origin and evolution of the biota of southeastern north america: evidence from the fossil plant...

Origin and Evolution of the Biota of Southeastern North America: Evidence from the FossilPlant RecordAuthor(s): Alan GrahamSource: Evolution, Vol. 18, No. 4 (Dec., 1964), pp. 571-585Published by: Society for the Study of EvolutionStable URL: http://www.jstor.org/stable/2406211 .

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ORIGIN AND EVOLUTION OF THE BIOTA OF SOUTHEASTERN NORTH AMERICA: EVIDENCE FROM THE FOSSIL PLANT RECORD1

ALAN GRAHAM2

Biological Laboratories, Harvard University, Cambridge, Massachusetts 02138

Accepted May 27, 1964

The biota of southeastern United States is a complicated assemblage of plant and animal communities. Some species are presumably derivatives of the original Meso- zoic vegetation that developed in the Ap- palachian highlands, while others were introduced later, as conditions became favorable for their migration and establishment. In this presentation an attempt will be made to characterize the older plant communities, and the conditions under which they evolved, and to demonstrate the existence of paleoenvironments suitable for the introduction of other components during subsequent periods of geologic time.

Consideration will be given primarily to the angiosperms, which first become abun- dant in eastern United States during the Cretaceous Period. This limits discussion to Late Mesozoic and Cenozoic vegetation, and only brief attention will be given to the plant communities of the Early Mesozoic and Paleozoic eras. The plants of these older periods belong mostly to extinct families and genera, and provide little direct evidence for interpreting the origin and evolution of the modern biota. As far as possible, emphasis will be placed on new data provided by pollen and spore studies, and certain geochemical and geophysical techniques currently being used to study past environments. This tends to minimize dependence on the older, and in some cases questionable, paleontological data.

THE PRE-TERTIARY PLANT COMMUNITIES

Paleozoic and Early Mesozoic.-During the Paleozoic and Early Mesozoic, ferns,

'An invited paper presented as part of the Society for the Study of Evolution's symposium, Origin and Evolution of the Biota of Southeastern North America, Chapel Hill, March 27, 1964.

2Present address: Dept. of Biology, Kent State University, Kent, Ohio 44240.

cycadophytes, and conifers, mostly of genera now extinct, dominated the land- scape of southeastern United States. These plants are distantly related to modern taxa and, although interesting from other points of view, contribute little to our knowledge of the present plant communities. The geologic record, however, provides some interesting information on the physical setting under which the biota developed. A series of paleophysiographic maps repre- senting conditions in eastern United States from the Mississippian to the Permian (cf. Schuchert, 1955, maps 43, 49, 55, and 56) illustrates the gradual uplift of the basins of deposition to form the mountain sys-tems of southeastern United States. By the end of the Permian Period the Appalachians were in existence, and from that time to the present (230 million years) there have been highlands in southeastern United States that provided a relatively stable environment for the development of the biota: Viewed from the standpoint of availability for occupation by flowering plants, the oldest large section of the region is the southern half of the Appalachian Upland, extending from central New York to northern Georgia and northern Ala- bama, and west of the Mississippi represented by the Ozark Plateau. Never, since it was first oc- cupied by angiosperms, has the Appalachian Upland of the United States (and Canada) been invaded by seas; and, except for its northern extension, it lies wholly south of the limits of the Pleistocene glaciation (Fernald, 1931).

In contrast to the plant and animal communities of the uplands, however, those along the margin were subjected to pronounced environmental stress. Landslides provided new and disturbed habitats as processes of erosion acted to reduce the mountain systems. The Cretaceous seas invaded the eastern, southern, and western margins of the highlands and then retreated to a position some miles east of the present coast. Most of the vegetation that de-

EVOLUTION 18: 571-585. December, 1964 571



572 ALAN GRAHAM

TABLE 1. Increase in percentage antgiosperms from the Patuxent (oldest) to the Patapsco (youngest)

Cretaceous formations of Maryland

Megafossils Microfossils

lormiiation No. No. Per cent Per cent ot angio- angio- angio-

species spernis sperms sperms

Patapsco 100 25 25 26 Arundel 30 5 16 10 Patuxent 100 6 6 7

veloped on this broad Cretaceous plain was inundated by reinvasion during the Terti- ary. The Tertiary seas did not extend as far inland as did the Cretaceous, however, and there resulted a borderland about 25 miles wide that has been available for plant and animal colonization for about 60 million years. From their maximum extent during the Eocene, the Tertiary seas progressively retreated, providing a gradual expansion of stable inland environments, while marginal communities continued to be subjected to changing physical conditions.

The orogenic activity taking place in the Appalachian region during the Paleozoic and Early Mesozoic played an important part in the evolution of the early angiosperm flora. In contrast to those portions of the North American continent that were later covered by Pleistocene ice, glacial outwash, or loess, inundated by Tertiary seas, or subjected to intense orogenic activities of Tertiary mountain building, the Appalachian highlands provided stable areas for the maintenance of the biota. From this upland reservoir, new plants were continually being introduced into an environment of change and stress in which the evolutionary development of new forms was favored.

Cretaceous vegetation.-The first record of angiosperms in eastern United States comes from the floras of the Lower to Middle Cretaceous Potomac Series of Mary- land. The progressive increase in percentage of angiosperms in the megafossil record of these floras has been known for some time, but only recently has a corresponding increase in angiosperm pollen been demon-

strated (Groot and Penny, 1960; table 1). Among the megafossils which have been recorded from the Potomac Series are Populus, Sassafras, Cinnamomum, Dios- pyros, Liriodendron, Magnolia, Myrica, Planera, Salix, Cornophyllum, Gledit- sio phyllumn, Laurophyllum, Ficophyllum, Quercophyllum, Juglandip/zyllum, Nelum- bites, Menispermites, Sapindopsis, and others.

