A Pleistocene Cloud Forest

Cloud forests are lush environments unique to high elevations located within tropical latitudes.  Vines cover evergreen trees and ferns carpet the ground.  Cloud forests occur along the Andes Mountains from Central America to Argentina at elevations between 3600-10,800 feet, and most are frost free due to the tropical latitude, though they are cooler than lowland forests.  The low seasonality of cloud forests allows for a diverse assemblage of flora and fauna.  Some common plant species found growing in South American cloud forests are elephant ear, strangler fig, and walking palm.  Over 400 species of birds reside in cloud forests including an astonishing 30 species of hummingbirds.  Mammals such as tapir, peccary, brocket deer, jaguar, cougar, ocelot, and spectacled bear roam cloud forests.  Even more species of reptiles and amphibians abound in the thick vegetation of the understory.  Huge beetles and a butterfly with see-through wings are just some of the countless insects that thrive in cloud forests.

Location of cloud forests around the world.

A cloud forest in Ecuador.

Walking palm trees.

Invisible wings make this butterfly hard for predatory birds to see.

A site with evidence of a Pleistocene-aged cloud forest was unearthed during the construction of an highway in Ecuador.  Scientists examined pollen, geochemistry, and charcoal excavated from strata here dated to between 45,000 years BP-42,000 years BP. During those 3000 years the site went through 3 successional stages.  Pollen evidence suggests during the initial stage that it was a valley floor swamp dominated by grass, aster flowers, and plants in the nightshade family.  This environment was replaced by a forest of holly and plants in the Melstomataceeae and Weinmannia families.  Melastomataceae is a family of tropical flowering plants, and the Weinmannia family includes 65 tropical plants.  This stage succeeded to an environment dominated by alder, myrtle, and plants in the hedyosum genus which includes 65 tropical species.  The latter 2 stages consisted of plant compositions that don’t occur in present day cloud forests.

The authors of this study also measured the amount of sporormiella in the sediment.  Sporormiella is a dung fungus and is used as a proxy for megafauna abundance when fossil evidence is not available.  The amount of sporormiella suggests megafauna were present but not abundant.  Ground sloths, giant armadillos, and gompotheres (a type of mastodon) compose part of the regional fossil record here.  These species were likely the source of the sporormiella in the 42,000 year old sediment.

Fire is rare in montane cloud forests, but there are plenty of other agents of change that cause the environment to go through successional stages.  Landslides on steep slopes after heavy rains can demolish a forest, opening an opportunity for pioneer plants.  Wind throws and forest dieback from old age, disease, or insect infestation also opens up space for pioneer species.  Megafauna probably had just a minor impact on Pleistocene cloud forests because they were not abundant here and plant growth is rapid.  The authors of this study did find volcanic ash in the sediment.  Volcanic-sparked fires do burn some cloud forests, forcing the environment to regenerate through several successive stages.

Reference:

Loughline, N.; et. al.

“Landscape Scale Drivers of Ecosystem Change in the Montane Forest of the Eastern Andean Flank, Ecuador”

Paleogeography, Paleoclimatology, and Paleoecology 2017

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The Oak Colonization of North America

Oaks are such an important part of the temperate forest ecosystem that it’s hard to imagine they originally evolved near the arctic circle.  During the Eocene about 45 million years ago the earth was mostly tropical and sea levels were much higher than they are today.  There were no ice caps, and climate at the poles was warm and temperate.  Nevertheless, for almost half the year the sun didn’t rise near the arctic circle, just as today night is nearly 6 months long in places like Alaska.  Seasonal darkness led to the evolution of deciduous trees that saved energy by dropping their leaves during winter when the sun didn’t rise.  This adaptation became a great advantage when worldwide climate cooled.  Deciduous trees pushed south because they were able to survive dormant cool seasons that began to occur during the start of the Oligocene ~33 million years ago.  Deciduous trees, especially oaks, replaced tropical species incapable of coping with winter frosts.  Deciduous trees didn’t waste energy with unnecessary growth during winter.

