Functional traits and soil water availability shape competitive interactions in a diploid-polyploid complex

Summary Even within those polyploid plant species that become established initially, only a few persist in the long term. Competitive interactions between polyploids and their ancestral cytotypes in secondary contact zones can contribute to local extinctions. Environmental factors such as water availability and functional trait divergence may influence these interactions. We conducted a greenhouse competition experiment with four cytotypes (2x, 4x, 6x, and 12x) of Dianthus broteri under two contrasting irrigation regimes. We estimated niche and fitness differences and predicted the pairwise competitive outcomes. Additionally, we explored the influence of leaf physiological functional traits (SLA, A N , g s , LDMC, F v /F m , and i WUE) on competitive interactions. Soil water availability modified the competitive dynamics between cytotypes and predicted competitive exclusion. Under high water availability, lower ploidy levels (2x and 4x) outcompeted higher ploidy cytotypes (12x, 6x), while the latter exhibited greater competitive abilities under low water availability. These differences were explained by functional traits related to competitive effects (SLA) and competition tolerance (A N , F v /F m , and i WUE). Synthesis: Our study emphasizes that the long-term fate of polyploids largely depends on water availability, with high polyploids having a competitive advantage in arid environments. This ultimately highlights the role of functional traits in shaping the competitive dynamics between cytotypes.