2.2 Interspecific relationships
Owing to their wide niche breadth, toxic weeds can successfully coexist with several other plant species (Ren, Zhao & An 2013; Cheng et al.2014a). Unlike the shallow-rooted graminoids whose roots horizontally extend in the surface soil (Wang et al. 2004), toxic weeds are mostly axial-root species which deeply root, and thus can absorb water and nutrients from much deeper in the soil compared to forages (Li, Niu & Du 2011; Maguire, Sforza & Smith 2011; Sun et al. 2014). Such interspecific differentiation in the acquisition of soil resources alleviates competition and permits co-existence with heterospecific plants (Fargione & Tilman 2006; Ryel 2010; Xin et al. 2012). Nevertheless, perennial toxic weeds are usually tall and thus superior competitors for light resources relative to shorter plant species (Hautier, Niklaus & Hector 2009; Craine & Dybzinski 2013; Li et al. 2016). In addition, individuals often aggregate to form patches that facilitate intraspecific cooperation, enhance their competitive ability and promote their expansion (Sun, Ren & He 2011; Gao & Zhao 2013; Ren, Zhao & An 2015). As a consequence, patches of heterospecific plants that are separated by toxic weeds often are not able to survive in the presence of competitively superior toxic weeds (Zhao et al. 2016).
The allelopathy of toxic weeds is an important competitive behaviour that inhibits the growth of receptor plants (Fig. 3). The primary phytotoxic mechanisms are regulated via the following two pathways. First, allelochemicals (e.g. flavonoids, coumarins and phenolic compounds) can inhibit mitosis (Yan et al. 2016), reduce chlorophyll content (Pan et al. 2015), disrupt root development (Yan et al. 2014), promote the overproduction of proline (Yanet al. 2016), inhibit germination (Cheng et al. 2011), reduce endogenous auxin content (Yang et al. 2011), and promote reactive oxygen species accumulation (Pan et al. 2015; Yanet al. 2015).The second pathway is the arrest of sexual multiplication by pollen allelopathy (Sun, Luo & Wu 2010). Interestingly, phytotoxic effects increase with age; that is, older plants are superior competitors compared with younger plants (Weiet al. 2017). Notably, the allelopathy effects of toxic weeds exhibit species specificity, for example, S. chamaejasme has strong inhibitive effects on some species including Setaria viridis , Amaranthus retroflexus (Pan et al. 2015),Pedicularis kansuensis (Hou et al. 2011), Festuca rubra L. , Medicago sativa (Guo et al. 2015),Melilotus suaveolens Ledeb (Wang, Zhou & Huang 2009) andOnobrychis viciifolia (Zhou et al. 2009b), while other species such as Agropyron mongolicum (Wang et al. 2008),Psathyrostachys juncea (Zhou et al. 2009a), Elymus dahuricus (Zhou et al. 2010) and Lolium perenne (Wang, Zhou & Huang 2009) show resistibility against the allelopathy effect ofS. chamaejasme . Therefore, these species can be used to restore degraded grasslands inhabited by toxic weed.