Environmental Engineering Reference
In-Depth Information
insects and birds increased in abundance after control
of invasive trees, an important fi nding given the con-
servation value of these remnant native forests (Heleno
et al
. 2010). The Asian invasive Tree of Heaven,
Ailan-
thus altissima
, was successfully controlled with tree
cutting plus herbicide application in Spanish wood-
lands, but the authors did not report vegetation recov-
ery (Const รก n - Nava
et al
. 2010 ). They recommended
afforestation to avoid future recolonization of
A. alt-
issima
. If invasive plants are controlled while their
populations are sparse or their areas are small, any
subsequent escapes, spreading or reinvasion can be
controlled with relatively small efforts (D'Antonio &
Chambers 2006 ).
Recolonization or re-establishment ability by
native
species
is a critical prerequisite to restoration follow-
ing invasive species control. In the study by Heleno
et al
. (2010), native seeds became the most important
component of the seed rain just one year after clearing
invasive trees, and the authors suggested this showed
a renewed
successional trajectory
leading toward a
native-dominated community. By contrast, Mediterra-
nean annual grasses were the most abundant species
to recolonize in California coastal sage scrub after
mowing and grass-specifi c herbicide that reduced the
alien grasses (E.B. Allen
et al
. 2005). Native shrubs had
limited establishment under the constant impact of
colonizing invasive species, and removal of invasive
species was recommended every 5 years to maintain
native dominance (E.B. Allen
et al
. 2005 ). Similarly,
the Mediterranean grass barbed oats (
Avena barbata
)
may compete with establishing native shrubs in aban-
doned farmland in Australia, and must also be periodi-
cally controlled (Standish
et al
. 2008 ). The Australian
site had low soil nutrients (Standish
et al
. 2008 ), while
the California site had high soil nutrients (E.B. Allen,
et al
. 2005), indicating that Mediterranean annual
grasses have a broad ecological amplitude for inva-
sion, especially barbed oats, which occurred in both
study sites.
Seed bank
density and longevity are also critical
factors in the invasion process. The seed bank density
of alien grasses was two orders of magnitude greater
than that of native forbs and shrubs in California
coastal sage scrub (Cox & Allen 2008). Fortunately, the
seeds of many alien grasses - especially the dominant
Bromus madritensis
and
B. diandrus
, both from the Med-
iterranean region - are short lived, so that carefully
timed spring fi re (before the seeds have dispersed) has
been a useful tool in restoring perennial grassland
(Gillespie & Allen 2004). However, given the ability of
these annual grasses to recolonize or re-establish, fi res
must be set every 5-7 years. This high frequency of fi re
is a good tool for restoring perennial, fi re - adapted
grasslands, but is incompatible with native shrub or
tree recovery, as frequent fi res will not allow woody
plants to establish or grow to maturity. Other means of
alien annual-grass control are needed for shrub and
woodland restoration, such as timed grazing in spring
to reduce annual seed production. Seed bank longevity
is also a factor in the successful control of
Tamarix
,
which has a short-lived seed bank; the few seedlings
that establish after mechanical or chemical control can
be readily eliminated with follow-up treatment.
The degree of soil or ecosystem alteration following
invasion will also determine the feasibility of restoring
invaded systems in MCRs. Invasive
Acacia saligna
, from
Australia, has caused elevated soil nitrogen of South
African fynbos, which impedes restoration efforts
because the native shrubland is adapted to low levels
of soil nutrients (Yelenik
et al
. 2004). Hand clearing of
the invasive shrub promoted colonization by
Ehrharta
calycina
, a nitrophilous native perennial grass. As a
result, it was recommended that fi re or mulch be used
to control high soil N and promote re-establishment of
native shrubs. Bark mulch was effective in immobiliz-
ing N and allowing establishment of native shrubland
that was invaded by alien Mediterranean annual
grasses in California (Zink & Allen 1998).
As mentioned earlier, Mediterranean Basin wood-
lands and shrublands are apparently more resistant to
biological invasions than those of other MCRs, and
thus are - in theory - more amenable to restoration.
Indeed, invasive plants from the Mediterranean Basin
often come to dominate in other MCRs because they
are more aggressive than the native species, or because
environmental conditions (e.g. fi re, grazing and soil
nutrients) have changed so that the native species are
no longer able to thrive. Some of the environmental
drivers may be controlled (e.g. reducing fuel for fi re or
reducing grazing intensity), while others are less easily
manipulated, such as reducing soil N on a large scale.
Another approach to restoration is to select native
species with physiological traits that inhibit invading
species, and establish vegetation that is resistant to
invasion (Funk
et al
. 2008). This approach will succeed
if ecosystem disturbance rather than species traits
have allowed invasion. However, if the environmental
drivers have been controlled and the invasion is still
ongoing, as has been observed in California coastal
