A taxonomic study of bitterbrush (Purshia tridentata (Pursh) DC.) in Oregon

Graduation date: 1977Extensive morphological and growth habit variations have been\ud reported for bitterbrush (Purshia tridentata), a widespread western\ud shrub frequently used in range revegetation projects. Hybridization\ud of this species occurs with cliffrose (Cowania stansburiana) at the\ud point of their overlapping range in Utah and is believed to be\ud responsible for expression of certain bitterbrush attributes.\ud The objectives of this study were to investigate specific morphological\ud and physiological characteristics of bitterbrush over a portion\ud of its geographical range and to determine which, if any, of the variations\ud could be attributed to selective forces of particular habitats.\ud Twenty-nine populations of bitterbrush in eastern Oregon and\ud three populations in northern California were selected for study to\ud account for maximum variation in associated veget3tion, environmental\ud parameters, and morphological expression. Field studies\ud were conducted to examine taxonomic and growth habit characteristics\ud of each population and for use in population comparisons.\ud Seed germination studies indicated that the slight variations in\ud population response to nine time treatments in a 3% thiourea solution\ud essentially disappeared with a 15-20 minute treatment. Longer\ud treatments resulted in deformed radicles which failed to elongate.\ud Size and shape of leaves and growth form responses from seedlings\ud grown in the greenhouse compared favorably to the parent populations.\ud Pubsecence and glandulation also appeared to be primarily\ud genetically induced and were expressed early in seedling growth.\ud Cluster analysis indicated that many populations studied tended\ud to build up integrated morphological patterns or seed, leaf, color,\ud growth form, and habit attributes, However, the nearly continuous\ud gene-flow in this out-breeding shrub resulted in extensive overlapping\ud morphology between populations which is in accordance with the nature\ud of clinal variations.\ud Trends for gene flows were followed in northeastern Oregon and\ud along the eastern front of the Cascades with ecotypes forming where\ud environmental conditions created unique habitats. Under lodge pole\ud pine (Pinus contorta) on pumice soils, a low, layering ecotype was\ud identified which retained small, glabrous leaves, heavy glandulation,\ud and a low growth habit under common greenhouse conditions. An\ud ecotype similar in habit but varying in leaf shape, seed size, and bark color was found on harsh sites at the Hart Mountain Antelope Refuge.\ud Tall, massive plants growing on deep soils in northern California near\ud Janesville and in northeastern Oregon near Durkee represented two\ud additional ecotypes which showed differentiation in seed and leaf\ud characteristics for their respective populations.\ud Paper chromatography for flavonoid compounds gave complex\ud spot patterns which indicated possible addition of cliffrose genes in\ud certain populations. Although morphological characteristics associated\ud with specific patterns were not found on an individual plant\ud basis, two major patterns of similarity between certain populations\ud were noted.\ud Seventeen populations were selected for chromosome studies and\ud represented a full range of morphological and environmental parameters.\ud A diploid number of 2nl8 was found for all populations.\ud For managers contemplating a bitterbrush seeding project,\ud choice of seed source should be carefully considered. A relationship\ud was observed between taller plants and frost-free growing season.\ud Seed from these plants may produce seedlings unable to survive where\ud frosts occur frequently. Since seed for re-establishment is generally\ud collected from these taller, high seed-producing plants, past seeding\ud failures may be related to selection of non-frost tolerant ecotypes.\ud If local seed cannot be obtained, it was suggested that associated\ud vegetation, soil type, and frost-free growing season of the seed\ud source be matched as nearly as possible to the site selected for\ud seeding.\ud Data from this study indicated that a 15-20 minute thiourea\ud treatment yielded a higher germination response than the five minute\ud treatment generally suggested in the literature