A Comparison of Rare and Common Grasses of the Stipeae. I. Greenhouse Studies of Growth and Variation in Four Species from Parapatric Populations

This is a comparative study of variation and growth rates between the widespread grasses Achnatherum (Stipa) lemmonii, Achnatherum (Stipa) thurberiana, and Achnatherum (Stipa) occidentale and their rare relative Achnatherum (Oryzopsis) hendersonii growing in shallow, frost-disturbed soils in central Washington. We assess two hypotheses: rare plants are less variable and do not compete as well as widespread plants. The former thesis will manifest itself in lower variation and more highly constrained integration; the latter by lower growth rates. The rare species usually has a lower growth rate and level of variation, especially in vegetative shoots, which are those most likely to affect survival. For reproductive shoots, A. hendersonii shows weaker integration in terms of the absolute level assessed, but integration appears stronger when relative levels are examined; the results for vegetative shoots are inconsistent. The lower growth rate of A. hendersonii may preclude its survival away from its typical habitat. The lower levels of variation may reflect selection at the native sites of A. hendersonii and thus contribute to its rarity. The lower level of variation in A. hendersonii contrasts with its local abundance at the study site, as compared with the more variable A. lemmonii, A. thurberiana, and A. occidentale. Local abundance combined with a low level of morphological variation may indicate A. hendersonii is genetically depauperate.