ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...

ORIGIN AND DIVERSITY OF EAST TEXAS FLORA/INTRODUCTION 205
eastern and western North America and eastern Asia (an East Texas example is the genus
Trillium). Molecular phylogenetic work suggests that for a number of genera, the eastern and
western North American species are sister groups, “which in turn are the sister of the Asian
species” (Xiang et al. 1998). Such a consistent pattern suggests a common geographic history
and can “be viewed as support for the long-standing hypothesis that the disjunction in eastern
Asia, eastern North America, and western North America represents the fragmentation of a once
continuous plant community” (Xiang et al. 1998). Unfortunately for the Tertiary floras of western
North America (and Europe as indicated above), geologic and climatic changes resulted in
the elimination of many species from those areas.
However, a significant number of genera have survived in only two geographically distant
Tertiary relict areas, eastern Asia and eastern North America. This striking distribution pattern
has long been of interest to botanists (e.g., Linnaeus in Halenius 1750—the dissertation of one
of his students; Gray 1846, 1859) and continues to be so today (e.g., Li & Adair 1994, 1997;
Xiang et al. 1998; Graham 1999; Wen 1999, 2001; Xiang & Soltis 1999; Donoghue et al.
2001). Because of Gray’s early work on this biogeographic pattern, it has sometimes been
referred to as the “Asa Gray disjunction” (Wen 1999). Some of the most recent research (Wen
1999) indicates that this disjunct distribution pattern occurs in 65 genera of flowering plants.
The genus Carya is one example (Fig. 120); other East Texas examples include Aletris,
Ampelopsis, Apios, Brachyelytrum, Campsis, Diarrhena, Halesia, Hamamelis, Lindera, Lyonia,
Menispermum, Nyssa, Parthenocissus, Penthorum, Phryma, Podophyllum, Sassafras, Saururus,
Stewartia, Tipularia, Trachelospermum, Triosteum, Wisteria, and Zizania (Li 1952; Little 1970;
Boufford & Spongberg 1983; Hamilton 1983; Hsü 1983; Wu 1983; Ying 1983; Cox & Moore
1993; Graham 1993a; Wen 1999). In fact, representatives of 33 of the 65 genera (51%) cited
by Wen (1999) as eastern Asia-eastern North America disjuncts occur in East Texas. In the
words of Graham (1993a), “It is well known that the broad-leaved deciduous forests of eastern
North America and eastern Asia are floristically related…. It results from the maximum
extension of the temperate deciduous forest in the mid-Tertiary and its disruption in western
North America during the Pliocene and in western Europe during the Quaternary.” This is one
of the most ancient components of the East Texas flora—by at least the Early Tertiary Period
(Eocene Epoch—55.5–33.7 mya), deciduous vegetation was present across the middle of the
North American continent, including such familiar genera as Acer, Celtis, Liquidambar, Populus,
and Rhus (Graham 1993a). In addition to present-day disjuncts between the two areas, fossils
of numerous present-day Asian genera (e.g., Ailanthus, Ginkgo, Metasequoia) have been found
in North America (Graham 1999).
FIG. 120/ WORLDWIDE DISTRIBUTION MAP OF CARYA (JUGLANDACEAE) SHOWING ITS DISJUNCT OCCURRENCE IN EASTERN ASIA AND EASTERN
NORTH AMERICA (FROM WU 1983).