The Kentucky Bluegrass Country

Some specimens excavated from the Little Kettle Creek fossil site in Wilkes County, Georgia suggest a climate phase at the time of their deposition similar to the present day climate of the Kentucky Bluegrass Country.  https://markgelbart.wordpress.com/2011/03/17/little-kettle-creek-the-only-pleistocene-fossil-site-in-the-piedmont-region-of-southeastern-north-america/       Bog lemmings reach their present day southern range limit in northern Kentucky, and red backed voles occur in cool north facing slopes of the southern Appalachians.  Yet, both of these species lived in central Georgia during some portion of the last Ice Age.  Moreover, evidence from catfish bones also implies much harsher winters than are known in Georgia today.  Rural areas of the Kentucky Bluegrass Country, particularly horse pastures with centuries old trees growing in them, are considered relics of the kind of savannah woodlands that predominated in southeastern North America during much of the Pleistocene.  Accordingly, this intriguing similarity between the present day Bluegrass Country and Pleistocene central Georgia inspired my research for this blog entry.   However, there are some important differences between the 2 regions.

Map of the Kentucky Bluegrass Country.  The present day climate here is a close analogue to some climate phases of the Ice Age in central Georgia.

The geology of the Kentucky Bluegrass Country causes a differing soil chemistry from that found in most of Georgia.  The inner bluegrass region rests on top of ancient limestone, making the soil basal while most of Georgia’s soil is acid.  The limestone orginated from Paleozoic Age coral reefs that ringed islands resembling those of the modern day Bahamas.  Over time geological processes turned all that coral into limestone.  The underlying limestone  makes the soil of the bluegrass region among the richest in the world.  The soil is high in calcium and other minerals that help plants grow and the animals that eat these nutritious plants are healthier than those from other regions, explaining why this is good land for raising horses.  Most of Georgia’s soil was never nearly this fertile, even before agriculture wore out the land.  The underling rock in the outer bluegrass region is shale.  Shale is merely fossilized mud.

Note the spectacular 400 year old bur oak in the top left photo. Bur oak is a characteristic species of the Kentucky bluegrass country.  I tried in vain to find as impressive a photo on google images, so I ripped this one off from the below referenced book.

Note the stand of giant bamboo cane in the bottom right photo.  Cane is a relic today, but at the time of European settlement it formed thickets that were as much as 20 miles long adjacent to streams and rivers.  Daniel Boone escaped from Indian captivity by running into one of these canebrakes.

The photo in the upper right corner is a typical feature of the Kentucky river with a steep bluff on one side and a floodplain on the other.  The photo in the upper left corner is of a spring that emerges above ground and seeps back underground.

The calcium rich soil fosters the growth of calciphiles–plants that grow best in this type of earth.  Pre-settlement forests consisted of sugar maple, black walnut, butternut, blue ash, white ash, Kentucky coffee tree, honey locust, black cherry, bur oak, chinquapin oak, Shumard’s oak, white oak, shellbark hickory, buckeye, hackberry, and mulberry.  Common undergrowth trees included great stands of pawpaw, crabapple, hazelnut, plum, twinleaf, blue cohosh, and slippery elm.  Many of these species are shade intolerant, and they grew in natural widely spaced groves similar to those described by William Bartram from his travels through Georgia in the 18th century.  The trees grew far enough apart to allow sunlight to reach the ground, allowing a wide variety of grasses and herbs to flourish.  A few of the common species found in the pre-settlement savannah woodlands were buffalo grass (Panicum sp.), the now endangered buffalo clover (Trifolium stoloniferum), wild rye (Elymus villosus and virginica, not the European grain), wild peavine (Amphicarpa bractenta), the edible wild bean (Phaeseolus polystachios), goldenrod (Solidago sphacelata), and many calcium-loving ferns, sedges, and wild flowers.  Giant bamboo cane (Arundinaria gigantea) grew in thick stands along streams and rivers.  One stand of bamboo was estimated to be 20 square miles in extent.

The Kentucky River washes away some of the rich soil on the floodplain, resulting in thinner, poorer soils along its margin.  This area sported a somewhat different composition of trees that included sycamore, white oak, elm, and tulip with an undergrowth of dogwood.  Red cedar, now a more common tree in second growth forests here, grew on rocky bluffs.

European settlers rapidly destroyed much of this beautiful environment.  They cut down most of the trees and converted most of the rich cane land into pasture.  They introduced species of grasses and herbs that outcompeted native plants.  They killed most of the sugar maple trees by cutting big chunks out of the trees to harvest the sap instead of carefully tapping the trees in a way that would keep them alive.  They allowed their livestock to overgraze between the trees, eliminating regenerating trees, reducing pawpaw stands, and wiping out hazlenut, so that the latter is completely absent from the region today.  However, many of the bluegrass pastures host ancient pre-settlement trees that are still standing.  A 400 year old bur oak still lives in Bourbon County.  Other pastures consist of groves of blue ash, bur oak, and Kentucky coffee trees ranging in age between 200-400 years old. The pastures are remnants of savannah woodlands.  Mowing is preventing regeneration and eventually these old trees will die.  For this reason these relics are endangered.

