Posts Tagged ‘Last Glacial Maximum’

Changing Forest Composition in Northwest Florida over the Past 40,000 Years

August 1, 2012

Camel Lake is an ancient body of water located 30 miles west of Tallahassee, Florida in Liberty County.  Geologists haven’t been able to determine if it’s a limestone sink lake or an aquifer positioned near the surface of the land.  It’s not a large lake being 500 yards in diameter, and it is sometimes referred to as a pond.  Today, open pine savannah consisting of longleaf pine, turkey oak, and wiregrass surround the lake.  Cypress trees, Spanish moss-covered live and laurel oaks, myrtle, and titi grow adjacent to the water.  In 1986 three scientists took a core of the lake bottom.  They counted pollen grains, catalogued plant macrofossils, and carbon dated differing levels of the core.  The information they gathered provides a diary of changes in plant composition in the vicinity of Camel Lake over the past 40,000 years.

Shoreline at Camel Lake, Florida.  A core of sediment taken at this lake found Pleistocene -aged pollen dating to as old as 40,000 years BP.  They found plant macrofossils in abundance as well including needles resembling those from longleaf pine, leaves similar to those of live oak, a species of quillwort no longer found south of North Carolina, and many other species.

The period of time from ~40,000-~31,000 BP is known as the Mid-Wisconsinian Interstadial.  The climate during this era was at least as wet as today, and maybe more so, because the glacial ice covering much of Canada was in a meltwater phase, and more moisture flowed into the atmosphere.  But it was still the Ice Age and temperatures were slightly cooler than those of today, reducing evaporation rates.  The cooler temperatures and increased precipitation fostered a mesic forest in northwest Florida dominated by high numbers of oak (comprising 40% of pollen totals), hickory, elm, ash, and hornbeam.  Cypress, sweetgum, sycamore, and myrtle were also common.  Chestnut didn’t range this far south in this part of Florida during Colonial times, but during the Mid-Wisconsinian Interstadial, chestnut pollen was abundant in the air.  The authors of the below referenced paper believe the pollen was from the American chestnut, rather than chinkapin which is a small bush.  The pollen from these 2 related species can’t be distinguished, but the amount suggests it originated from great forests of chestnut, not small understory bushes.  At the beginning of the mid-Wisconsinian Interstadial pine was still common, but pine pollen dropped to less than 20% as the climate warmed.

Pollen graph from the below referenced paper.  Shows the pollen levels from the last 40,000 years.  Click to enlarge.

Between ~31,000-~29,000 a transitional phase occurred.  The climate rapidly began to become colder and drier as the Laurentide Glacier over Canada resumed expansion.  Pine increased in abundance and oak remained common but chestnut, beech, and hornbeam disappeared.  Marsh elder, a plant that thrives in fluctuating water conditions, was common, indicating dramatic dry/wet seasons.

The Last Glacial Maximum, the coldest stage of the most recent Ice Age lasted from ~29,000 BP-~14,000 BP.  Pine pollen increased to over 80%.  Oak, hickory, and cypress were still present but at low levels.  A low diversity and density of plants covered the landscape.  Shallow water plant species macrofossils, such as quillworts and small pondweed, dominate this level of the core, showing lower water levels within Camel Lake.

A sudden change in climate is evident in the part of the core dating to between ~14,000 BP-~12,000 BP.  Pine pollen immediately drops from over 80% to less than 20%.  Hickory increased to an astonishing 25%.  A forest represented by this much hickory has no modern analogue.  Spruce represented 8% of the pollen and was soon joined by beech also representing 8%.  A hickory-spruce-beech forest interspersed by small prairies where grass, ragweed, wormwood, composites, and marsh elder grew composed the landscape.  Scientists initially misinterpeted the results of their study from this time period and proposed that this phase was the coldest of the Ice Age in Florida, colder even than the LGM.  However, this was before the discovery of Critchfield’s spruce, an extinct species of spruce thought to have grown in temperate climates.  Though Camel Lake produced no macrofossils of Critchfield’s spruce, it was probably the species exuding the pollen discovered in the core.  Critchfield’s spruce macrofossils have been found in Louisiana and southwest Georgia (https://markgelbart.wordpress.com/2011/09/03/the-extinction-of-critchfields-spruce-picea-critchfieldii/) .  With this in mind a better interpetation of the data indicates temperatures only slightly cooler than those of today and with increasing atmospheric moisture during this time period.

