Two Agrostis taxa occur in geothermal habitats of Yellowstone National Park: Agrostis rossiae is reported to be endemic to the thermal areas and A. scabra occurs both in thermal and nonthermal habitats. Thermal populations of both taxa are always surrounded by a nonthermal population of A. scabra that is reproductively isolated from the thermal plants by its later flowering time (summer rather than spring). Since Agrostis species are well known for their ability to form edaphic ecotypes, we used common garden and greenhouse experiments to test the hypothesis that ‘‘soil type’’ (a complex variable including factors such as soil chemistry and soil biotic community) was responsible for the separation of the thermal and nonthermal populations. In addition, field monitoring and growth chamber experiments were used to determine whether soil temperature affected the local distribution of these taxa. The results indicate that the thermal and nonthermal taxa are affected similarly by both soil temperature and soil type. The separate distribution of the thermal and nonthermal taxa is not due to the greater tolerance of one of these taxa for a particular factor, but instead to the interaction between seasonal fluctuations in soil temperature and the life history of the thermal and nonthermal plants. Nonthermal Agrostis plants, which are perennial, are excluded from the thermal habitats by lethal summer soil temperatures that limit the duration of the growing season, effectively selecting for precocious flowering and an annual habit. The thermal plants, in contrast, do not grow in the nonthermal matrix in prevailing cool field temperatures, but do grow in these habitats under artificially elevated temperatures. The thermal taxa may have reduced competitive ability at cooler temperatures. Thermal Agrostis have adopted a ‘‘stress avoidance’’ strategy, unlike previously studied thermal plants in Yellowstone, e.g., Dicanthelium lanuginosum, which are stress tolerant.
Key words: Agrostis; bentgrass; geothermal; Gramineae; heat tolerance; life history; Poaceae; soil temperature; stress avoidance; stress tolerance; thermal; Yellowstone.