Posts Tagged ‘(Pinus resinosa)’

Pleistocene Soil Cycles

February 24, 2012

In his book The Natural Environments of Georgia Dr. Charles Wharton suggests Ice Age coniferous forests consisting of boreal species built many of the soils in the mountains and piedmont of Georgia.  This is fodder for contemplation of Georgia’s ecology.  Pleistocene soil composition must have gone through cycles that paralleled the climate cycles of stadial to interstadial and glacial to interglacial.  Soils became thin during stadials but were enrichened during interstadials and interglacials.  I am aware of no studies investigating the origins of topsoils in Georgia, and this kind of study is not even possible now because almost all of Georgia’s original topsoil has eroded or blown away, thanks to poor agricultural practices.  Nevertheless, speculation on the ecology of the Pleistocene soil cycle is another fruitful topic for this blog.

Fossil evidence from Bob Black Pond in Bartow County shows that a forest composed of jack pine, red pine, white pine, white spruce, Critchfield’s spruce, and paper birch grew in north Georgia during the Last Glacial Maximum ~21,000 BP.  This probably represents a common dominant forest in north Georgia during climate phases of the Ice Age known as stadials–times of arid cold when the Laurentide Glacier expanded to the north and locked up much of the planet’s atmospheric moisture.  To contemplate a full cycle of Pleistocene soil development, let’s go back farther in time to about 30,000 BP.

30,000 years ago, an interstadial that had lasted for about 4,000 years was coming to an end.  Interstadials were warmer, wetter climate phases within Ice Ages.  Studies of the pollen record show oak pollen always increased during interstadials, while pollen from coniferous trees decreased.  The leaves and debris from oaks and other hardwoods build up a healthy, thick topsoil, usually taking about 100-200 years to do so.  After 4,000 years most of the topsoil in the region must have been particularly rich.

Ice Age climate fluctuated rapidly.  Imagine now, that an ice dam on the St. Lawrence River melted enough during the warming trend of the previous 4,000 years to collapse, sending a torrent of freshwater and ice bergs into the North Atlantic.  This flood of cold freshwater shut down the thermohaline current that had kept the climate warm for millennia.  Climate changed immediately to colder, more arid, and windier conditions.  CO2 levels plummeted as well.  After a few decades many of the oaks and other broadleaf trees that had spread to upland habitats began to die from drought and wind and lower CO2 levels.  Grasses and coniferous trees compete better than hardwoods under these conditions.  Plants need CO2 for respiration.  During stadials CO2 levels fell so low that even some coniferous trees became starved for CO2.  Fossil juniper from the La Brea tarpits, for example, show evidence of CO2 starvation.  Under these conditions broadleaf trees only persist near rivers and streams.  Grasslands and brush thrive in the shade free environment, but the burgeoning bison, horse, and mammoth populations overgrazed the vegetation, leaving bare soil which blows away in the wind and much of the topsoil is thinned or lost.

Jack pine forest in Michigan. The landscape much resembles that of an open pine savannah in the coastal plain of Georgia.  Like an open pine savannah, jack pine forests are fire dependent.  Jack pine grew in the mountains and the piedmont of Georgia during stadials, the coldest stages of the Ice Age, but is completely absent from the state today.  The hilly terrain likely made for a more varied environment though in Georgia than this photo indicates.

Today, Eastern jack pine (Pinus banksiana) grows no farther south than northern Michigan and is a common tree on sandy outwashes in Ontario, Canada.  It’s a pioneer species, able to grow on thin sandy soils.  During the driest coldest phase of stadials, jack pine colonized dry upland sites in Georgia where many oaks could no longer dominate.  Jack pine forests are rich environments.  They grow thinly allowing light to hit the forest floor.  This promotes the growth of grasses and berry bushes.  Kirtland’s warblers, upland sandpipers, bluebirds, cowbirds, deer, bear, snowshoe hare, and rare prairie plants such as Allegheny plum, rough fescue, and Hill’s thistle all thrive in jack pine forests.  In Pleistocene Georgia many of these same species with the addition of extinct grass-eating mammals  were probably also abundant.  The rare Kirtland’s warbler (now summering in only a few counties in Michigan) winters in the Bahamas which were expanded in size due to lowered sea levels during the Ice Age.  (https://markgelbart.wordpress.com/2011/11/11/banana-hole-fossil-sites/) I suspect this bird was more widepread then and may have occurred in Georgia because it is dependent on jack pine forests.  Perhaps not coincidentally, fossils of upland sandpipers have been excavated from Bartow County where the jack pine fossils were found.