From the Upper Cretaceous floras of the Raritan and Magothy formations of Long Island, Steeves (1959) identified pollen of the following taxa: Magnoliaceae, Ha- loragaceae (Myriopphyllum?), Juglandaceae (Platycarya?), Ficus, Myricaceae, Myrta- ceae, Anacardiaceae, Cyrillaceae, Symploca- ceae, Euphorbiaceae, Lauraceae, Fagaceae, Nyssaceae, Rhamnaceae, and Sparginiaceae. Megafossils from the same floras have been studied by Hollick (1904, 1906, 1912) and Berry (1905, 1911a, 1911b). They report such genera as Acer, Rhus, Cordia, Vibur- num, Celastrus, Cornus, Diospyros, Kal- mia, Quercus, Ilex, Juglans, Laurus, Persea, Sassafras, Baulinia, Cassia, Illicium, Lirio- dendron, Magnolia, Ficus, Comptonia, Myrica, Platanus, Rhamnus, Crataegus, Amelanchier, Populus, Salix, Sapindus, and others.

McLaughlin (1957) has studied the pollen and spores of the Late Cretaceous Bruhn Lignite of western Tennessee. He found pollen similar to that of the Erica- ceae, Nymphaeaceae (Nymphaea or Nu- pkar), Juglandaceae (Platycarya, Engel- hardtia), Betulaceae, Fagaceae (Castanea), Theaceae (Ternstroemnia), Leguminosae, Sapotaceae, and others.

The composition of these eastern Ameri- can Cretaceous floras provides information relating to the origin of an interesting pattern of floristic relationships evident in the southeastern flora. Table 2 is a listing of certain plant families presently found in southeastern United States and southeastern Asia. It is evident from the table that floristic affinity is expressed at various taxonomic levels. In some cases the families are identical between the two regions, while in others the genera and species are



BIOTA OF SOUTHEASTERN NORTH AMERICA 573

the same. For example, the Taxodiaceae occurs both in southeastern United States and in southeastern Asia, but there are no common genera. In contrast, the Araceae and Phrymaceae have identical species in each area. It may be assumed, a priori, that non-weedy taxa similar between eastern United States and other regions of the world once had a continuous, or more continuous distribution and that differences in the level at which floristic affinity is expressed are due to (a) the duration of geographic separation and (b) to differences in evolutionary rates among the various taxa. For the first of these assump- tions to have a basis in fact, however, it is necessary to demonstrate widespread distribution of plant species during some early period of angiosperm development, to ac- count for affinities at the higher taxonomic categories, and opportunity for interchange at later intervals, to explain specific and some generic affinities.

Table 3 shows the distribution of some families represented in the Cretaceous floras of Texas, Wyoming and Colorado, Delaware and Maryland, Tennessee, western Canada, and Alaska. Many families

TABLE 2. Floristic relationships between eastern United States and eastern Asia, arranged according to the taxonomic level at which affinity is ex-

pressed (columns 4 and 5)

No genera in: No. No. Family East East genera species

U. S. Asia common common

Taxodiaceae 1 7 0 0 Saururaceae 2 2 1 0 Schisandraceae 1 2 1 0 Aceraceae 1 2 1 0 Hippocastanaceae 1 1 1 0 Nyssaceae 1 3 1 0 Clethraceae 1 1 1 0 Salicaceae 2 3 2 0 Juglandaceae 2 6 2 0 Magnoliaceae 2 2 2 0 Fagaceae 3 5 3 0 Betulaceae 5 6 5 0 Berberidaceae 5 5 5 0 Araceae* 1 Phrymaceaet 1

* Symplocarpus foetidus. t Phryma leptostachya.

TABLE 3. Distribution of plant families (dicot- yledons) among Cretaceous floras of North America. 1, Texas (MacNeal, 1958); 2, Wyominig and Colorado (Dorf, 1942); 3, Delaware and Maryland (Raritan and Magothy Megafossil floras, as listed in Steeves, 1959); 4, Tennessee (McLaughlin, 1957); 5, western Canada (Bell,

1956); 6, Alaska (Hollick, 1930)

Family 1 2 3 4 5 6

Aceraceae x x x Anacardiaceae x x x Araliaceae x x x x x Caprifoliaceae x x x Celastraceae x x x x x x Cornaceae x x x x x Ebenaceae x x x Ericaceae x x x x Euphorbiaceae x x x Fagaceae x x x x x Juglandaceae x x x x Lauraceae x x x x x x Leguminosae x x x x x Magnoliaceac x x x x x x Menispermaceac x x x x x Moraceae x x x x x x Myricaceae x x x x x Myrsinaceae x x x Myrtaceae x x x x x Nymphaeaceae x x x Platanaceae x x x x x x Proteaceae x x x Rhamnaceae x x x x x x Salicaceae x x x x x Sapindaceae x x x Sapotaceae x x x x Sterculiaceae x x x x Tiliaceae x x x Vitaceae x x x x

now characteristic of the lower latitudes of the northern hemisphere ranged as far north as Alaska (cf. columns 1 and 6 of table 3). Comparisons at the generic level, and with floras from different parts of the world, further reveal the widespread occur- rence of many plant families during the Cretaceous. For example, Teixeira (1948, 1950) has studied some of the Mesozoic floras of Portugal and lists several of the same families and genera found in Creta- ceous deposits of Tennessee, Texas, Alaska, and other parts of the world: Lauraceae (Cinnamomum, Sassafras), Magnoliaceae, Cornaceae (Cornus), Vitaceae (Cissites), Nymphaeaceae, Araliaceae (Aralia), Meni- spermaceae (Menispermites), and others.



