By

Donald R. Hammer
©  August 19, 1981

As early as December 1895 in the  “The American Geologist”, the term “Ozark Uplift” included the Shawnee Hills in Illinois and Kentucky, the Ozark Plateau and St. Francois mountains in Missouri, the Boston mountains and Ouachita Mountains in Arkansas. The uplift area extends from southern Illinois to Eastern Oklahoma. It stands as divide between the coastal plain, as represented in the Mississippi embayment, and the Great plains. The middle of the uplift is a typical high plateau; around the margins are the areas of more pronounced topographic diversity known by the different local names given above. Each is considered separately. The Shawnee Hills are ridges and represent structural features. The St. Francois mountains are made up of isolated peaks irregularly clustered. The Boston Mountains form a range of steep-sided elevations lying between the White and Arkansas rivers. The Ouachita Mountains show numerous anticline ridges.

The Ouchitas, in particular, are believed to belong structurally as part of the Southern Appalachian belt which is known as the Ouachita Orogeny.  These mountain ranges formed during Cambrian times as part of a landmass collision between Laurentia and the Acatlán Complex originally attached to Gondwana. Gondwana and the complex eventually slammed into North America, about 300 million years ago. Much of the Boston Mountains and the Ozark Plateau formed on the leeward side of the Ouchitas, along with the interconnecting Appalachian Plateau then.  Evidence of this bears out in structural similarities of the two regions as well as the comparative paleo-magnetism of the rock itself.  

By 225 million years ago, all the landmasses came together to form the super-continent of Pangaea.  During this time, plant life was evolving and invading the different reaches of the super-continent.

Moving forward to the time of the Jurassic period, 150 million years ago, the North American Continent started to separate from the Eur-Asian Continent.  However, by that time, plants had 250 million years of evolution behind them, and were well advanced in development and structure to survive.

 By the time of the Cretaceous period, there were seas separating the western half of North America from the eastern half.  The Ozark Uplift was very much surrounded by seas then, with only a connecting bridge through the Shawnee Hills to the Southern Appalachians and Appalachian Plateau.  It would appear that by this time, the Ozark Uplift would have accommodated a very diverse plant life community, since temperatures would have been moderated by the surrounding seas, and it would have had ample moisture.

During the Late Cretaceous to Tertiary era, from 80 million years to 55 million years ago, there were still land bridges connecting the Eur-Asian Continent with North American continent, but only in the very north latitudes.

 The Eocene period of the Tertiary, (57.8–36.6 Million years ago), was of great importance to plants because of the high amount of carbon dioxide in the atmosphere, and the earth appeared to have been warm and free of glaciation at these high latitudes. The Eocene has been considered by many as an unusually warm period in Earth's history. Latest Paleocene Thermal Maximum rocks record a methane release, severe winter cooling in Polar Regions. This event resulted in a dramatic 4–8 °C increase in deep-ocean, high-latitude, and continental temperatures, marking the onset of warm conditions that extended into the Eocene.
 This period also significant because it represents the first fossilized evidence we have that the family of Castanea and other closely related fagaceous genera include Fagus and Castanopsis.  They were abundant both in China, in Western North America, Eastern North America, throughout Europe and the Western Middle East in the vicinity of what is now the Black Sea..  Fossilized Eocene evidence of Castanea ungeri pollen and leaves have been found as far north as 70 degrees North latitude in Atanikerdluk Greenland, and in areas of Canada and Alaska.

During the time of the Pleistocene, from about 2 million to within 8000 years ago,  the climate changed to cause massive glaciation.  Consequently, glaciation affected major changes in the range of the plants, and isolated many plant communities.  There were changes in the dominance and plant associations too. Other plants either died out or were able to migrate towards the warmer climates to the south via “successive generational dispersion”.  The areas where glaciation occurred were scoured down to bedrock, so much of the fossilized evidence was obliterated or carried away as sediments from the glacial melt.  It is known that the Illinoian Glacial advance stopped just short of the Shawnee Hills in southern Illinois.  So whatever species of Castanea may have been present along that “bridge” before Illinoian glaciation, (850,000 years ago), was not there by 35,000 years ago.  The last glacial advance was the Wisconsin glaciation.  Its initial advance was 30,000 years ago, was at its peak 18,000 years ago, and came to an end approximately 8,000 to 10,000 ago.  These periods of time would have effectively broke most links to C. Ozarkensis in the Ozarks with any other family members of Castanea to the east, except for isolated colonies in Alabama or northern Mississippi.    It is apparent by the geography and the warming influence of the near by Gulf of Mexico, along the southern flanks of the Ouachitas then, the C. Ozarkensis could have very easily survived isolated in protective enclaves of the Boston and Ouachita Mountains for much of the Pleistocene era.  But, the large volume of water flowing from the Ohio and Mississippi rivers from glacial melt waters would have been an effective barrier for the spread for any of the Castanea family, if they were not already established in those regions before glaciation.   Much of the Eastern half of Arkansas was part of the Gulf of Mexico before glaciation, but much of the Mississippi sedimentary glacial deposits has since filled in that part of Arkansas with sedimentary soil.

