Phenotypic integration
In spite of the recognized importance that changes in the correlation structure can have for evolutionary change (Lande & Arnold, 1983), we still know surprisingly little about how the environment influences levels of phenotypic integration. Despite much recent progress on this topic (Pigliucci & Preston, 2004; Schlosser & Wagner, 2004), most empirical studies have only studied patterns of phenotypic integration in a single environment (Pigliucci & Preston, 2004; but see Liu et al., 2007; Pigliucci et al. , 1995). In this study, we found an increase in coefficient of integration (CI) with density at 50 d (Gianoli, 2004; Schlichting, 1986; Wylde & Bonduriansky, 2020), and decrease in the number of correlations (NC) with density at 70 d (Badyaev et al. , 2005; Mallitt et al. , 2010; Pigliucci & Kolodynska, 2002). The stage-dependence responses in CI suggested the strength of response of integration to density decreased over time. It is hypothesized that the increase in the number and strength of correlations among functionally correlated traits (phenotypic integration) is related to the extent of environmental stress (Gianoli, 2004; Schlichting, 1986), and endow plants the ability to effectively respond to such stress (Chapin III, 1991). At 70 d, density effects on plants became attenuated, due to small individuals being obsoleted (Wang et al. , 2017). This may lead to weakened responses in trait correlations. It is reported phenotypic integration contributed to differences between native and invasive species less than phenotypic plasticity (Osunkoya et al. , 2014), but differences in phenotypic integration plasticity were more distinguishable than differences in plasticity for clones ofDaPhnia magna (Plaistow & Collin, 2014). It suggested phenotypic integration plays a more important role in plant adaptation to environmental stresses.