Botanists are uncertain whether Poa pratensis, the species of bluegrass for which this region gets its name, is a native or an introduced species.  Bluegrass is also known as English grass, and it ranges into Eurasia and select areas of the Rocky Mountains.  The first settlers reported seeing bluegrass growing in Kentucky in the mid 18th century, and there is no ship’s record of it being imported that early.  This species may have had a natural Holarctic range, meaning that millions of years ago it spread across the Bering landbridge.

Ecologists think the trampling and grazing of great herds of bison and elk  shaped the savannah woodland environment here, and that fire was rather infrequent.  The rich land and abundant mineral licks attracted unusually large numbers of ungulates to this area.  The bison, elk, deer, bear, and passenger pigeons kept tree saplings naturally thinned, fostering the growth of widely spaced trees.  Fire must have occurred only rarely because sugar maple (a fire intolerant species) was formerly common here.  This ancient savannah woodland environment originated during the Pleistocene, though the plant and animal composition varied, depending upon the climate phase.  During interstadials and interglacials when deciduous trees were abundant, mastodons, giant ground sloths, deer, and long-nosed peccary thrived.  Both honey locust and Kentucky coffee trees bear leguminous pods that mastodons would have spread across the landscape in their dung.  Perhaps their abundance here along with pawpaws is evidence that mastodons made their last stand in the bluegrass country.

There is an excellent Pleistocene age fossil site in the region–Welsh Cave.  It was excavated in 1965.  Here’s the list of fossil speciments found here.  *denotes extinct species. # denotes species no longer found in the region

*dire wolf

#grizzly bear (southeasternmost known occurrence)

#badger

least weasel

*mammoth

horse

*flat-headed peccary

water shrew

pygmy shrew

short-tailed shrew

eastern mole

#snowshoe hare

#red squirrel

#spruce vole

yellow-cheeked vole

red-backed vole

meadow vole

pine vole

#porcupine

#thirteen lined ground squirrel

#pocket gopher

brown bat

pipistrelle

The radiocarbon date from the fossils found here was ~13,000 BP which translates to ~15,000 calender years BP.  The faunal composition suggests a prairie environment with fingers of boreal forests consisting of pine, spruce, birch, and northern hardwoods.  Superficially, it would have resembled the lowlands of the present day Yellowstone National Park in appearance.  Obviously, the preponderance of species that prefer (or preferred in the case of extinct species) open spaces indicates the fossils were deposited during the Last Glacial Maximum when hardwood forests were on the wane and grasslands were expanded because of the cooler drier climate.  The Welsh Cave site is the southeasternmost known occurrence of the grizzly bear.  13-lined ground squirrels lived in Kentucky until as recently as 600 AD.  Their remains have been found in Indian middens dating to then.

Two More Interesting Notes About Pre-Settlement Northern Kentucky

–A passenger pigeon roosting site in 18th century Shelbyville, Kentucky was estimated to be 120 square miles in extent.

–The Kentucky River, a tributary of the Ohio River, once was home to a freshwater species of cod, now restricted to more northerly localities.  The American burbot, also known as the eel pout, is called the “poor man’s lobster.”  Along with 2 species of sturgeon and the paddlefish, it’s been eliminated from Kentucky’s waters.

Reference:

Wharton, Mary; and Roger Barbour

Bluegrass Land and Life

The University of Kentucky Press 1991

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The Cross Timbers Ecoregion. An Analogue for Georgia Environments during Some Stages of the Pleistocene?

The cross timbers is a North American ecoregion that exists between the eastern forest and the tall grass prairie.  Much of the cross timbers forest is still intact because the quality of the wood is so poor it was never clear cut.  Acreage never cleared for agriculture but used for pasturage still hosts plenty of really old trees.  400 year old post oaks and 500 year old red cedars are not unknown or even rare.  The cross timbers is also known as post oak/blackjack oak uplands, named for the 2 dominant tree species.  Neither species produces quality wood, explaining why, unlike in the eastern forests, lumber companies left this region alone.

Map of the cross timbers ecoregion.

Oak savannah in the cross timbers.  The flora is influenced by fire, drought, and tornado.  This is the southern part of tornado alley.

The cross timbers ecoregion is bounded by tall grass prairie to the north-northeast, oak-hickory-pine Ozark highlands to the east, and mixed grass plains to the west.  Steep hills, low mountains, rough escarpments, and 4 sizeable rivers shape the topography.  Young forests form impenetrable thickets of scrub oak, greenbrier, and sumac.  Mature forests become oak savannahs, influenced by frequent fire and tornado.  In addition to post oak and blackjack oak, bur oak, and black hickory (Carya texana) grow on the uplands, while big bluestem and Indian grass thrive between the widely spaced trees.  River bottomland forests consist of river birch, mockernut hickory, cottonwood, sycamore, black walnut, hackberry, and buttonbush.  Live oak is a component of the western part of this region.  Red cedar becomes a dominant cross timbers tree when fire is absent or suppressed.