Fires were rare during stadials because lightning storms were rare.  Jack pines require fire for regeneration.  Other species of pine less dependent on fire such as red pine (Pinus resinosa) and white pine (Pinus stroba) encroached into jack pine forests in the absence of fire.  Post oaks which are among the most fire resistant and drought resistant oaks also move into these pioneer forests.  Gradually, the needles and debris from Ice Age coniferous forests added humus and thickened he topsoil.  When the next interstadial began (~15,000 BP) the climate warmed, precipitation increased, CO2 levels increased, and oaks and other broadleafed trees expanded from their refuges along waterways and once again colonized their old territory.  Jack pine is the most shade intolerant boreal species and was the first to be completely replaced, retreating to the north where it was able to take advantage of newly deglaciated sandy soils.  Next, red pine retreated, mostly toward New England, though relic populations remain in West Virginia.  Of the boreal species of pine, white pine was the least shade intolerant, so it still persists in north Georgia, though it’s much less common than it was during the Ice Age.

Young mixed boreal and hardwood forest.  At the beginning of interstadials when climate became wetter and warmer, oaks and other hardwoods rapidly displaced boreal conifers in the Georgia mountains and piedmont, shading the pines out.  These climate phases probably fostered the greatest variety of wildlife because northern species of plants and animals would still be present but southern species would begin colonizing the new habitat.

The greatest diversity of wildlife likely occurred during transitions from stadial to interstadial and vice-versa.  Environments in transition harbored a greater variety of habitats that animals and plants of northern and southern affinities would have found favorable.  A study of forest succession in the Georgia piedmont found that bird species abundance peaked at the stage when oaks began replacing pines.

There’s no evidence that northern species of pines ever extended their range into south Georgia.  Central Georgia was probably a transition zone where northern species of pines mixed with southern species of pine in environments that have no modern analog.  Shortleaf pine, the southern pine best adapted to cooler weather, was probably the most common pine species, though some northern pines ranged into the piedmont.  But much of south Georgia became brush, grassy deserts during stadials and much of the topsoil there blew away.  Eolian sand dunes rolled across the landscape, and the wind scooped out depressions and created Carolina Bays–a subject for a future blog entry.

If I could live during the Pleistocene (part two)

September 17, 2010

As I noted in last week’s blog entry, I don’t like roughing it.  If I’m going to live 41,000 years BP, I want to live in a nice sturdy house that would keep me safe from hungry bears, big cats, wolves, and rough weather.  I’d build a big adobe brick house with a wall around ten acres behind it where I could have a garden, fruit orchard, grain fields, and room to raise livestock such as milk cows, chickens, ducks and geese.

If I could live 41,000 years BP, I’d reside in an adobe house.  Adobe bricks are simple to make, only requiring mud, grass or sand, and sun.

My adobe house would have double thick walls, and raised windows with bars in front of them to prevent beasts from breaking into my abode and making a meal of me.  (For more about adobe houses see this link–     http://desertphile.org/adobe/adobe.htm)

In front I would have a raised platform or balcony for wildlife viewing, and on occasion to provide a place for hunting when I need meat.  Most of my home would be one story, but I’d have a tower room, built not unlike a lighthouse, which would afford a panoramic view of the surrounding landscape.  To improve the view, I’d clear a circle of land around my dwelling which would also serve as a firebreak.  My water supply would come from a well.  A dry toilet, or clivis multrum, would take care of my waste.  

Woodstoves would keep me warm in the winter, but I don’t think I’d need an air conditioner because this is the Ice Age, and summers are comfortable.  Solar units, and a generator, using wood alcohol that I would manufacture,  provide my electricity.  Of course, I’d have all necessary machines–bulldozers, bushhogs, trucks, boats, etc.  All engines would be modified to run on wood alcohol.

My Pleistocene adobe house is located in what’s now Elbert County on a hill one mile north of the Broad River and two miles west of the Savannah River.  In my opinion the Georgia piedmont (at least in the still rural areas) is the prettiest region of the state.  I even like it better than the north Georgia mountains.  I’d locate my home relatively close to a river for the easy source of protein–fish, turtles, freshwater mussels, and crayfish.  It would be necessary for me to maintain a dirt road between my house and the river.

