Late Pleistocene Megameanders

Glacial recession during the end of the last Ice Age caused dramatic changes in the climate and river drainages of southeastern North America.  All that melting Canadian ice released moisture into the atmosphere and average annual precipitation in the region tripled to an estimated 47 inches.  But average temperatures were still cooler than they are today and as a result evapotranspiration rates were lower. This increased precipitation and reduced evaporation caused rivers to meander more than they do today.  These supermeanders eroded scars that are still visible in satellite photographs.  Recently, scientists studied 6 paleomeander scars adjacent to the Oconee, Ogeechee, Black, Neuse, Pee Dee, and Congaree Rivers.  These rivers are located in Georgia, North Carolina, and South Carolina.

Satellite photo of a paleomeander scar located along the Oconee River.  From the below referenced paper.

The scientists found these paleomeanders dated to between ~17,000 years BP-~11,000 years BP.  The scar next to the Oconee River was radio-carbon dated using a 17,000 year old pine log.  The supermeanders cut through the former braided channels that existed during the Last Glacial Maximum when rivers shrank and became clogged with sandbars due to aridity.  Eventually, when present day climatic conditions began to predominate, the supermeanders became cut-off from the main flow of the river.  For awhile they existed as oxbow lakes but then filled with clay and sand.  Scientists estimate the supermeanders were 2-5 times larger than modern meanders and the discharge was up to 4 times larger.  The typical flow was equal to a modern day 5 year flood event.  Scientists aren’t sure of the exact mechanism that caused supermeanders.  It was likely a combination of increased precipitation, low evapotranspiration rates, and seasonal monsoons.  The distance between the frigid air over the Laurentide Ice Sheet and tropical air was much smaller during the Ice Age, and this could have caused an increase in major storm events.

I hypothesize canebrakes and river bottomland forests really expanded during the supermeandering phase.  Canebrakes are a now nearly extinct environment consisting of pure stands of bamboo cane.  Canebrakes formerly occupied hundreds of square miles of river bottomland in the southeast, but European settlers cleared them for agricultural purposes.  They were the most fertile pieces of land in the region.  Canebrakes depend upon a complex regime of flood and fire.  Suppression of either results in the growth of river bottomland forests that shade bamboo out.  During the late Pleistocene canebrakes attracted herds of bison and horses which fed on the nutritious bamboo.  Newly arrived humans facilitated the spread of canebrakes by setting fire to the landscape.  However, canebrakes must have also existed along the braided rivers of the Glacial Maximum, perhaps growing on the sandbars in the middle of partially  dried up rivers. The supermeander oxbow lakes likely hosted the last North American capybaras and giant beavers (Casteroides sp.) before they were hunted into extinction by people.

Reference:

Suther, Bradley; David Leigh, George Brook, and L. Yann

“Megameander Paleochannels of the Southeastern Atlantic Coastal Plain, USA”

Paleogeography, Paleoclimatology, and Paleoecology July 2018

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

The Congaree National Park, Home of the Giants

The Congaree National Park in South Carolina still held some surprises for me, even though I traversed this biggest last stand of bottomland forest in 1988.  Then, it was just a National Monument but in 2003 was upgraded to National Park status.  The park hosts 17 national and/or state record trees, including an 156 foot tall loblolly pine; and record shumard, southern red, and overcup oaks; as well as champion water hickory, Carolina ash, holly, box elder, persimmon, and paw paw.

Loblolly Pine–Pinus taeda

Loblolly pine.  The state record tree located in this park has a circumference of 16 feet.  This one looks to be at about 9 or 10.

The tree on the right is the loblolly pine from the previous photo.  The tree to the left is a red maple.  Some of the stands of loblolly pine in the park were found to be 227 years old.  The same study found that this species sprouts following hurricanes which open up the forest canopy.  The ages of stands corresponded with the history of hurricanes. (I took all the photos for this blog entry.)

This species is also known as old field pine because it reseeds so easily in lots devoid of other trees.  It has become a dominant tree throughout the southeast since much of the cotton and corn fields and horse pastures, which 100 years ago made up the face of the southern lands, were abandoned and went fallow.  Before European settlement it was merely a common component, but today it grows in pure stands as well as mixed with other species.  Therefore, it’s sometimes given the name bull pine due to its predominance.  Not only does it grow in upland locations, but it reaches prodigious size in wet areas from whence it gets its name, loblolly, which means mud puddle.  In the Congaree they grow in wet mud.

Sweetgum–Liquidamber styraciflua

Forked sweetgum trunk.

