Mixed evidence for shifts to faster carbon capture strategies towards range edges of two coastal invasive plants in eastern Australia

Understanding shifts in traits across the course of an invasion can significantly increase our understanding of mechanisms underpinning range expansion. For example, shifts to traits associated with faster growth may be advantageous in range edge populations of invasive species to decrease generation time and thus promote rapid range expansion. We tested whether populations at the expanding range edges of two coastal plant species invasive in eastern Australia (Gladiolus gueinzii Kunze and Hydrocotyle bonariensis Lam.) possessed different carbon capture strategies compared with range core populations where they were first introduced. Pairwise leaf trait relationships between specific leaf area (SLA), photosynthetic rate (A_mass), foliar nitrogen (N_mass) and foliar phosphorus (P_mass) were investigated for range edge and range core populations using standardised major axis (SMA) regression. Across species, SMA regression slopes for range core and range edge populations for all pairwise comparisons did not differ significantly from each other, suggesting that each species has a similar carbon capture strategy across its range. However, at a species level, H. bonariensis displayed significant shifts in trait values along a common regression slope for many pairwise comparisons. Range edge populations were found to have higher values for N_mass, A_mass and SLA compared to range core populations and displayed greater nutrient use efficiency, suggesting that range edge populations are positioned further along the leaf economics spectrum towards faster growth strategies. In contrast, for G. gueinzii, leaf traits were positioned along a common regression slope with no difference in the trait values of range core and range edge populations. Our results suggest that there is selection for faster carbon capture strategies at range edges for some introduced species undergoing range expansion and this may be a contributing factor in explaining rapid range advance.