I can take an educated guess as to what kinds of plants and animals I would encounter around my house.  There are only two Pleistocene-aged fossil sites in central Georgia (Nodoroc and Little Kettle Creek), though there are many more to the north and south.

Nodoroc is a bog that formerly was a mud volcano, last erupting in 1810 with a massive release of carbon dioxide.  These mysterious types of eruptions have also occurred in African lakes within the last few decades.  Nodoroc is a Creek Indian word meaning gateway to hell because the Indians used to execute criminals and toss them in this bog.

Scientists found plant macrofossils and pollen here, dating to 28,000 years BP, during a brief weak interstadial just before the Last Glacial Maximum.  The forest around the site consisted of an interesting mix of northern and southern species of pine as well as oak.  Northern species of pine such as white, red, and jack tend to have smaller grains of pollen, while southern species, such as shortleaf, tend to have larger grains.  Both size variations were found here, though it’s not possible to identify exact species, based on pollen.  But some plant macrofossils, though not in good enough condition for certain identification, compared favorably to red and/or jack pine; others compared favorably to shortleaf pine.  Because both northern and  southern species of pine occurred here then, the climate must have had mild summers and mild winters

Current range map of the red pine (Pinus resinosa).  Most of where it currently ranges was under glacial ice during the Ice Age, so it must have occurred south of this area then.  I propose that northern species of pine such as red and jack (Pinus banksiana) spread throughout the upper south following cold arid climate cycles when river beds dried out and wind blew the sand into large eolian sand dunes.  Scrub oak and grass initially colonized these dunes, but when precipitation increased as an interstadial began, lightning-induced fires burned the scrub oaks forests and grasslands, allowing fire-adapted pines to colonize these areas.  Eventually, as the climate continued to get warmer and wetter, hardwood trees outcompeted and replaced these shade-intolerant species.  Insterstadials never lasted long enough for hardwood forests to completely outcompete northern pines–a return to cold arid conditions would’ve probably killed many deciduous trees, allowing pine to regain territory.  But the current interglacial we live in now has lasted long enough for broad-leafed forests to shade out red and jack pines in the upper south, except for isolated relic populations of the former in small areas of West Virginia.

Hickory, spruce, and fir pollen were also common; chestnut, beech, and maple were present in low numbers.  The understory consisted of alder, blueberry and/or rhodadendron, and hazlenut.  Enough ragweed, grass, and sedge pollen was present to suggest the presence of large meadows or small prairies, making up to 25% of the landscape.

Little Kettle Creek is the only Pleistocene-age animal fossil site in the entire piedmont region of southeastern North America.  Teeth and bones of mammoth, mastodon, bison, white tail deer, (cf) southern bog lemmings, (cf) red backed voles, and catfish were recovered here.  The two rodents no longer range farther south than Kentucky, again indicating cooler summers for central Georgia during the Ice Age.  Mammoth and bison grazed the meadows;  mastodon and deer foraged the forest edge and streamside woodlands.  Growth rings on the catfish bones are evidence of colder winters than those of today because modern day catfish in warm southern states don’t have dormant growth cycles like fish found in northern states.

Fossil sites to the north and south of the piedmont have more species and most of them probably also lived in what’s now central Georgia as well.  Around my Pleistocene house I would also expect to see Jefferson’s ground sloth, elk, horses, tapirs, llamas, peccaries, dire wolves, jaguars, saber-tooths, bears of at least once species, giant beavers, and many smaller species of extant mammals that no longer occur in state but still live to the north and west.  Examples of interesting small species I’d expect to see are the hognosed skunk, red squirrels, and the extinct noble chipmunk.  I’d also expect to see a much greater variety of birds than I’d see today in an unspoiled wilderness devoid of human habitat destruction and pesticide use.  I’d be on the lookout for northern ravens, magpies, prairie chickens, upland sandpipers, terratorns, California Condors, and extinct species of vultures and eagles.  Birds that are rare or extinct today but were common then include bald eagles, ivory-billed woodpeckers, Carolina parakeets, passenger pigeons, swans, and cranes. 

That abundance of wildlife is the reason I really wish I could move into my Pleistocene home.

One final thought for today: This Ice Age ecosystem I describe was the norm.  Today’s interglacial ecosystem is an aberration because Ice Ages last ten times longer than interglacials.