in low-lying areas (Vlok
1988
). This increase in
