The Geography of Phytochemical Races

3.1 North America 139
allele frequencies: (1) a form ranging from the northern extent of the range to southern
parts of the Appalachian uplands, (2) an isolated Florida peninsula form, and (3) an
intermediate form that occurs along the southeastern coastal plain. The peninsular and
intermediate forms are similar in distribution to the southern ecotype noted above.
Essential oils are commonly occurring constituents of conifers but are less frequently
encountered in hardwood species. Examples of trees that do produce these
compounds include species of Sassafras (Lauraceae) and Magnolia (Magnoliaceae).
To this list one can add Liriodendron tulipifera, which A. L. Smith et al. (1988)
have shown to produce a rich array of common monoterpenes: α-pinene, β-pinene,
camphene, myrcene, 3-carene, limonene, β-phellandrene, cis-β-ocimene, γ-terpinene,
terpinolene, and bornyl acetate (see Fig. 3.7 for structures). The analyses
were done on twig samples collected from 11 provenances, representing sites in
Alabama, Georgia, Louisiana, Mississippi, Ohio, South Carolina (coastal plain
and piedmont), Tennessee, and Virginia. With the single exception that the piedmont
specimen from South Carolina lacked β-phellandrene, all 11 compounds
were present in all specimens. Variation was signifi cant in most cases with the
following ranges for concentrations (expressed in % of dry wt × 10 −3 ): α-pinene,
5.00–16.10; β-pinene, 5.14–24.78; camphene, 1.03–5.58; myrcene, 2.23–18.26;
3-carene, 0.51–2.57; limonene, 3.31–18.77; β-phellandrene 0.00–24.56; cis-β-ocimene
33.30–75.21; γ-terpinene, 0.08–2.37; terpinolene, 1.85–12.20; and bornyl
acetate, 2.40–21.84. There appears to be no clear geographic pattern in the terpene
concentrations.
Restriction endonuclease site analysis, including results from the earlier study
(Parks and Wendel, 1990), revealed fi ve sequence changes within populations of
L. tulipifera. Of these, three occur exclusively in the “northern” haplotype and two
only in the “southern” haplotype. Based upon estimates of sequence-divergence
time (Parks and Wendel, 1990), it was suggested by Sewell et al. (1996) that the two
forms diverged about 1.2 million years ago. To put this time estimate in perspective,
it is necessary to refer to comments by Parks and Wendel (1990) on the fossil record
for L. tulipifera. Liriodendron was present as an element of a temperate deciduous
forest during early to mid-Miocene. Later cooling resulted in a southward migration
of the temperate forest effectively isolating the Asian and North American populations.
During Pleistocene glaciation, the temperate forest is thought to have survived
in only a limited number of areas in the southeastern part of the continent:
(1) Nonconnah Creek, Tennessee (Delcourt et al., 1980), at which fossil evidence
of L. tulipifera was found; (2) Goshen Springs, Alabama (Delcourt, 1980); and
(3) Sheelar Lake, Florida (Watts and Stuiver, 1980). Parks et al. (1994) suggested
that possible refugial areas were likely to lie along the bluffs of the Apalachicola
River, where the northern haplotype might have survived, and the Ocala highlands
area of north-central peninsular Florida, where the southern haplotype could have
survived. This latter area, which corresponds with the peninsular allozyme group,
existed as an island during the Pliocene (Stanley, 1986). With the retreat of the last
glaciers, populations of the two types came into contact, allowing hybridization to
occur, the result of which was the intermediate type seen along the southeastern
coastal plain.