574 ALAN GRAHAM

The contribution of these Cretaceous paleobotanical studies has been to demonstrate the existence of a rather uniform vegetation during this period, allowing the establishment of similar taxa in different parts of the world. This affords at least a partial explanation for the common oc- currence of families and representatives of other higher taxonomic units in areas that are presently isolated geographically. As these areas, and their associated communities, later became isolated into smaller units, and subjected to various environmental influences, genera and species evolved that now characterize different geographic regions. The similarities between the families and other higher taxonomic categories of these regions, however, find at least partial explanation in the relatively uniform vegetation of the Cretaceous Period.

Cretaceous paleoenvironments.-The recent application of geochemical and geophysical techniques to problems of paleobi- ology has resulted in a better understanding of Cretaceous environments. In 1951 H. C. Urey described a technique whereby the temperatures of past geologic times could be determined. The element oxygen (016) has an isotopic form (018) which is incorporated into the developing shells of certain marine organisms. As these shells become part of the sedimentary record, the ratio of 016 to 018 is preserved. The amount of 018 in biogenic calcium carbon- ate varies with the temperature of the waters from which it is precipitated. By measuring the amount of 018 in the fossil shells of such invertebrates as belemnites, which are widespread in Cretaceous deposits, it is possible to determine the temperature of the water in which the organisms lived. The principles, limitations, and other details of the technique have been described by Urey et al. (1951).

Lowenstam and Epstein (1954) have obtained paleotemperature data from a series of belemnites collected from Scandi- navia (Sweden, Denmark) and England south through Holland and Belgium into

the Paris Basin. Other specimens were obtained from Algeria, India, Japan, Aus- tralia, and the United States. Their results substantiated conclusions based on previous paleontological evidence, that uniform temperature conditions prevailed during the Cretaceous and that the equatorial and warm-temperate belts were broader than at present. The lowest temperature records were obtained from Denmark (ca. latitude 56? N) and Australia (ca. latitude 25? S), indicating that incipient temperate conditions existed in the extreme northern and southern latitudes. These data are significant in determining the place of origin for the temperate deciduous habit of angiosperm communities. If further investigations substantiate the lower temperatures for high latitude regions during the Creta- ceous, this would support the concept of a holarctic origin for temperate biotas.

One group of belemnite specimens from the Upper Cretaceous Coon Creek tongue of the Ripley Formation at the type locality in western Tennessee yielded paleotemperatures within the range of 20 to 28? C. Since data from belemnites are believed to represent the lower limits of variation, and the results slightly below the yearly aver- age, temperatures of the shallow waters of the seas in western Tennessee during the Cretaceous were probably about 27-30? C. Waters of comparable temperatures are presently found around islands of the At- lantic, such as Bermuda (310 N latitude; Lowenstam and Epstein, 1954). The maximum summer temperature of Bermuda is near 940 F, the winter minimum near 440 F, and the mean annual near 710 F. The rainfall is approximately 58 inches per year and evenly distributed. Paleotemperature data, and corresponding climates of comparable land areas, suggest that the Creta- ceous climate in southeastern United States was warm temperate to subtropical.

The fossil flora of the Upper Cretaceous Ripley Formation of Tennessee (Berry, 1925) provides further evidence for warm temperate to subtropical conditions in the region. The most common families of the




Ripley flora, in order of abundance, are Leguminosae, Lauraceae (Cinnamomum, Laurus), Myricaceae, Moraceae (Ficus, Artocarpus), Celastraceae, Apocynaceae, Euphorbiaceae, Rhamnaceae, and Myrta- ceae. Although these families have representatives in the present southeastern flora, the largest number of species in most instances, and the areas in which they most commonly occur together, are in the regions farther to the south.

Variations in climate during the Creta- ceous are evident from paleotemperature analysis of belemnites distributed from the Lower Cretaceous (Albian, Cenomanian) through the Middle Cretaceous (Turonian, Coniacian-Santonian) into the Upper Cre- taceous (Campanian, Maestrichtian). Ac- cording to Lowenstam and Epstein (1954), there was a "progressive rise in temperature from the Albian through the Coniacian- Santonian, followed by a decline through the early Campanian and by an ill-defined drop to leveling off in the Maestrichtian" (p. 227). The drop in temperature in early Maestrichtian times may have resulted in a retreat of the subtropical and warm-temperate vegetation southward, but during the Early Tertiary, tropical species were again introduced into southeastern United States, as evidenced by the Eocene floras of the Mississippi Embayment.

THE TERTIARY PLANT COMMUNITIES

Before considering the Tertiary and Quaternary history of the southeastern vegetation it is necessary to outline the floristic relationships of its various components. These elements were incorporated into the biota during various periods of the Cenozoic Era, and their presence provides the framework for considering the time and magnitude of Cenozoic plant migrations in response to changes in the environment. The geographic affinities of many species in the southeastern biota lie with one or more of four regions (table 4): tropical, subtropical, and warm temperate regions to the south; eastern Asia and western North America; western Europe; and boreal

America. Climatic changes after the Cre- taceous Period provided opportunity for each of these elements to be introduced into southeastern United States.

The general succession of plant communities during the Tertiary epochs has been investigated most intensively in western United States. The results of these studies indicate that during Eocene time (extending from 58 to 45 million years ago) floras of tropical aspect existed at several widely separated localities. The Goshen flora, located in west-central Oregon, near latitude 440 N, and the flora of the Green River shales of Wyoming and Colorado contain numerous species with tropical to subtropical affinities. These floras constitute part of the Neotropical-Tertiary Geo- flora, which persisted in western United States until about Mid-Oligocene (30 million years ago).

