Agriculture Reference
In-Depth Information
the higher-elevation sites have not had sufficient time for adequate propagule
dispersal (Rejma´ nek
1989
). Alternatively, character syndromes of most successful
invasive species in California include rapid growth rates in high light environ-
ments. The low light environment on the forest floor of conifer forests may not be
compatible with the life history of most potential invaders.
Disturbance cycles are frequently tied to alien plant invasions and so some of
the basis for limited alien presence in montane conifer forests is tied to a century of
management policy that has excluded fire. This fire suppression policy in most
coniferous forests in the western USA has until relatively recently been highly
successful in excluding fire, extending the fire return interval to far longer periods
than under pre-Euro-American conditions (see
Chapters 5
and
13
). This policy,
while promoting a number of problems of dense stands of understory saplings and
accumulated large biomasses of downed litter, nevertheless restricted the invasion
of alien grass species by maintaining a dense canopy cover. Active management
today to reinstate more natural fire return intervals through prescribed burns has
restored more natural ecosystem processes but has also enhanced forest vulner-
ability to alien invasions. This has been a notable problem in prescribed burns in
ponderosa pine forests of the Sierra Nevada where the open conditions created by
burning, although similar to natural conditions, has greatly promoted invasion of
cheatgrass (
Bromus tectorum
) into these communities (Keeley & McGinnis
2007
).
This condition has led to a management challenge in choosing between restoring
natural fire regimes or altering those fire regimes to favor communities of native
species, with the latter having the potential for long-term impacts on forest
structure, increased vulnerability to crown fires and potential cascading effects
on new invasions of alien species (Keeley
2006b
).
Fire and Alien Species in Mediterranean-type Climate Arid Lands
Prescription burning in sagebrush ecosystems at the desert margins of the MTC of
California has been widely used to increase rangeland resources for livestock grazing
(Keeley
2006b
).
Artemisia tridentata
, the dominant shrub in these ecosystems, is
intolerant of fire and is quickly replaced by more palatable herbaceous plants.
Natural recovery of these stands is very slow and requires decades (Harnis &Murray
1973
). Massive invasions of cheatgrass into these sagebrush ecosystems degraded by
fire have created another classic grass fire feedback cycle which promotes mainten-
ance of cheatgrass dominance by shortening fire return intervals (Mack
1981
).
The widespread invasion of
Bromus madritensis
into the western Mojave and
Sonoran Deserts in California began in the twentieth century and has continued,
producing a widespread cover of flammable grass (Brown & Minnich
1986
;
Rogers & Vint
1987
; Brooks
1999
). Areas that formerly had no continuity of fuels
to carry a fire now experience large fires thousands of hectares in size. Many
native shrub species and succulents have little or no resilience to fire and are
strongly and negatively impacted by the introduction of a disturbance regime that
includes fire (Brooks & Minnich
2006
).
