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

 

Tags: 13-lined ground squirrel, 720 lb. black bear, black-tailed jackrabbit. 60 mile long herd of bison, cross timbers, Fox Squirrel, hog-nosed skunk, post oak-black jack uplands