How did a forest of hickory, beech, and an extinct species of spruce frequented with prairie openings become the dominant type of flora for 2,000 years here?  I propose that megafauna and passenger pigeon foraging suppressed the spread of oaks which normally dominate in this type of climate.  Llamas, deer, peccaries, bison, bears, and passenger pigeons gobbled up the acorns.  But hickory nuts have hard shells, beech can grow from sucker roots, and spruce cones were not as palatable.  These defense mechanisms gave them an advantage over oaks. Increased frequency of thunderstorms spawned lightning-induced wild fires that created the prairie openings.  I regard this time period as harboring a particularly interesting environment in northwest Florida, the likes of which may never be seen again.

From ~12,000 BP-at least ~10,000 BP an increase in oak, sedge, and grass suggests a drier warmer climate.  The climate became so dry between 10,000 BP-7760 BP that no sediment was deposited and there is no pollen record for that time period here.  Since ~7760 BP the modern composition of plants as discussed in the first paragraph of this essay has predominated.  Climate has changed little over the past 7700 years following the final dissolution of the massive glacial Lake Agassiz in Canada…at least in comparison with Ice Age climate fluctuations. (See also http://markgelbart.wordpress.com/2011/01/06/temporal-correlations-between-lake-agassiz-the-okefenokee-swamp-and-ancient-flood-myths/)

Reference:

Watts, W.A.; et. al

“Camel Lake: A 40,000 Year Record of Vegetational and Forest History from Northwest Florida”

Ecology 73 (3) pp. 1056-1066 June 1992

Sagponds Contain a Partial Pollen Record of Ice Age Vegetation in Northwest Georgia

February 2, 2012

In the southern Appalachian mountains when groundwater dissolves underlying limestone the ground slumps to a granite bottom, creating an interesting environment known as a sagpond.  These differ from sagponds located in California which are formed from strike-slip faults.  Georgia sagponds form from the same geological principle that creates limesink lakes in Florida.  Dr. Charles Wharton counted as many as 96 sagponds per square mile in Bartow County.  They range in size from a few feet across to several acres.  Some go seasonally dry; others retain water year round.  Scientists recognize 4 types of sagponds.  Dry sagponds drain within an hour after rain stops.  Young sagponds, where the ground hasn’t finished slumping, hold water during winter and spring.  Mature sagponds are partially filled with colluvial silt and retain water year round.  Extinct sagponds have been completely filled with silt but still might be moist enough to support a marsh.

Topographical map of Green Pond.  Sagponds often form on mountains and provide wetland habitats in otherwise dry upland environments.

These low wet areas dotting the otherwise dry upland ridge and valley region host disjunct populations of at least 49 species of plants that are normally found on the coastal plain, including the evergreen laurel oak, gallberry (a type of holly), fetterbush, rock rose,  joint grass, panic grass, beaked rush, and many others.  Common aquatic plants found in sagponds are tupelo, buttonbush, red maple, and sedges.  Buttonbush and tupelo tend to grow in the middle of a sagpond; grass and sedge grow on the edges; than there is a heath zone ringed by a lowland woods consisting of loblolly pine, laurel oak, and red maple.

Page from the book Natural Environments of Georgia with a photo of a dry sagpond in Bartow County.

Photo by Alan Cressler of a sagpond on Keel Mountain in Madison County, Alabama.  I couldn’t find any photos on the web of sagponds in Georgia, though there are supposed to be a lot of them.

