Relative Competitive Ability of Helianthus paradoxus and Its Progenitors, H. annuus and H. petiolaris (Asteraceae), in Varying Soil Salinities

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Abstract
The effect of soil salinity on the growth and competition among Helianthus paradoxus, H. annuus, and H. petiolaris was examined in a greenhouse experiment to determine the role competition might have in leading to the narrow endemism of H. paradoxus. Helianthus paradoxus (the Pecos sunflower or puzzle sunflower) is a threatened annual species that is federally listed and only found in a few areas in west Texas and New Mexico. It is a diploid hybrid species that occurs in saline soils where its progenitors, H. annuus and H. petiolaris, are absent. The response of the target species to the competing species was usually dependent on soil salinity. Helianthus paradoxus was the better competitor in high‐saline soil and H. annuus the better competitor in low‐saline soil. Aggressivity values in low‐saline soil indicated the following competitive hierarchy: H. annuus > H. paradoxus > H. petiolaris. In the higher‐saline soils the competitive hierarchy was H. paradoxus > H. annuus > H. petiolaris. The ability of H. paradoxus to tolerate higher‐saline conditions and perhaps even restrict the more geographically widespread H. annuus in saline soils may allow H. paradoxus to survive in inland salt marshes. Data presented here indicate that while H. paradoxus can grow in low‐saline soil, competition from H. annuus could restrict it, leading to its narrow endemism within salt marshes.