Alien Species and Fire - Fire in Mediterranean Ecosystems - page 339

Fig. 12.3 Fuel break on USFS lands in southern California illustrating the inevitable type
conversion from native shrublands to alien-dominated annual grasses and forbs. (Photo by
Richard W. Halsey.)
establishment of alien grasses and forbs in adjacent wildland areas ( Fig. 12.3 ).
These fuel breaks may act as corridors that bring alien plants into wildland areas.
Other corridors such as railroads are likewise implicated as sources of alien plant
invasions (Bangert & Huntly 2010 ).
Despite the key role that fire plays in the spread of alien species in California,
there are management agencies that advocate the use of prescription burning as a
means of controlling some alien species. Although dependence on this or any other
eradication method alone is doubtful as a means of sustainable control, it may
have some short-term value ( Box 12.1 ).
Alien Plant Invasion in Montane Conifer Forests
Alien plant threats are greatest in the foothills and coastal plain; they decline
markedly with elevation (Mooney et al. 1986 ; Schwartz et al. 1996 ) and are
generally relatively minor in undisturbed closed-canopy conifer forests (Keeley
et al. 2003 ). In closed-canopy forests invasives are likely to increase following
high-intensity fires (Keeley et al. 2003 ) but in more open forests invasives increase
with a decrease in severity (Franklin 2010a ). Both historical and niche-based
factors may be responsible for this pattern. Mountain habitats have been occupied
by humans for a relatively short time in California; thus, disturbances conducive
to invasion have not been available until relatively recently and so time may be
a factor. Less intensive land use in mountain habitats of the Old World also
may be responsible for fewer potential invasive species in high-elevation sites,
although this has not been demonstrated. A related historical explanation is that