The Geography of Phytochemical Races

144 3 After the Ice
support this scenario. In contrast, A. concolor tends to occupy drier habitats such as
occur east of the Cascades.
Abies procera and A. magnifi ca and intermediate forms constitute a large complex
stretching from north central Washington to the southern Sierra Nevada. Abies
magnifi ca tends to grow at higher elevations with A. procera occupying somewhat
lower sites. The question has been raised as to whether the intermediate forms are
the result of introgression between two “good” but similar species or whether they
are the result of recent evolutionary differentiation. Zavarin et al. (1978) addressed
this issue by examining the essential oil components of 352 individuals collected at
35 sites. Four compounds were found useful in studying the transitional populations,
3-carene, limonene, β-phellandrene, and α-pinene. The chemical data allowed the
specimens to be sorted into three groups: (1) north of 44° (A. procera); (2) between
44° and 40° (transitional); and (3) south of 40° (A. magnifi ca). Transition-like populations
were also found in the southern Sierra Nevada and in the vicinity of Mt. Shasta.
The authors also compared published values for seed weight and cotyledon number
and found that those data paralleled the trends seen within the chemical transitional
region. The authors pointed out that neither the chemical nor morphological observations
clearly differentiate between the two hypotheses, whereas paleobotanical
evidence suggests that the transitional forms are the product of recent evolutionary
change.
Abies lasiocarpa (Hook.) Nutt. is a western North American taxon of considerable
signifi cance. Differences in terpene composition among extensive collections
led Hunt and von Rudloff (1977) to suggest that the variation should be refl ected in
the recognition of three taxa, A. lasiocarpa, A. balsamea (L.) Miller, and A. bifolia
Murr. Subsequent studies of volatile components of material from populations in
California, Oregon, and Colorado (Cope, 1983) reinforce that view. The populations
from Colorado differed from the others in having higher concentrations of santene,
α-pinene, and camphene, and smaller amounts of β-phellandrene and limonene.
3.2.2 Cupressus bakeri (Cupressaceae)
The next example is the Baker cypress, Cupressus bakeri Jepson, which occurs in
scattered populations in the Siskiyou, Cascade, and Sierra Nevada Mountain ranges
of northern California and southern Oregon (Fig. 3.8). Despite the limited range of the
species, there have been suggestions that the morphological variation observed is best
handled by recognizing two subspecies, subsp. typica C. B. Wolf from the southern
part of the species’ range and subsp. matthewsii C. B. Wolf from the northern part
(Wolf and Wagener, 1948). Two studies have also been directed toward documenting
chemical variation within this species, one using megagametophytic fatty acids (Rafi i
et al., 1992b), and one using mono- and sesquiterpenes (Rafi i et al., 1992a).
The fatty acid composition of the Baker cypress is comparatively complex. The
major saturated acids were identifi ed as palmitic (16:0) [chain length:number of double
bonds], stearic (18:0), and arachidic (20:0) with only minor contributions from