From Mid-Oligocene to Lower and Middle Miocene, environments were transitional between subtropical and temperate conditions. The Arcto-Tertiary Geoflora had developed in the high northern latitudes and migrated southward in response to climatic changes. The ecotone between this community and the neotropical geoflora was determined primarily by physiography, and where variations in physical conditions produced a diversified environment, temperate and subtropical species grew in the same community.

By the Middle and Late Miocene the Arcto-Tertiary Geoflora was the dominant plant community in the middle to high northern latitudes. It was distributed across boreal Asia, North America, and Europe, and extended southward into regions where climate and physiography were favorable (i.e., southeastern and western United States, eastern Asia, and central Europe). During the Pliocene (beginning about 13 million years ago) the uplift of the Cascades and Coast ranges created arid conditions to the east of the mountain systems and the Madro-Tertiary Geoflora evolved in response to the xeric environment. This paleocommunity has no coun-



576 ALAN GRAHAM

TABLE 4. Floristic affinities of the vegetation of southeastern United States with four geographic regions: tropical, subtropical, and warm-temperate regions to the south; eastern Asia; boreal Amer-

ica; and Europe

I. Tropical, subtropical, and warm-temperate re- II. (continued) gions to the south (Dressler, 1954; Miranda and Diphylleia cymosa D. grayi Sharp, 1950; Sharp, 1951; Watson, 1891). The Caulophyllum thalic- C. robustrum species listed in the first column occur in eastern troides United States and in the regions listed to the right. Menispermum canadense M. dauricum

Temperate to warm temperate Schisandra glabra S. repanda, S. bicolor

Pinus strobus Mexico (Chiapas, Calycanthus floridus C. chznensis Oaxaca, Puebla), Sassafras albidum S. tzumu, S. ran- Guatemala daiense

Guatus florida M emala Penthorum sedoides P. chinense Cornus florida MexicoDeuaibraa D.snss Epidendrum conopseum Mexico (Morelos) Decumaria barbara D. sinensis Illicium floridanum Mexico (Vera Cruz, Cladrasts lutea Cl adastis (

Tamaulipas) Cladrastas lutea Cladrastis (4 species) Liqiiidabar styaciflua Mexico outh toPachysandra procum- P. terminalis, P. axil- Liquidambar styraciflua Mexico south to ben lrsP. stls

Nicaragua bens laris, P. stylosa Menispermum canadentse Mexico (Mexico and Stewartia malacoden- Stewartia (ca. 6 spe-

Nuevo Le6n) dron, S. ovata cies) Nyssa sylvatica Mexico (Hidalgo) Panax trifolius, P. Panax (3 species) Panicum villosissimum Mexico (Nuevo quinquefolius

Le6n) Mitchella repens M. undulata Polygonum virginianum Mexico (Hidalgo, Shortia galacifolia S. uniflora, S. sinensis

Puebla) Acer rubrum, A. saccha- A. pycnanthum Prunus serotina Mexico, Guatemala rinum

(different vars.) Carya myristiciformis C. cathayensis Fagus grandifolia Mexico (Hidalgo, III. Boreal America. The species listed occur in

F. mexicana) northeastern United States and as disjuncts or Epifagus virginiana Mexico (Hidalgo) southern extensions in southeastern United States.

Tropical Abies fraseri (paired with the northern A. Mastichodendron f oe- West Indies balsamea)

tidissimum var. Picea rubens foetidissimum Thuja occidentalis

M. foetidissimum var. British Honduras, Acer pensylvanicum gaumeri Mexico (Cam- Acer spicatum

peche, Yucatan) Alnus crispa Ardisia escallonioides Bahamas, Cuba, His- Betula alleghantietsizs

paniola, Mexico, Betula papyrif era var. coidifolia Central America Drosera rotundifolia

Dipholis salicifolia West Indies, S Mex- Gaultheria hispidula ico, British Hon- Juncus trifidus duras, Guatemala Scirpus caespitosus var. callosus

Licaria triandra West Indies (related Vaccinium macrocarpon species in Central Viburnum alnifolium and South Amer- V. cassinoides ica) Zygadenus glaucus

Nectandra coriacea West Indies, Mexico (Yucatan), British IV. Western Europe. The species listed occur in Honduras, Guate- eastern United States and in Europe as paired mala species (first half of the list), or as identical and

II. Eastern Asia (Li, 1952). The east American widespread species (second half of the list). species, given in the first column, are paired with Paired or closely related species the Asian species listed in the second column. American species European species American species Asian species Anemone quinquefolia A. trifolia

Saururus cernuus S. chinensis complex Buckleya distichophylla Buckleya (3 species) Convallaria montana C. majalis Pyrularia pubera Pyrularia (2 species) Oxalis montana 0. acetosella Nelumbo lutea N. nucif era Identical, widespread species Trautvetteria carolin- T. japonica Cassandra (Chamaedaphne) calyculata

ensis Drosera rotundif olia Jeffersonia diphylla J. dubia Juncus trifidus




terpart in the vegetation of eastern United States. As a result of conditions created by the rise of the western mountains, many Arcto-Tertiary species were eliminated from western United States. The Pleisto- cene glaciations further disrupted the continuity of the circumboreal temperate vegetation, leaving remnants of the Arcto- Tertiary Geoflora isolated in eastern North America, eastern Asia, Europe, and to a lesser extent, in western North America.

The changes in the western American Tertiary vegetation are thought to have been in response to climatic trends of regional or worldwide magnitude. Floras of tropical aspect existed in the northern latitudes of many parts of the world and were followed by a more temperate vegetation that was modified by climatic changes culminating in the Pleistocene glaciations. The fact that the Cenozoic climatic trends evident in the floras of western United States are documented in floral sequences from other regions means that a similar pattern should be reflected in the Tertiary floras of eastern United States. Lower Tertiary communities should contain a relatively high portion of tropical to subtropical species, while those of the Middle Tertiary should be somewhat intermediate, and those of the Upper Tertiary temperate in composition. The establishment of a general sequence of floral composition is useful in interpreting the Tertiary communities in eastern United States, where the floras have not been studied as intensively, or as recently, as those of the west.

