Phenotypic integration predicts phenotypic plasticity in the invasive species Deyeuxia angustifolia but not the native shrubby species Rhododendron aureum

Phenotypic plasticity is essential for plant adaptation to environmental change and competition, yet the mechanisms that constrain or enhance it remain unclear. In alpine tundra ecosystems experiencing rapid climate change, we examined how phenotypic integration and within-environment trait variation jointly shape plasticity in the invasive herb Deyeuxia angustifolia and the native shrub Rhododendron aureum. Along an elevation (2050–2200 m) and encroachment gradient on Changbai Mountain, we measured 42 functional traits to assess whether integration limits plasticity, how this relationship is mediated by within-environment variation, and how trait relationship influences these dynamics. We found that in D. angustifolia, synergistic trait networks—characterized by high edge density, unique correlations, and a predominance of positive interactions—enhanced plasticity, whereas in R. aureum, trade-off–dominated networks imposed structural constraints that limited plasticity. Within-environment trait variation was the primary driver of plasticity in both species, with a stronger influence in D. angustifolia, particularly at higher elevations. This variation enabled D. angustifolia to exploit micro-environmental heterogeneity more effectively, while R. aureum’s limited variation and trade-off constraints reduced its adaptive capacity. Our results reveal that the combination of high trait variation and synergistic integration confers D. angustifolia a competitive advantage, facilitating its upward encroachment. In contrast, R. aureum’s restricted plasticity may hinder its persistence under ongoing environmental change, highlighting the importance of trait network structure and within-environment trait variation in shaping species responses to global change.