Context dependence
Plant traits have been assessed for usefulness to predict invasibility of plants in introduced ranges (Lambdon & Hulme 2006; Pyšek & Richardson 2008; van Kleunen et al. 2010); however, conclusions are equivocal. Thompson & Davis (2011) argue that invasive plants do not possess traits significantly different from native species, therefore, traits are not useful to predict invasibility of plants. In contrast, (van Kleunen et al. 2010; Van Kleunen et al. 2010)) oppose this argument. Recent studies concluded that it is challenging to use traits to predict invasibility of plants, in part, because it is context dependent; interaction between a trait and environmental factors most likely determines invasibility of a given non-native species (Alpert et al.2000; Leffler et al. 2014). Pyšek et al. (2012) concluded that traits of an invasive plant can be useful to assess their impacts on ecosystems, but this is also strongly context dependent. This likely also applies to traits determining pathogen susceptibility. For instance, a non-native plant may have a higher chance to be infected by pathogens if the introduced range has phylogenetically closely related species (Agrawal & Kotanen 2003; Parker & Gilbert 2007; Parker et al. 2015). We emphasize that the importance of plant traits may also be context dependent for predicting susceptibility of plants to pathogens.
Many invasive species tend to form near mono-specific patches, which is less common for native species (Burdon et al. 1989; Levine et al. 2003; Hejda et al. 2009). Studies have shown that density of host plant populations was positively correlated with disease levels in agricultural (Burdon & Chilvers 1982) and natural systems (Cobb et al. 1982; Jennersten et al. 1983; Alexander 1984; Burdonet al. 1995). While this tendency of invasive species may in part be due to escape from enemies in the introduced range, it may also make them more susceptible to population decline if a compatible pathogen is encountered (Clay et al.2008; Mordecai 2011). This density-dependent pathogen severity suggests that invasive plants with high density may decline due to pathogens over the time.
Evolution of both non-native host plants (Maron et al. 2004) and pathogens (Parker & Gilbert 2004) is another factor that may regulate susceptibility of plants to pathogens. Recent studies indicate that some invasive plants have the ability to rapidly adapt to new environments modifying their traits such as total biomass, SLA, or flowering time (Flory et al.2011b; Colautti & Barrett 2013). Furthermore, levels of phenotypic plasticity and ontogenetic change in traits is another factor to consider in determining disease susceptibility (Boege et al. 2007).
There is unlikely a single predictor of pathogen accumulation and there will be context dependence complicating observed patterns. This context dependence may be related to different types of pathogens, species interactions, and/or environmental conditions. For example, it has been suggested that responses of pathogens to plant N status may depend on the types of pathogens (Hoffland et al.1999, 2000). High tissue N content is predicted to increase disease severity by obligate or biotrophic pathogens but reduce severity of facultative or necrotrophic pathogens (Hoffland et al. 2000; Dordas 2008; Veresoglou et al. 2013). Similarly, plant senescence is also related to different types of pathogens. Early senescence reduces impacts of biotrophic pathogens while delayed senescence acts as defense against necrotrophic pathogens (Häffner et al.2015). Furthermore, pathogen dispersal mode may influence relationships with plant traits. For example, taller plants may be more susceptible to wind dispersed pathogens, whereas plants with foliage low to the ground may be more affected by pathogens from the soil surface (Marquis et al. 2001; Robert et al. 2018; Vidal et al. 2018), suggesting that traits conferring tolerance, resistance, and escape may be pathogen dependent.