The Wilcox flora of the Gulf Coast does reflect a tropical Eocene vegetation. The flora reportedly contains such genera as Anemia, Lygodium, Zamia, Canna, Ficus, Manihot, Cedrela, Banisteria, Sterculia, Artocarpus, Banksia, Coccoloba, Magnolia, Hiraea, Cinnamomum, Oreopanax, Aristo- lockia, Nyssa, Nectandra, Prunus, and Liquidambar. This community extended as far north as southern Illinois (latitude 370 N), and its presence indicates a climate with minimum winter temperatures above 32? F. By comparison, the present mini-

mum winter temperature recorded at Cairo, Illinois within the last 40 years is -14? F.

In evaluating the paleoenvironments of southeastern United States during the Eocene, however, it is important to recog- nize that the Wilcox flora is essentially a strand community. As the Tertiary seas progressively retreated southward, the fossil record was biased toward tropical plants growing close to the basins of deposition. An accurate concept of Eocene vegetation and environments will ultimately depend upon a better understanding of the inland communities. The numerical representation of megafbssils in a flora is determined by the proximity of the plant to the site of deposition, height of the plant, and deciduous vs. evergreen habit. The mirco- fossils, being wind distributed, tend to give a more regional picture of the vegetation, and include taxa unlikely to enter the megafossil record (i.e., low-growing herba- ceous plants with leaves that are retained on the stem, such as temperate representatives of the Compositae, Gramineae, Um- belliferae, Amaranthaceae, Chenopodiaceae, and others). A combined study of both the mega- and microfossils provides the most complete basis for reconstructing paleoenvironments and it is unfortunate that few such studies are available for the Lower Tertiary floras of southeastern United States. Gray (1960) has made a brief study of the microfossils of the Clairborne (Eocene) flora of Alabama and found a number of temperate pollen types repre- senting plants that presumably grew inland from the more tropical marginal communities. She lists the presence of Tilia, Alnus, Ulmus, Ostrya-Carpinus, Carya, Castanea, Celtis, Fagus, Juglans, Liquidambar, Lirio- dendron, Myrica, Nyssa, Quercus, and others. Further studies may reveal that by the Eocene the temperate Arcto-Tertiary Geoflora was the dominant community in southeastern United States, and that tropical species grew in favorable habitats that were atypical of the total biota.

The only Oligocene flora from eastern United States that has been studied re-



578 ALAN GRAHAM

TABLE 5. Comparison of climates between cen- TABLE 6. Distribution of percentage native gen- tral Florida (Fort Lauderdale, Everglades, Bar- era in the eastern deciduous forest, from Eocene tow), and Rutland, Vermont. Present conditions through Pliocene times (data from Barghoorn, in peninsula Florida are probably similar to those 1951)

in central Vermont during the Oligocene Age Unidentified Identified Identified

Jan. avg. July avg. Annaal Flora (106 yr) or extinct exotic native Locality and and rainfall genera genera genera

mininmum maximum (inches) Wilcox 58 47 34 19

Fort Lauderdale 68.9, 28 81.4, 99 62.9 Alum Bluff 26 16 42 42 Bartow 61.8,18 81.6, 102 55.7 Calvert 19 10 33 57 Everglades 67.4, 28 81.8,100 55 Citronelle 5 0 14 86 Rutland 19.8,-30 69, 98 37.4

cently is the Brandon Lignite flora of Vermont. The beds are underlain by Pre- cambrian sediments and overlain by Pleisto- cene drift, hence the exact age of the flora is not known. On the basis of floral composition, it is estimated to be Late Oligo- cene or possibly Early Miocene in age. Traverse (1955) has studied the pollen and spores of the Brandon Lignite:

Several of the groups of plants now shown to be present by the palynologic studies intensify the subtropical, exotic aspect of the flora and increase the likelihood of lower Tertiary, presumably upper Oligocene, age for the deposit. These finds are: Glyptostrobus, a taxodiaceous Chinese ecologic equivalent of Taxodium; Jus- siaea, an onagraceous plant distributed mostly in wet locations in the tropics and subtropics; Engelhardtia, a juglandaceous genus of subtropical and primarily southeastern Asiatic modern distribution; Planera (Ulmaceae) and Liquidambar (Hamamelidaceae), indicators of subtropical or warm temperate climate and moist to swampy conditions; Alangiumn, a tropical to subtropical genus of Old World distribution in the modern flora. Most important was the discovery of the Manilkara-Mimusops group of the Sapotaceae. This group is now confined mostly to southern Florida in its modern distribution in the continental United States. The exact plant association most nearly equivalent to the Brandon flora is that of the bays and similar swamps of southeastern United States. Many of the important genera of the Brandon flora are found in these swamps (p. 1).

Climatic conditions presently prevailing in peninsular Florida approximate those of Vermont during the Upper Oligocene. The climate of Fort Lauderdale, Bartow, and Everglades, Florida is compared with that of Rutland, Vermont (15 miles southeast of Brandon) in table 5. The Brandon flora,

like many Oligocene floras from other parts of North America, is transitional between a tropical and temperate community.

