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).