Appendices & Glossary - Botanical Research Institute of Texas

1206 APPENDIX TEN/EAST TEXAS AS A UNIQUE HABITAT
standpoint of biogeography, eastern deciduous forest elements are one of the most fascinating components
of the East Texas flora. The fossil record shows that many plants once had distributions across
much of the Northern Hemisphere—for example, temperate forests with tropical elements occurred
very broadly and reached their maximum extension in the mid-Tertiary Period (the Tertiary extended
from 65 million years ago (mya) to 5 mya). According to Graham (1999), “there was a belt of vegetation
similar in structure and composition extending around much of the northern hemisphere.” This
widespread flora has been variously referred to as the Arcto-Tertiary flora, the Tertiaro-mesophytic
flora, the boreotropical flora, or a mixed mesophytic forest. Geohistorical events from the mid-Tertiary
to the present have included alterations in the shapes of the northern land masses, plate movements,
fluctuations in sea levels, mountain building, and profound changes in the climate. Specific examples
include land bridges across what are now the Bering Sea and the North Atlantic (due to lowering of
sea level) and fossil evidence of periods of global climate much warmer than at present—e.g., broadleaved
deciduous forests in polar areas, and glaciation. As a result of these factors, there have been considerable
changes in both the composition and the disposition of the flora, and the ranges of many
plant groups have been greatly restricted (e.g., eliminated from Europe and w North America). A number
of species of the once widespread Northern Hemisphere Tertiary flora have survived in one or
more of four main Tertiary relict areas—1) eastern Asia; 2) eastern North America; 3) western North
America; and 4) southeastern Europe—however, many survived in only two of these areas, eastern
Asia and eastern North America. According to Wen (1999), ca. 65 genera of seed plants have this disjunct
eastern Asia–eastern North America distribution. East Texas examples include, but are not limited
to, Aletris, Ampelopsis, Apios, Brachyelytrum, Campsis, Carya, Diarrhena, Halesia, Hamamelis,
Lindera, Ly onia, Menispermum, Nyssa, Parthenocissus, Penthorum, Phryma, Podophyllum, Sassafras,
Saururus, Stewartia, Tipularia, Tr achelospermum, Triosteum, and Zizania. 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 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. This biogeographical
pattern has sometimes been referred to as the “Asa Gray disjuction” in honor of the Harvard
botanist who was one of the earliest to recognize its significance. It should be noted that this disjunct
distribution pattern has complex and multiple origins (e.g., dispersal across both the Bering and North
Atlantic land bridges), with similar present day distributions differing in time and manner of origin.
However, the consensus is that the eastern Asia-eastern North America pattern is in general a relict of
the maximum development of Northern Hemisphere temperate forests (with tropical elements) in the
Tertiary, with greater survival in eastern Asia and eastern North America and higher rates of extinction
in Europe and western North America due to such factors as glaciation and mountain building
(Li 1952; Little 1970; Wood 1970, 1972, Graham 1972, 1993a, 1999; Wolfe 1975, 1987; Boufford &
Spongberg 1983; Hamilton 1983; Hsü 1983; Wu 1983; Ying 1983; Tiffney 1985a, 1985b; Upchurch &
Wolfe 1987; Cox & Moore 1993; Wendt 1993; Xiang et al. 1998; Wen 1999, 2001; Dilcher 2000; Xiang &
Soltis 2001).
A related floristic relationship is the similarity between some isolated forests in the mountains of
Mexico and those in the eastern United States. Numerous East Texas deciduous forest genera (e.g., Acer,
Alnus, Carpinus, Cercis, Crataegus, Cornus, Epifagus, Fagus, Fraxinus, Juglans, Liquidambar, Magnolia,
Mitchella, Myrica, Nyssa, Ostrya, Pedicularis, Platanus, Prunus, Quercus, Rhus, Smilax, Tilia, Ulmus,
and Vaccinium) and even a few species (e.g., Carpinus caroliniana, Epifagus virginiana, Fagus
grandifolia, Nyssa sylvatica, Liquidambar styraciflua, Mitchella repens) occur broadly across the eastern
United States as far west as eastern Texas and then reappear in the Mexican highlands and in some
cases even in Guatemala (Miranda & Sharp 1950; Martin & Harrell 1957; Thorne 1993d; Graham 1993a,
1999). Most of the deciduous forest “temperate” genera occur in “isolated patches of humid montane
forest, typically in the Sierra Madre Oriental,” usually above 1,000 m elevation (Martin & Harrall 1957).
These occurrences in Mexico represent a disjunction, across a zone of arid grassland and thorn scrub,
of at least 500 kilometers from the nearest East Texas locations, and in some cases much more (Martin
& Harrall 1957). At one time, the similarities were thought to be the result of the migration of vegetation
zones associated with Pleistocene glaciation (Dewey 1949). However, the fact that this relationship