I think the cross timbers region may be a near, but of course not exact, analogue to some parts of southeastern North America during some climatic stages of the Pleistocene, particularly the eoWisconsinian.  The eoWisconsinian is not precisely defined in the literature but generally is thought to be the early stages of the Wisconsinian Ice Age, perhaps roughly dated from 118,000 BP- 70,000 BP.  It encompasses 3 stadials (cold stages) and 3 interstadials (warmer stages).  The climate gradually became cooler and drier during this time period as the north polar ice cap began to reform and expand following its complete dissolution during the Sangamonian Interglacial.  This gradual cooling was interupted by sudden reversals when the climate turned wet and warm.  Temperatures at the beginning of the eoWisconsinian may have been as warm or warmer than they are today, but by the end average temperatures were below those of the present day.

The 3 pollen studies of southeastern North America that date to this time period suggest an environment dominated by oak and grass during interstadials.  The main difference between the Oklahoma cross timbers and the eoWisconsinian of southeastern North America is the considerable presence of pine in the latter, especially during cold stages.  Spruce was present but didn’t become a significant compoenent of southeastern Ice Age forests until after the eoWisconsin.

I also consider the Oklahoma cross timbers an analogue to eoWisconsinian environments of the southeast because of the intermingling of western and eastern fauna.  The fossil record shows that several species of extant mammals and birds today restricted to the west used to live in the south during the Ice Age.

13-lined ground squirrel

Current distribution of 13-lined ground squirrel.  This species lived in the southeast during the Ice Age.

The rangemap above clearly shows the 13-lined ground squirrel (Spermophilus tridecemlineatus) is conspicuously absent from the southeast.  Yet, fossil specimens of this species have been recovered from Yarbrough Cave in north Georgia and the Turtle River in south Georgia–evidence it was widespread in the state during the Ice Age.  Ground squirrels hibernate for an astonishing 8 months.  The evolutionary advantage of being dormant during long winters might explain why they no longer live in south where winters are short.  However, the growing season in the Oklahoma cross timbers is over 7 months long.  I hypothesize its absence in the south today is due to a severe reduction in southeastern grasslands in the early Holocene when forests expanded. Later,  Indians regularly began setting fires here to enhance grassland development, but ground squirrels have been unable to recolonize the area.  It would be interesting to do a little experimental human- aided transport to see if they could live in the present day south on suitable habitat, but I’m sure ecologists would consider it an invasive species and object.  Along with ground squirrel specimens, other western species such as badger, northern raven, upland sandpiper, and magpie fossils often turn up in southeastern fossil sites dating to the late Pleistocene.

Fox squirrels are far more common in the cross timbers than gray squirrels.  The latter prefer dense young forest where they can jump from tree to tree to avoid predators.  Fox squirrels are larger and less agile and prefer to run on the ground between widely spaced trees when escaping predators.  Therefore, I hypothesize they were the more common squirrel in Georgia’s interstadial oak and grass savannahs.

Specimens of hog-nosed skunk (Coneputus mesoleucus) appear in some Georgia and Florida pleistocene fossil sites, but it too is absent in the present day south.  It’s not even a denizen of the cross timbers but occurs just west of that region in arid habitats.  I hypothesize patchy tracts of desert-like environments persisted throughout the south during most of the Ice Age, expanding during stadials but still existing as relics during interstadials.  The complete disappearance of these desert-like tracts may explain this species more restricted present day range.

Black-tailed jackrabbits, pronghorn antelopes, prairie dogs, and grizzly bears reached their easternmost range limit in the cross timbers during colonial times.  There’s no fossil evidence that jack rabbits and pronghorns ever recolonized the south after the mid-Pleistocene but they did occur in the region in the early Pleistocene and the Pliocene.  Probably, a large forest grew along the Mississippi River, forming an unsuitable ecological barrier that prevented their southeastern recolonization when climatic changes allowed favorable habitat to redevelop there.  Prairie dogs never lived in eastern North America as far as we know, but Pleistocene-age fossils of grizzly bears have been uncovered in Welsh Cave, Kentucky.  Why grizzly bears never colonized the rest of the southeast is a bit of an ecological mystery.

Incidentally, early white explorers reported the cross timbers to be rich in game.  In the early 19th century they saw mixed herds of thousands of bison, mustangs, deer, elk, and pronghorns.  One report mentions a bison herd that was 60 miles long.  In 1823 A.P. Chouteaux shipped the black bears skins of 300 females and 150 cubs from his trading post in northeast Oklahoma in just 1 season.  As late as 1911 a huge 720 lb black bear was killed in the cross timbers region.

Reference:

Claire, William; Jack Tyler, Bryan Glass, and Michael Mares

Mammals of Oklahoma

University of Oklahoma Press 1989