Non-native, invasive plant species pose a challenge for land-management practitioners because of their potentially adverse effects on restoration success (reviewed in D’Antonio & Meyerson 2002) and the perception that reconstructed grasslands will harbor invasive species source populations (addressed in Hirsh et al. 2013, this issue). Many studies have addressed factors that help to make some communities more resistant to invasion than others (reviewed in Hector et al. 2001) and this special issue highlights several studies that have applied these concepts in a restoration context. However, as several authors in this issue point out, a deeper understanding on how planted community structure affects invasion is needed to improve restoration practices.
The propagule pool within sites and additional propagule pressure from the surrounding matrix pose challenges for practitioners as they work to establish and maintain grasslands. Early in the reconstruction process, practitioners take steps to mitigate effects of the propagule bank accumulated from often decades of agricultural production and many techniques (such as cover crops and chemical control) are used to mitigate effects of the local propagule pool before and during establishment. Non-native, invasive species pose an additional threat to successfully established plantings as a result of secondary invasions stemming from subsequent disturbance, management, or environmental fluctuations (Alpert et al. 2000). In a recent study, over 80% of the seed bank in established reconstructions contained non-native and potentially invasive plant species (S. Rossiter and K.A. Yurkonis, unpub. data) which could establish after soil disturbance or severe drought (Yurkonis and Meiners 2006). Managing these secondary invasions is more challenging due to difficulties associated with targeting a specific species in an established community and may require site re-seeding efforts. While successful techniques exist to manage initial and subsequent populations of invasive plant species, resistance may be best maximized by investing in seed mixes and seeding regimes tailored to the mechanisms that contribute to grassland invasion resistance.
This special issue outlines several approaches designed to maximize desired species establishment and minimize undesired species in newly developed grasslands. These approaches mainly center around manipulating focal plant density and richness (DiAllesandro et al. 2013, Goldblum et al. 2013, Nemec et al. 2013, this issue). While these factors certainly affect invasion resistance, there are other ways in which invasion resistance arises and understanding these effects offers opportunities for developing new, more effective management approaches.
Future experimental plantings need to further address: 1) how resident plant identity and interactions contribute to invasion resistance; 2) how sown species pattern affects invasion resistance; and 3) whether soil biota can be manipulated to improve invasion resistance.
Plant Identity and Interactions
There is a clear consensus that local invasion resistance can be achieved by maximizing fine-scale focal plant richness (Hector et al. 2001, Hooper et al. 2005). Several studies in this issue alone demonstrate the validity of using species-rich seed mixes in reconstructions to reduce undesired species (DiAllesandro et al. 2013, Nemec et al. 2013). However, we have yet to develop seed mixes that specifically test how invasion resistance arises in more species-rich communities.
Species-rich plantings may be more effective against undesired species as a result of two pathways. They may be more effective because they contain species that are better competitors against invaders (plant identity; selection effect) or they may be more effective because the planted species collectively pre-empt resources that invaders need to establish (plant interactions; complementarity effect) (Loreau & Hector 2001, Dukes 2002, Fargione & Tilman 2005). While both factors may inhibit invasion (Nemec et al. 2013, this issue), they have different implications for how we should design seed mixes for invasion resistance. If traits of the resident species drive invasion resistance, then we should tailor seed mixes to include specific species that are most competitive against undesired species (as in Norland et al. 2013, this issue). This could be achieved by investigating invasion into monospecific stands of the [End Page 120] native species. Alternatively, if invasion resistance primarily arises when resident species more completely use available resources as a result of collective interactions among species, then we should tailor seed mixes to include suites of species which are...