By the beginning of the Miocene the Eastern Deciduous Forest had assumed essentially its present structure and composition. The rate of development of the biota is illustrated by comparing the floras of the Wilcox Group (Eocene; Berry, 1930), Alum Bluff (Oligo-Miocene; Berry, 1917); Calvert (Middle Miocene; Berry, 1916a), and Citronelle (Pliocene; Berry, 1916b). The Wilcox flora consists of approximately 100 genera of megafossil plants. Of this number, 19 are native to southeastern United States. The Citronelle flora contains an equal number of genera, and 86 are native. The change from a flora in which the tropical exotic element was common to one which was essentially modern and temperate took about 53 million years (table 6). Later events in- volved (1) expansion of the temperate biota as Tertiary seas continued retreat toward their present position, (2) natural evolutionary development and speciation, (3) rearrangement of communities during the Pleistocene, and (4) modifications re- sulting from the influence of man.

Cenozoic land connections.-Climatic barriers between the biotas of southeastern United States, eastern Asia, and Europe are temporal, and are not characteristic of long periods of geologic time. Changes in climate, as documented in the geochemical and geophysical properties of the sediments, as well as in the fossil record, have provided numerous opportunities for the




interchange of species between regions now isolated geographically. The vegetation of the Cretaceous Period was relatively cos- mopolitan in distribution and many floristic similarities, presently expressed at the family and, in some cases, the generic level, probably originated during this time. De- rivatives of this early vegetation constitute an original core of plant species to which various elements have been added during the Tertiary and Quaternary periods. Tropical climates afforded opportunity for the introduction of southern species from Cretaceous times through Mid-Oligocene, a duration of about 60 million years. Tem- perate elements from Asia and Europe had suitable climates for southward migration from the north during the Cretaceous and Tertiary periods, about 85 million years, and boreal elements from continental United States could have been introduced during the Pleistocene, until about 11,000 years ago.

Climate, however, is not the only factor influencing interchange between biotas. The distribution potential of the propagules, and continuity of land mass, are also important. The present arrangement of continents and islands in the northern latitudes suggests the likelihood of a connection across the present Bering Sea into Eurasia, and across the North Atlantic into Europe. The existence of such a land connection during the Cenozoic would facilitate exchange between the eastern American flora and the vegetation of Eu- rope and Asia.

The Bering Land Bridge.-The village of Wales, on the Seward Peninsula of Alaska, is separated from Uelen, on the Chukchi Peninsula of Siberia, by approximately 60 miles of water that varies in depth from 100 to 500 feet. Several lines of evidence indicate that North America and eastern Eurasia were connected across this region several times during the Cenozoic. It is interesting to compare the conclusions of different authors concerned, respectively, with the paleobotany, zoogeography, and geology of the Bering Sea area.

Chaney (1930) secured a small collection

of fossil plants from St. Lawrence Island, located about 40 miles east of Siberia and 100 miles west of the Seward Peninsula, near latitude 63? N. The matrix was a non-marine sediment of Eocene age. Fos- sils similar to several modern genera were identified (Platanus, Alnus, Populus), but the most common was reported as Sequoia langsdorfi, now known to represent remains of Metasequoia. The latter genus is presently an endemic to China, but its widespread distribution in Tertiary floras of western United States and Europe suggests that "St. Lawrence Island represents the remnant of a land bridge which connected Asia with North America-a bridge over which the redwood forest was essentially continuous during at least the first half of the Tertiary, and across which not only land plants but land animals were able to migrate from one continent to another" (Chaney, 1930).

Simpson (1947) has made an extensive comparison of the fossil and modern mammalian faunas on either side of the Bering Sea, and notes that the faunas show peaks of similarity, alternating with periods when they were more distinct. He attributed this pattern to alternating periods of land continuity and discontinuity, and concluded that major faunal interchange took place across a continuous land area during the early Eocene, late Eocene, early Oligocene, late Miocene, and middle-to-late Pliocene, as well as several times during the Pleisto- cene Epoch.

Hopkins (1959) has studied the geology of the Bering area, with particular reference to the stratigraphy and structure of Wran- gell Island north of Siberia and Lisburne Peninsula in northwestern Alaska. He finds that "The geological evidence indicates quite clearly that Siberia and Alaska represent segments of a single continental mass, separated by a segment only temporarily submerged, the Bering-Chukchi platform" (p. 1519) and that "We may conclude that the area of the Bering and Chukchi seas lay above sea level throughout most of the last 50 or 60 million years."

The agreement in results of these various



580 ALAN GRAHAM

studies indicates that interchange between the biotas of North America and Asia was possible several times during the Cenozoic Era.

The North Atlantic Land Bridge.-The present separation between eastern North America and Europe is more pronounced than across the Bering Sea, and there is little evidence of complete continuity during the Cenozoic Era. The connection was sufficient, however, to allow many plants now growing in eastern United States and Europe to cross via such Arctic Islands as Spitsbergen, Ellesmere Island, Greenland, and Iceland. A pollen and spore study of Lower Tertiary sediments on these islands has been made by Manum (1962), who also provides a summary of the earlier megafossil studies of Heer and other in- vestigators. Identified genera include Ginkgo, Metasequoia, Nymphaea, Cercidi- phyllum, Hamamelis, Acer, Aesculus, Alnus, Betula, Corylus, Juglans, Planera, Ulmus, and others. The presence of these plants indicates a temperate environment similar to that presently prevailing in southeastern United States and southeastern Asia. The presence of Ginkgo and Metasequoia, both in these deposits as well as in Europe, dem- onstrates opportunity for interchange between the biotas of eastern United States and Europe during the Cenozoic Era.

THE PLEISTOCENE PLANT COMMUNITIES

During the Pleistocene Epoch, plant communities were influenced by climatic variations correlated with the advance and retreat of the glaciers. Considerable discussion has centered around the extent of this influence on the vegetation of regions beyond the glacial boundary, such as southeastern United States.