Another surprise for me.  I didn’t know this species could grow with its root system submerged in water, but I saw plenty of individuals in the Congaree growing in 6-12 inches of water.  Some had roots spread in structures similar to those of cypress and tupelo, enabling them to balance without tipping over in muddy soils.  Sweetgums, like loblolly pines, reseed readily in fields and are a common tree in upland sites.  But they’re adaptible enough to thrive in low muddy land as well.

Cypress–Taxodium distichum and Tupelo– Nyssa sp.

Cypress tree trunk.

Look at all of these cypress knees.  Scientists are unsure whether the cypress knees, which are part of the root system, grow for respiration or balance.

Up close view of a cypress knee.  Cypress trees are closely related to California red woods.

There are a lot of interesting microhabitats within a cypress-tupelo swamp.  Areas flooded in shallow water provide homes for fish, reptiles, amphibians, crayfish, and aquatic insects and spiders.

Flooded land in the cypress-tupelo swamp.

Fallen hollow logs and standing snags provide dens from everything from bats and possums to snakes and birds.

This hollow log is a possum mansion.

This standing snag could be hiding bats or flying squirrels.

When the water recedes lush grass and bamboo cane grow.  I can just imagine such Pleistocene mammals as long-horned bison, horses, and mammoths feeding in these grassy glades which often come about when a mighty old tree topples over allowing more light to reach the forest floor.

Note the grassy glade in the background.  There is plenty of grass and cane for a small population of bison and horses in the Congaree.  Too bad they’ve been extirpated.  The only big game left is deer and feral hogs.

A stand of cane.

When the mighty behemonths do topple over, their roots rip caverns in the forest floor.  Those often fill with water forming deep pools that are free of fish.  Amphibians can breed here without fish preying on their eggs and tadpoles.

Upturned tree root.

Pool formed in a hole from where a tree toppled over.

Beech–Fagus grandifolia

Beech tree canopy

Beech grows on sites that are near water but normally stay high and dry.  I love beech, and I wish I lived in a forest of these beautiful trees.  Their bark is white, the leaves turn a lovely yellow in fall, and they produce delicious little nuts.  Beech trees were common in the south during certain climatic stages of the Pleistocene, and as I mentioned in my blog entry “Pleistocene Passenger Pigeon Populations,” I think it’s a clue that passenger pigeon populations skyrocketed when beech was common because beech can grow and spread from sprouts.  The absence of passenger pigeons feeding allows more oak acorns to survive, and oaks outcompete beech.

The Congaree Swamp is probably as old as the Okefenokee Swamp

No studies on the age of the Congaree Swamp have been conducted as far as I know, but I assume it formed about the same time as the Okefenokee which became a swamp between 7,000-8,000 years ago following the final dissolution of the Canadian glacial Lake Agassiz when the water table rose all across the continent.  Throughout much of the Pleistocene, the region around the Congaree River was likely a mixture of upland oak and pine forests and savannahs with only scattered marshes perhaps near creeks and beaver ponds.  Particularly dry climatic stages even hosted oak scrub and sand dune environments.  It’s possible this region has repeatedly converted and re-converted to swamps with every full blown interglacial.

A comparison between my 1988 trip to the Congaree with my experience in 2011

The park system has considerably upgraded the facilities since I was here last in 1988.  Then, there was a gravel parking lot, and nothing else other than some gray paint marks on trees to demarcate the trails.  I hiked by myself in late July and was impressed with the giant trees.  This was before 1989 when Hurricane Hugo flattened many of the trees, but even then there were quite a few felled trees.  I saw a deer resting on a log, a scarlet king snake, and about a billion orb-weaving spiders that built webs across the entire trail at about 1 foot intervals.  After about 5 hours of enjoying nature, I became paranoid that I didn’t know what trail I was on, and I began jogging because I didn’t want to get lost in the park after dark–I feared the potential threat from feral hogs and rabid raccoons.  I didn’t see a single person while I was there until I got back to my car and met a park ranger.

Today, the Harry Hampton Visitor center is located at the entrance of the park.  It’s an air conditioned haven with clean lavatories.  Though the park is still not crowded, we did cross paths with many people.  Moreover, there is a pleasant 2.4 mile boardwalk which made the park accessible for us because my wife is wheelchair bound.

The lower boardwalk goes through a cypress-tupelo swamp.  The upper boardwalk goes through a bottomland forest dominated by sweetgum, loblolly pine, red maple, river birch, and ash.  I only saw a few oaks here–water, swamp chestnut, and willow; though in other areas of the park they’re more common.  Holly trees are common in the understory, and there are occasional patches of palmetto.  I saw no paw paw trees here, but I remember there were many on the trails.

The trails are now color coded, so there is little danger of becoming confused as to which trail one is on as I did 23 years ago.  The paint marks are on trees for the hiker to follow.