Pleistocene Soil Cycles

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.

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2 Responses to “Pleistocene Soil Cycles”

  1. Robert Endlich Says:

    I have a question WRT para 4 of this post, “CO2 levels plummeted as well,” What data confirm the decline of the deciduous trees when the climate got colder during the Wisconsin Glacial and at what did that occur? I agree with the post, I am looking to see at what age (23,000 years BP?>, and at what CO2 concentration that occurred in Georgia. I have seen the abstract concerning the severe and sustained decline of the Junipers reported at the La Brea Tar Pits in Los Angeles during the LGM.

    • markgelbart Says:

      Historical atmospheric CO2 levels are determined from glacial ice core layers in Greenland and Antarctica. So when we’re talking about Georgia’s CO2 levels, we’re talking about worldwide CO2 concentrations.

      Off the top of my head, the best study I know of that correlates tree composition with atmospheric levels of CO2 is this:

      Heusser, Linda; and D. Oppo
      “Millenial and Orbital Scale climate Variability in the Southeastern U.S. and the neotropical Atlantic Ocean during MIS 5: Evidence from Pollen and Isotope In ODP Site 1059A”
      Earth and Planetary Science Letters 214 2003

      I wrote an article about this study: https://markgelbart.wordpress.com/2011/05/09/ocean-drilling-project-1059a-found-a-treasure-for-paleoecologists/

      There’s a really neat graph from this study that I scanned and placed in my article. Click on the graph to enlarge and note the correlations between fluctuating CO2 levels and tree species composition. This study covered the time period between 140,000 BP-50,000 BP which included a change from full glacial to interglacial and back again.

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