 Widespread Late Quaternary species- and genus-level extinctions of mammals in North America and Europe have been documented since the early 19th Century. These extinctions were concentrated during the last deglaciation 16,000 to 9,000 years ago, during a period of rapid and often abrupt climatic and vegetational change.  In contrast to mammals, few Quaternary plant extinctions are documented.  Most are concentrated at the Pliocene/Pleistocene boundary, approximately 1.6 million years ago, although a few occurred in Europe during the Early to Middle Pleistocene. No Late Quaternary plant extinctions have been reported, leaving the impression that few or no extinctions occurred during the last glacial/interglacial cycle. Most knowledge of late Quaternary vegetational and floristic change comes from fossil pollen preserved in lake and wetland sediments. Species extinctions may be masked by taxonomic smoothing in the pollen record, which rarely permits species-level differentiation. Plant macrofossils often support species-level identification, but few sites have been studied in detail, and few studies have included the detailed morphological and anatomical analyses required to determine whether fossil material is unequivocally assignable to extant or extinct species.  Based on pollen samples, we do know that spruce, like the now extinct Picea critchfieldii, inhabited the lower reaches of the Mississippi Valley, along with megafossils of temperate taxa (Quercus spp., Juglans nigra, Acer sp., Carpinus caroliniana, Fagus grandifolia, Carya spp., Ulmus americana, and Juniperus virginiana) also occur in the deposits. There is also a great deal of pollen evidence that Castanea, occupied the Memphis area 15,000 years ago during this Pleistocene era too. Although the pollen samples were never speciated,  the question begs: Could the Memphis area have been occupied by the C. Ozarkensis, or was it C. Dentata and C. pumila, or all three species?   These were much different plant communities than what we are accustomed to seeing today.  These particular plant associations that once occupied the Ozarks during the Pleistocene era, strongly resembles the plant communities found in Labrador in Canada today. Same can be said for the Late Holocene invasion of the Quercus - C. dentata forests in New England.  It appears from the C. dentata pollen samples collected within that area of the country was no more than 2000 years old.  Around 1100 years ago, Tsuga and Fagus decreased and Quercus and Castanea were significantly increasing in dominance from Pennsylvania up into New England.  Unfortunately, we have no comparable pollen records to show how C. Ozarkensis may have held out in the Ozarks or advanced after glaciation.

As a side note: Another unique geologic feature that was covered up by this glacial sediment was the New Madrid Rift Zone. Prior to glaciation, the rift zone would have been under water, as this region was within the extension of the Gulf of Mexico.  This rift is presumed to have originated more than 600 million years ago.   However, about 200 million years ago, a new episode of rifting took place along the east half of North America as the Atlantic Ocean began to open, resulting in the continent crust being stretched or extended, and in the New Madrid Rift being pulled apart. This is a very extensive rift, which interconnects to the Walbash fault system, and up through the St. Lawrence River Rift area to the north, and as far south as to the Balcone fault system in Texas.  It appears to be responsible for separating the Ozark Dome from the Appalachian Mountains and the Appalachian Plateau over the years.  This too would have made an effective physical barrier for plants to colonize across from the south.

Most of the precursors to Angiosperms used the wind, rather than insects, to pollinate. The entire family of Fagaceae has rather primitive flowers compared to most other angiosperms, and pollinates in this fashion., which logically implies most all the members of Castanea have been around much longer than most other angiosperms, but certainly not as old as the gymnosperms.

In order for Castanea and all its related members to have covered to the broad extent it has in the Northern Hemisphere during the Eocene time, could only mean that the Castanea lineage must have been well established before then, and goes back farther in evolution than 80 million years.  Some evidence suggests the Castanea lineage may be much older than 127 million years old, before the continental breakup of Pangaea occurred.  That is because, the Castanea nuts would never survive a salt water crossing to another continent.

Isolated stands of Castanea dentata has been record in the Shawnee Hills of southern Illinois and Kentucky.  This indicates a logical pre-glacial land bridge connection to the entire Castanea family east and west of the Mississippi River.  The Castanea was apparently around on the North American continent in geologic time before the formation and southernly flow of the Mississippi River.   All the current landmass east and south of the Ouachitas in Arkansas was underwater up until the Pleistocene era, so colonization from the south would have been impossible.

