Agriculture Reference
In-Depth Information
12
Alien Species and Fire
A large diversity of alien plants is found in most mediterranean-type climate
(MTC) regions and fire is sometimes closely linked to their ability to invade
natural ecosystems. This is a concern because aliens often upset natural ecosystem
processes, and thus are a major management concern. These five regions not only
differ in their contributions of non-native plant species to other regions, but also
vary in their susceptibility to invasion by alien species, something often referred to
as a community's invasibility.
Fire is a key factor behind plant invasions into natural plant communities
and particularly critical is the timing of propagule availability and characteris-
tics of the fire regime. Fire also interacts with geology in dictating functional
types that become pernicious invasive problems. For example, on coarse-textured
low-fertility soils in two of the southern hemisphere MTC regions, shrubs
and trees are among the most aggressive invasives, and are capable of invad-
ing seemingly undisturbed intact shrublands. However, on more fertile soils
such as in California and Chile, grasses and other herbaceous species are
bigger threats, but invasion typically requires disturbance and under some
circumstances fire can effect
type conversion
from woody vegetation to alien-
dominated grasslands.
One of the important characteristics of many invasions in MTC regions is
the fire-promoting capacity of the invading species (Brooks
et al.
2004
). These
species are favored by fire and have vegetative traits that promote further fire in
the system. Such fire-promoting feedback processes characterize a
grass fire cycle
where initial invasion promotes further success of fire-promoting grasses
(D'Antonio & Vitousek
1992
). However, there are actually two very different
fire-promoting grass invasion processes that are functionally quite different, one
being more typical of subtropical ecosystems and the other of MTC ecosystems
(
Box 12.1
). In some cases aliens may change fuel structure sufficiently to alter
fire behavior from surface fires to crown fires (e.g.
Fig. 12.1
).
As with other interactions between fire and community characteristics in
MTC ecosystems, different patterns are evident with respect to alien plant
invasion (Fox
1990
); South Africa and southwest Australia show similar
patterns due to infertile soils, whereas the moderately fertile soils of California
and Chile promote quite different patterns, and the Mediterranean Basin stands
alone in that it is a major source of invasive plants for other MTC regions and
