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).