Posts Tagged ‘Mid-Wisconsinian Interstadial’

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 https://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