Targeting weed biomass
The removal of dense weed biomass is critical for reducing competition
for naturally recruiting native’s species, or those added via
revegetation efforts. Weeds are often fast growing and form dense
canopies, which reduces light to the soil and can thus restricts the
germination and subsequent growth of native seeds or seedlings. Further,
many annual grassland weeds have higher nutrient requirements than
native perennial grasses, thus creating a highly competitive environment
for natives to establish. The most commonly used methods for reducing
weed biomass include hand removal, herbicide application, and fire.
Grubbing, or hand weeding, is a restoration technique that completely
removes unwanted plants (Tikka et al ., 2001). While highly
effective, this method is also very labour intensive, and is usually
only appropriate for smaller scale projects (Gibson-Roy et al .,
2007). That said, every three years, community efforts have successfully
removed 34% of the invasive perennial grass, N. trichotomathroughout Canterbury, New Zealand, which has contained the population
from further expansion (Bourdot & Saville, 2019). Grubbing is the best
solution for sites where weeds are newly emerging and easy to remove, or
where only a few individuals have established, such as roadsides.
Grubbing can prove a critical tool for post restoration management by
quickly removing reinvading weeds. Grubbing is one of the most effective
methods to reduce competition for space, light and soil nutrients as the
whole plant is instantly removed.
Herbicide application is often an economically viable and effective
solution for reducing weed competition. Herbicide works most effectively
when integrated with other treatments, as seen by Johnson et al .
(2018) who observed spot-spraying weeds with glyphosate significantly
improved the establishment of native forbs seeds when combined with
fencing, and the removal of leaf litter. Aerially spraying clopyralid
(at a rate of 37.4 L/ha) was successful in reducing woody weed
encroachment and enhancing plant diversity when combined with prescribed
burning (Ansley & Casellano, 2006). Waller et al . (2016) also
observed significantly improved native establishment when herbicide was
combined with fire, tillage and rodent exclusions. In a degraded grazing
exclusion zone, Huddleson & Young (2005) identified herbicide
application on its own was effective for not only reduced annual weed
competition by 40%, but increased native establishment ten-fold.
In some cases, herbicide application was ineffective at improving native
establishment (Cole et al ., 2005; Conrad & Tischew, 2011).
Spot-spraying Snapshot (a pre-emergent herbicide containing trifluralin
and isoxaban) at 2.5kg per 100m2 significantly reduced
the emergence of native forbs compared to the controls in South Africa
(Musil et al ., 2005), however was effective for controlling
invasive annual grasses. In New Zealand, boom-spraying flupropanate at
1.49kg a.i/ha reduced native pasture grass by 89% (Lusk et al .,
2017). In these cases, using herbicides selectively can enhance
restoration outcomes. Selective herbicides are used to kill the unwanted
weeds, while the desirable species remain unharmed, and this can be
attributed to; plant chemistry, physical growth parameters and plant
physiology (Sutton, 1967). It is important to note that the constant use
of herbicides within an ecosystem can promote the emergence of herbicide
resistant populations, thus reducing its long-term effectiveness.
Resistance to arguably the world’s most important herbicide, glyphosate,
has already been observed in several weeds (Powles 2008), including
Conyza spp. (Feng et al ., 2004; Urbano et al ., 2007) and
Lolium spp. (Baerson et al ., 2002; Yanniccari et al .,
2017). It is considered important, therefore, that herbicides should be
used selectively and in combination with other control methods in order
to secure their effectiveness for the long term.
Fire is one of the most effective tools for restoring temperate
grasslands that are dominated by weeds. Historically, grasslands are
ecosystems that are accustomed to frequent fire events, and altered fire
regimes in Australia (Stuwe & Parson, 1977), New Zealand (Mark, 2007;
Standish et al ., 2009), the United States (Foster & Gross 1998;
Stromberg 2007), and South Africa (Sankaran & Anderson, 2009) have been
linked to the modification of these landscapes (Archer et al .,
1988; Knicker, 2007). Fire quickly creates available space for heat
resistant seeds to germinate and grow relatively free of competition
(Meyer & Schiffman, 1999), and a number of studies have observed that
fire significantly reduces weed species and promotes native recruitment
(Huddleson & Young, 2005; Prober et al ., 2005; Bryant et
al ., 2017). Lipoma et al . (2018) identified fire to
significantly reduce the viable number of seeds in the soil compared to
pre-burnt conditions, and as most weeds often have dense seedbanks, this
can be beneficial in reducing at least the surface seedbanks of some
species (Peltzer & Douglass, 2019). In contrast, some species,
particularly broadleaf weeds such as Echium plantagineum , are
promoted by fire (Prober et al ., 2004). Heat tolerance in seeds
has been linked to seed shape, with more rounded seeds demonstrating
higher resistance than thinner seeds in European temperate grasslands
(Ruprecht et al ., 2015). This suggests that follow up weed
management of burnt sites is critical for the successful establishment
of native species. In Australia, a summer wildfire was observed to kill
90% of the standing native spear-grass (Austrastipa spp.), which is
considered relatively fire tolerant (Sinclair et al ., 2014). Fire
also offers soil manipulation services as carbon and nitrogen volatize
at 180 and 200OC respectively (DiTomaso et al .,
2006), therefore hot fires can remove soil nutrients that advantage
annual weeds and further inhibit their re-establishment (Knicker, 2007).
Strategically burning when problematic weeds are actively growing can
effectively prevent seed set for that season (Prober et al .,
2005). The complexity of fire effects suggests that post management
plans should be specific for the site in order to promote the
establishment of a healthy native grassland community (Musil et
al ., 2005; DiTomaso et al ., 2006).