It may be that Castanea Ozarkensis is a descendent of the most western progression of the now extinct  Castanea ungeri.  It has many attributes of both C. pumila and C. Dentata, like capable of producing either one or three seeds per bur.  Also, it takes on a shorter tree like structure of C. dentata rather than the scrubby C pumila.  C. Ozarkensis is primarily an upland tree which requires well drained soils, like C. dentata, whereas, C. pumila seems to have evolved to accept soils that are poorer in drainage.  Since the main population of C. Ozarkensis appears to have been isolated from the rest of the Castanea family for at least the last  850,000 years, could C Ozarkensis have a more directly associated lineage to Castanea ungeri?  If so, that would make it one of the oldest species of deciduous Castanea on the continent.   Only genetic analysis would tell.

Primarily because of suitable soils needed for invasion.  It is strictly an upland tree, which requires well drained soil.  The soils to the north and west are heavy in clays and poorly drained.  The soils to the south are sedimentary and very saturated.  It can only make its escape eastward, but then there is the poorly drained soils within Mississppi River Valley and Ohio River Valley which dissects the Ozarks from eastward soils able to support it  The St. Francois Mountain/Eastern Ozark Plateau areas generally lack the nutrients and humus in the soil compared to what is found in the Boston Mountain region, and is dominated by the dense red clayey subsoils. To have the upland C. Ozarkensis isolated from the rest of its genera in the east is indicative of severe erosion cycles of the desirable upland soils in recent geologic time.  It is suspected that, even though the Pleistocene glacial ice sheets did not extend into the Ozarks, the climate and the glacial drainage was severe enough to obliterate the C. Ozarkensis range and family connections to the east by greater than 200 miles!
  
 As for the colonies of C. Ozarkensis found in north Mississippi and Alabama, it is not well understood why these colonies remained isolated there, only to eventually subjected to the Chestnut blight infestation, and die off.   But it is suspected that these colonies were essentially growing in  “upland islands” of well drained soils, remnants of a pre-glacial ecosystem, before much of the suitable soils eroded away and eventually became surrounded by the poorer heavier clayey laterite subsoils..

An Extraction from Hazel R. Delcourt © 1979published in: Ecological Society of America.

Vegetation change during the past 25 000 years in and near the present Mixed Mesophytic Forest Region is inferred from pollen and plant macrofossil analyses of sediment cores from two sites on the eastern Highland Rim of Middle Tennessee, USA. Anderson Pond, in White County, dates from 25 000 radiocarbon years BP to present. During the Farmdalian Interstadial, 25 000 yr BP, northern Diploxylon pines, spruce, and deciduous trees were present at Anderson Pond, indicating a cool, moist climate. From 19 000 to 16 300 yr BP, during the Late Wisconsin glacial maximum, boreal taxa of jack pine, spruce, and fir were dominant. Late-glacial climatic amelioration began in Middle Tennessee at about 16 300 yr BP, with replacement of jack pine-spruce-fir forest by deciduous forest. Ash, ironwood, hickory, birch, butternut, willow, and elm increased in importance first, followed by beech and sugar maple. Mixed mesophytic forest taxa were most abundant during the early Holocene, between 12 500 and 8000 yr BP. A warming and drying trend between 8000 and 5000 yr BP is reflected by high influx values for pollen of oak, ash, hickory, swamp alder, and buttonbush, and by diminishing importance of mixed mesophytic forest taxa. Mingo Pond, Franklin County, Tennessee, contains a pollen record estimated to extend back to 14 000 yr BP. Mixed mesophytic forest taxa were abundantly represented at Mingo Pond during the late glacial and early Holocene, along with pollen representing prairie vegetation. Oak and sweetgum dominated the mid- and late Holocene record from Mingo Pond. Stratigraphic sites from the Missouri Ozarks to the Atlantic Coastal Plain date the full glacial in the southeastern United States between 23 000 and 16 500 yr BP. During the glacial maximum, boreal-like coniferous forests of spruce, jack pine, and fir extended southward to 34@?N latitude. Mesic deciduous forest persisted through the full glacial at 35@?N at Nonconnah Creek near Memphis. Full-glacial refuges for deciduous forest species may have also existed in south-facing gorges of the Cumberland Plateau and southern Appalachian Mountains in addition to bluffs along major streams in the southeastern United States. During the late glacial, beginning about 16 500 yr BP, boreal-like coniferous forest was replaced in midlatitudes (34@? to 37@?N) by cool-temperate coniferous-deciduous forest. In the early Holocene, between 12 500 and 8000 yr BP, cool-temperate mixed mesophytic forest prevailed between 34@? and 37@?N. The Mixed Mesophytic Forest Region assumed its present distribution in the mid-Holocene.