Dr. Wharton reports that Bloody Pond in Chickamauga Battlefield Park has a large stand of willow oaks.  (While researching this topic, I discovered that there are at least 2 other Civil War era battlefields with areas known as “Bloody Pond.”)

Scientists looking for ancient pollen and Pleistocene-aged plant macrofossils have excavated and cored several sagponds–Bob Black Pond, Quicksand Pond, Green Pond, Pigeon Mountain Marsh, and Lookout Mountain Marsh.  The pollen record from all these sites combined cover a timespan of between ~34,000 Bp-~10,000 BP in calender years.  Some of the studies date to before scientists realized carbon-dating gave dates too young when compared to tree rings.  The data I give in this discussion mention dates I’ve adjusted.  They’re just rough estimates based on the inaccurate dates from the early studies.

Pollen from Green Pond, believed to date to the Farmdalian Interstadial, suggests an environment in northwest Georgia consisting of dry oak and hickory woodlands with prairie openings.  Dr. W. A. Watts (of Penn State but now retired) conducted this study in 1973 and gave carbon dates of 29,630 BP- 25,000 BP which should be adjusted to about 34 ka BP-30 ka BP.  Pine pollen was scarce.  ~28,000 years ago pine and spruce pollen began to increase, indicating an abrupt change to a cooler drier climate.  Surprisingly, pond cypress was locally present.  This shows that individual plant species ranges didn’t always change in predictable patterns.  Pond cypress is absent in this region today, despite a warmer climate.  Shortly after this date, the pond filled with sediment and the pollen record ends for this site.  Conveniently, this is when the pollen record from sediment cored from Bob Black Pond, which Dr. Watts also studied in the early 1970’s, begins. 

Dr. Stephen Jackson of Wyoming University also studied Bob Black Pond over 20 years later.  He found boreal forest species dating to about 22,000 BP.  In addition to pollen he found plant macrofossils of white pine, red pine, jack pine, white spruce, Critchfield’s spruce, and paper birch.  Critchfield’s spruce is an extinct species thought to be adapted to temperate climates, and white pine still grows on the mountains of north Georgia, but the other 4 species no longer range much further south than Canada.

Jack pine present day range map.  This species lived as far south as northwest Georgia during the LGM.  Today, it lives no further south than northern Michigan.  Red pine is restricted to New England.  White spruce lives across Canada as does paper birch.

There’s no doubt the climate in north Georgia during the Last Glacial Maximum was cooler than that of today, but it wasn’t necessarily as cool as present day southern Canada, despite the presence of Canadian species of trees.  Scientists may have originally misinterpeted this data when they assumed the climate here was comparable to today’s southern Canada.  Pine and grass grow better in atmospheres with lower CO2 than broad-leafed trees.  So moderately lower temperatures combined with lower CO2 levels allowed northern species of conifers to compete better with oaks, maples, and other broad-leafed trees for space in the southern Appalachians.  It’s a good thing for the boreal species too because their present day range was completely under miles of ice then.

Fossil pollen from Pigeon Marsh in Walker County dates to approximately the LGM.  Jack and red pine pollen ranges from 25%-45%, oak pollen ranges from 30%-40%, and spruce pollen ranges from 1%-2%.  Hickory and chestnut were present but not abundant.   After the LGM, as the climate remained cool but became more moist, beech became the dominant tree here until the modern floral composition took over.

During the Pleistocene sagponds would’ve been prime foraging grounds for herds of hungry mastodons.  Sagponds supported the kinds of aquatic plants (especially buttonbush and pond cypress) that we know mastodons ate from analysis of their coprolites.  And indeed, fossil evidence of mastodons has been found in the ridge and valley region of Georgia and Tennessee.

References:

Watts, W. A.

“The Vegetation Record of a Mid-Wisconsinian Interstadial in Northwest Georgia”

Quaternary Research 3 (2) 1973

Wharton, Charles

The Natural Environments of Georgia

Georgia Department of Natural Resources 1980


Follow

Get every new post delivered to your Inbox.

Join 287 other followers