One line of evidence indicates a significant modification of the southeastern communities during the Pleistocene Epoch. In 1938 Brown reported megafossils of Larix laricina, Picea glauca, and Thuja occidentalis in Pleistocene deposits of Louisiana. Spruce pollen has also been recorded in peat bogs from Florida. These early re-

ports served to emphasize the importance of the discovery of Potzger and Tharp (1943, 1947, 1954) of 11% spruce and fir pollen in a bog from central Texas. In addition to the pollen of these boreal species in the lower levels of the bog, pollen percentages of grass, Alnus, Quercus, Carya, and Castanea varied and were interpreted to indicate climatic changes from cool (spruce-fir), to warm-dry (Quercus and grasses), to warm-moist (Alnus and Cas- tanea), to warm-dry (Quercus, Carya, grasses). The pollen record not only showed a significant displacement of boreal conifers beyond their present distribution, but reflected major changes in vegetation during Late Glacial and Post Glacial times.

In 1949 Deevey, relying in part on the paleobotanical studies cited above, provided an extensive summary of information relating to Pleistocene biogeography. Re- garding the effect of Pleistocene climates on the vegetation to the south, he concluded that "glacial chilling in the southeastern States must have been fairly extensive." Furthermore, if forests of spruce and fir grew in Florida, Louisiana, and Texas, the present temperate deciduous forest must have been forced far to the south and survived "in peninsula Florida and in Mexico, and have subsequently migrated to their present localities." A critique of this concept has been published by Braun (1955).

In contrast to the theory of pronounced vegetational changes in the Southeast during the Pleistocene, other lines of evidence indicate only a slight modification. A corollary to the theory of significant southward displacement of the southeastern biota is that certain similarities between this community and those of Mexico date from the Pleistocene. This would require re- placement of the presently effective barrier of xeric and semixeric vegetation of south Texas and northern Mexico by more mesic communities. Martin and Harell (1957) have considered this possibility in a study of the humid montane forests found along the escarpments of the Mexican and Guate-




malen plateaus. They believe that if plants and animals migrated from southeastern United States into refugia in Mexico during the Pleistocene, "we may expect unmis- takable faunal evidence of this event. However, if the forest connection is more ancient, as Braun asserts, any residual faunal evidence should accordingly be less obvious and at a higher taxonomic level." The distribution and ecological preference of several animal groups were studied (amphibians, reptiles, mammals, birds). In each instance the evidence favored interchange between the biotas of eastern United States and the Mexican highlands during the Tertiary, rather than the Pleistocene, and suggested that the desert barrier of south Texas and northern Mexico was not obliterated during the Pleistocene.

In a study made by Graham and Heimsch (1960) a pollen profile from a bog in central Texas, near the one studied by Potzger and Tharp, failed to demonstrate any significant change in the plant communities during the last 12,000 years. This necessitated a restudy of the material of Potzger and Tharp. It was found that in their samples boreal coniferous pollen con- stituted only 2% of the microflora, rather than 11%, and that the presence of Abies and Castanea could not be verified. Dif- ferences were also found in the pollen percentages reported for various taxa in the upper levels of the bog. Although 2 % spruce pollen in Texas still requires explanation, the revised profile does not suggest a pronounced change in the southern and southeastern vegetation during the Pleistocene.

On the basis of paleobotanical evidence as it existed at the conclusion of the investigations of Potzger and Tharp (in 1954), it was possible to conclude that a boreal forest of spruce and fir extended from Florida, across Louisiana, into central Texas, and that the temperate deciduous forest was displaced southward into refugia in Mexico and peninsular Florida. A re- evaluation of the evidence in recent years, however, does not agree with such a con-

clusion. It is more likely that although the ranges of certain species were modified during the Pleistocene, the general effect of climatic changes on the biota of southeastern United States was not extensive.

THE POSTGLACIAL PLANT COMMUNITIES

Broecker et al. (1960) have presented evidence for an abrupt change in climate close to 11,000 years ago. Surface temperatures of the Atlantic Ocean increased by several degrees centigrade; deep-sea sedimentation rates decreased; bottom waters of the Cariaco Trench stagnated; and pluvial lakes in the Great Basin shrank from maximum volume to nearly present size. The magnitude of this change was a 6-10? C increase in surface water temperatures and occurred over a period of less than 2,000 years. These events presumably mark the transition from glacial to postglacial times.

After 11,000 years B.P., climates in southeastern United States were essentially those of the present. This does not mean, however, that new introductions into the biota have ceased. The present diversity in physical conditions provides outliers of environments that approximate each of the climatic phases of the Tertiary and Quater- nary periods. For example, the present environments of peninsular Florida are similar to those that were widespread during the Lower Tertiary. The southern portion of the Appalachian region is warm temperate and grades through cool-temperate to cold-temperate conditions north- ward, while bogs and high-altitude areas in the south provide habitats favorable for the growth of species typical of more northern latitudes. Thus it is possible that plants from the four major regions with which the southeastern biota shows affinities (eastern Asia; boreal America; western Europe; and tropical, subtropical, and warm-temperate regions to the south) are being introduced at the present time via long distant trans- port. The climatic trends of the past provided greater opportunity for the establishment of these plants by periodic expansion



582 ALAN GRAHAM

of favorable environments, but physio- graphic diversity has perpetuated relicts of each paleoclimate into modern times.

Examples of introduction during Recent and Post Glacial times are numerous. Some are the result of inadvertent dispersal through human activities, such as weeds introduced with ship ballast along the Atlantic seaboard. Others originated as escapes from cultivation that have persisted and become naturalized, such as the three Asian species of Eleagnus presently growing in eastern United States (E. multi- florus, E. pungens, and E. umbellatus). Further examples include the introduction of Lonicera japonica from Asia, Eichornia crassipes from South America, and Poly- gonum convolvulus from Europe. In some instances the species are known to be of recent introduction, but the dispersal vector has not been determined. Cuphea glutinosa was first collected in the United States in 1884 from a population in Vermilion Parish, Louisiana. It is known elsewhere only from central and southern South America (Argentina, Brazil). Similarly, C. carthagenensis was not reported from the United States until the early 1900's, when plants were found along the Florida coast. Since that time C. carthagenensis has spread to Louisiana and along the Atlantic coast to North Carolina.

