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