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No influence of water limitation on the outcome of competition between diploid and tetraploid Chamerion angustifolium (Onagraceae)

Polyploid plants often occupy different geographic ranges than their diploid progenitors, but the causes of this segregation are poorly understood. Differential competitive abilities of cytotypes across an environmental gradient could be responsible for these observed geographic range differences. Cytotypes of Chamerion angustifolium (Onagraceae) are mostly allopatric, and prior research indicates that tetraploids are more physiologically tolerant of water limitation and occupy drier habitats than diploids. We hypothesized that tetraploids are stronger competitors than diploids in soils where water is limited, which allows them to persist in dry habitats while diploids cannot. We grew both cytotypes together in competition under water‐limited and well‐watered conditions. We varied both total plant density and the relative frequency of cytotypes among pots, which allowed us to separate the effects of intra‐cytotypic and inter‐cytotypic competition. Both diploid and tetraploid plants were smaller in the water‐limited treatment than in the well‐watered treatment. Nevertheless, there were no differences in the relative strength of intra‐cytotypic and inter‐cytotypic competition experienced by either cytotype across the watering treatments, indicating that diploids and tetraploids had equal competitive abilities in both treatments. Synthesis. Competition for limiting resources is often proposed as a mechanism causing ecological and geographic segregation between diploid and polyploid cytotypes. Our results do not support the hypothesis that tetraploid Chamerion angustifolium plants are stronger competitors than diploids when water is limited. A differential ability to compete for water is likely not responsible for the observed ecological and geographic segregation between cytotypes in this species. Competition may not be a general mechanism that causes segregation between diploid and polyploid cytotypes in nature.