Another modification of the southeastern flora occurring in Recent times is the elimination of certain species. The best known example is Franklinia alatamaka, described from McIntosh County, Georgia, in 1785. It was last seen in the wild in 1803, and presumably exists now only in cultivation.

These few examples illustrate that the plant communities of southeastern United States continue to undergo changes in composition during Post Glacial and Recent times. This means that information concerning the origin of the southeastern biota can be obtained by studying the taxonomy, ecology, natural history (i.e., pollinators, seed-dispersal mechanisms), distribution, and processes of evolution currently operat-

ing within the plant and animal communities. The present biota, however, is a product of influence and response operating through long periods of geologic time. The factors that influence the development of the biota in Recent and Post Glacial periods (human activity and glaciation) are not the same in kind or degree as those of the Tertiary (climatic trends toward cool to cold-temperate conditions, progressive retreat of epicontinental seas), the Meso- zoic (origin and spread of the angiosperms), or the Paleozoic (orogenic changes in the earth's surface). Consequently, a more complete understanding of the origin and development of the southeastern biota will depend not only on continued studies of the modern plant and animal communities, and factors presently influencing their evolution, but further research into the past history of the biota.

SUMMARY AND CONCLUSIONS

1. Studies on the Cretaceous floras of eastern United States reveal that a portion of the species comprising the modern biota probably have evolved more or less in situ from older Mesozoic vegetation. Examples include species of genera known to have been present in eastern North America during the early period of angiosperm evolution: Certain species of Acer, Betula, Jug- lans, Quercus, Populus, Salix, and others.

2. The first supplement to derivatives of the original angiosperm flora occurred during the Eocene and Early Oligocene. Land connections and climate were favorable for the introduction of tropical vegetation into the Southeast during the Early Tertiary. Once these species were introduced, later Tertiary trends toward colder climates produced one of three responses:

a. Some tropical species were eliminated from the biota as a result of lower minimum winter temperatures and reduction in annual rainfall. Examples are to be found among the fossil plans reported from the Eocene Wilcox flora, but no longer represented in the southeastern flora: Manihot,




Cedrela, Sterculia, Artocarpus, Hiraea, Cinnamomum, Oreopanax, and others.

b. Other tropical species persisted in refugia such as peninsular Florida. Ex- amples may be found among such species as Rhizophora mangle, Laguncularia race- mosa, Schaefferia frutescens, Tillandsia setacaea, Avicennia germinans, Annona glabra, Dipholis salicifolia, and others.

c. The remainder of tropical species introduced during the Early Tertiary evolved into types capable of existing under temperate conditions. Examples of this response are found among plants that are widespread in eastern United States but belonging to genera and families more typical of tropical regions. The family Thymelaeaceae, for example, contains about 40 genera and some 500 species, primarily of tropical and subtropical distribution. Only a single species, Dirca palustris, occurs in southeastern United States, but it is widely distributed and well adjusted to growth in a temperate environment. Simi- larly, Diospyros virginiana is the northernmost representative of the predominately tropical to subtropical family Ebenaceae, and Asimina triloba is the northernmost representative of the Annonaceae. Another example is Cuphea viscosissima, widespread in the Southeast. The genus is represented in Latin America by approximately 250 species.

3. A second group of plants was inter- changed between the southeastern biota and Europe and Asia during the Tertiary via migration across the Bering and North Atlantic land bridges. These are derivatives of the Arcto-Tertiary Geoflora and probably include species of such genera as Calycanthus, Schisandra, Cladrastis, Pachysandra, Stewartia, Mitchella, Pla- tanus, Quercus, Acer, Juglans, Ulmus, and others.

4. During the Pleistocene Period certain boreal elements were introduced that presently persist in bog and high-altitude habitats, or in other favorable environments. Examples of boreal species in the southeastern flora include Picea rubens, Vac-

cinium macrocarpon, V. alnifolium, Betula papyrifera var. cordifolia, and Gaultheria hispidula.

5. The last significant modification of the southeastern biota has occurred during the Post Glacial and Recent times, and particularly within the last 300 years as a result of human activity. These recent modifications include the introduction and spread of weedy species (i.e., Polygonum convolvulus), escapes from cultivation (Eleagnus), elimination of certain species (Franklinia alatamaha), and parceling of vegetation into smaller units as a result of land use. The latter effect tends to limit the size of the breeding systems, reducing the number of variations possible and the chances of their establishment in the population.

As a result of these recent modifications, it has become difficult in many instances to determine whether a particular species is a recent introduction or a relict of earlier migrations in response to climatic change. In general it may be assumed that if a well-established, non-weedy disjunct species has no visible means of effective long- distant dispersal, its distribution can most likely be related to changes in the earth's surface and to paleoclimates. Occasionally the distribution pattern itself can provide information as to the time of introduction. For example, plants disjunct in peninsular Florida that are confined to or occur primarily on coastal Quaternary limestones probably are of recent introduction, rather than relicts of a tropical Eocene vegetation. Studies on the distribution potential of the propagules and edaphic requirements of the plant could, in this example, be used to determine the most probable time and mode of introduction. Further studies are needed to establish accurate patterns of distribution, both for modern species and their fossil counterparts, and to clarify the taxonomy and ecology of the plants. On the basis of continued refinement of the floral and faunal evidence, more detailed explanations concerning the origin and evolution of the southeastern biota will eventu- ally be possible.



584 ALAN GRAHAM

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