Revegetation
Establishing dense competition from desirable species is the most
effective way to reduce weeds and return natural ecosystem
functionality. In the case that natural regeneration is an unviable
option, competition can be introduced using a variety of methods
including direct seeding (Thomas et al ., 2019; Cole et
al ., 2005), transfer of threshing material (Baasch et al ., 2016)
or hay (Sengel et al ., 2016), direct drilling (Bakker et
al ., 2003) and plant plugs (Tikka et al ., 2001). Hedberg &
Kotowski (2010) reviewed the effectiveness of different revegetation
options for fragmented grasslands and found that direct seeding (sowing
and broadcasting) to be the most widely used and most effective for
introducing species back to semi-natural systems. However, they
specifically recommended the use of plant plugs for the establishment of
rarer species (Hedberg & Kotowski, 2010).
The effectiveness of species richness in seed mixes has been explored
for grassland restorations, with both high and low rates demonstrating
beneficial results dependent on the projects scope (Prober & Thiele,
2005; Wortley et al ., 2013). The determined species mix is often
reflective of the goals of that particular restoration project; for
example, Conrad & Tishew (2011) found a high seed mix of 35 species
achieved their goals of increasing species diversity as well as
establishing target species, whilst in another area, Huddleson & Young
(2005) used a mix of only three native grasses to successfully
outcompete weeds. Further, it appears that high species diversity
improves the establishment of native species (Barr et al ., 2017),
long-term resilience to weed reinvasion (Carter & Blair, 2012; Scotton,
2016), and provide habitat for recolonization of threatened wildlife
(McDougal & Morgan, 2005). Nemec et al . (2013) has demonstrated
seed diversity to be a more important factor than seed rate for
achieving reasonable competition for weeds. While high seed rates can
improve the chances in successfully outcompeting weeds (Tikka et
al ., 2001; Bakker et al ., 2003; Barr et al ., 2017), this
approach can waste seeds as a result of higher intraspecific
competition, and the associated high costs can make it unpractical
(Sheley et al ., 2006; Wagner et al ., 2011). Seed mixes low
in diversity and density can promote spontaneous secondary succession,
and this can stimulate ecosystem processes more quickly (Lengyelet al ., 2012). We note that the failure of sown seeds to
establish can be linked to several factors, including herbivory, adverse
weather conditions, and species competition (Gibson-Roy et al .,
2007), therefore implementing pre-sowing management that minimises these
threats is critical. Whilst it is clear that the introduction of seeds
or seedlings is often critical for the restoration of many degraded
temperate grasslands it is also clear that the best implementation
method will be dependent on the site, scale and funding available to the
project (Prober & Thiele, 2005).