4. Discussion
Human activities are profoundly reshaping the Earth to the point that scientists proposed to define a new geological epoch, the Anthropocene (Lewis & Maslin, 2015). The continuous global emission of highly toxic pollutants taking place during this epoch (Vallack & Rypdal, 2019), challenges the capacity of plants to respond to the rapid shifts in environmental conditions. Organisms can accommodate to challenging environmental conditions through phenotypic plasticity (Bradshaw, 2006; Nicotra et al., 2010), i.e. the ability of a single genotype to express different phenotypes under diverse environmental conditions (Bradshaw 1965, 2006; Kelly, Panhuis, & Stoehr, 2012; Schlichting, 1986; Sultan, 1987, 2000). Indeed, a now classic expectation is that plasticity of functional traits may contribute to the survival of plants in highly heterogeneous environments and changing environmental conditions (e.g. Henn et al., 2018; Lázaro-Nogal et al., 2015; McIntyre & Strauss, 2014; Richards et al. 2010;), particularly in clonal plants or during the invasion of new habitats where genetic diversity can be limited (e.g. Drenovsky et al., 2012; Geng et al. 2007; Richards, Bossdorf, Muth, Gurevitch, & Pigliucci, 2006; Richards et al., 2008; Turner, Fréville, & Rieseberg, 2015; Williams, Mack, & Black, 1995). Hence, plasticity could be a critical component of population and, ultimately, species persistence under the current context of global change (Nicotra et al. 2010; Parmesan & Hanley 2015).
In order to understand whether phenotypic plasticity contributes to individual, population and/or species survival, we need to better understand the levels of phenotypic variation in response to specific environmental drivers in different plant taxa (Arnold, Kruuk, & Nicotra, 2019; Nicotra et al. 2010; Parmesan & Hanley 2015). In this context, we investigate the patterns of intraspecific variation for heavy metal accumulation and tolerance in two bryophyte species with contrasting affinity for heavy metals and discuss the mechanistic basis for such differences. The present investigation adds to current knowledge on the extent and patterns of phenotypic variation in a group of plants highly tolerant to environmental stress but whose response mechanisms are relatively understudied.