Evidence of the ancient forests where oaks originated exists near the arctic circle at a site known as Axel Heiberg Forest.  Today, this site is a polar desert, but wind erosion is gradually uncovering the forests that existed here 46 million years ago.  A series of floods, perhaps 1 every 10,000 years, covered these forests in sediment, so there are layers of tree stumps, roots, and fallen logs continuously being revealed, as winds strip the sediment away.  Sediment covered the forests rapidly during these catastrophic floods.  It is not a petrified forest because the geological conditions did not favor fossilization.  So once exposed to air, the ancient wood begins to decay, though the process is slow in cold arid conditions.  Scientists think the environment was a warm seasonal rain forest.  Tree composition consisted of dawn redwood, Chinese cypress, hemlock, pine, spruce, larch, gingko, and extinct species of birch, alder, sycamore, walnut, hickory, and oak.

Location of Axel Heiberg forest–site of the oldest subfossil remains of oaks. Today, it is a polar desert, but during the Eocene it was a temperate seasonal rain forest.

Subfossil wood from Axel Heiberg forest.

Comparison between white oak leaves (top) and red oak leaves (bottom).  White oaks and red oaks ecologically complement each other and colonized North America at the same time.

Oaks are classified into 2 groups–red oaks and white oaks.  Genetic evidence suggests red oaks diverged from white oaks about 33 million years ago when they both began to colonize latitudes south of the arctic circle.  Red oaks produce crops of bitter acorns every other year, while white oaks produce more palatable acorns annually.  The strategic difference in acorn production is an ancient ecological balance, attracting squirrels and other seed distributors equally.  Genetic evidence also shows eastern red and white oaks are sister species to western red and white oaks.  Mexican oaks are sister species to eastern oaks, having diverged between 10-20 million years ago.  Oaks colonized eastern and western North America at the same time, then later eastern oaks invaded Mexico.

Mexico has more species of  oaks than any other region in the world (154 species).  If a region has more species of a genus, it usually is thought to be the region where that genus originated.  Instead, scientists believe Mexico has a greater number of oaks species because of differences in elevation in mountains closer to the equator.  Mexican mountains host many different ecological niches causing frequent speciation among oaks.  This explains why Mexico is home to more species of oaks than any other region in the world, though it is not where they originated.

Reference:

Hipp, Andrew; et. al.

“Sympatric Parallel Diversification of Major Oak Clades in the Americas and the Origin of Mexican Species Diversity”

New Phytologist September 2017

Inner Coastal Plain Deserts of the Ice Ages

A new study reinforces evidence, indicating some regions of southeastern North America were harsh environments during climatic phases when the ice sheets that covered Canada were expanding.  The scientists who wrote this paper took cores of sediment from 2 Carolina Bays (Jones and Singletary Lakes) located in Bladen County, North Carolina. Carolina Bays are elliptical depressions found on the Atlantic Coastal Plain that were formed during Ice Ages.  They were created by a combination of peat fires, and wind and water erosion.  The peat fires lowered the elevation, wind blew out the dried unconsolidated sediment, and wind-driven water shaped them into elliptical formations.  Jones and Singletary Lakes were also studied in the early 1950s in 1 of the first paleoecological studies of late Pleistocene environments of the south.  The new study analyzed pollen composition, charcoal abundance, and biomass; and the authors compared their results to the earlier study.  The data was dated using radio-carbon dating.

Location of Bladen County, North Carolina.  This is the site of the study areas discussed in this blog entry.

Photo of Singletary Lake, a Carolina Bay.  Scientists took a sediment core at the bottom of this lake and analyzed pollen, charcoal, and biomass abundance over the past 50,000 years.

Between ~60,000 years BP-~30,000 years BP climate fluctuated drastically between warm wet interstadials and cold arid stadials.  The glaciers covering Canada advanced then retreated then advanced again in fits and starts.  During glacial expansion more of earth’s atmospheric moisture became locked in glacial ice, causing prolonged droughts, but this moisture was released when glaciers were in a meltwater phase.  Oak and grass pollen increased during meltwater phases, and so did charcoal abundance.  An increase in vegetation meant there was more biomass to ignite and burn during electrical storms.  Oak and grass were fairly abundant from ~43,000 years BP-~32,000 years BP.  The environment mostly consisted of woodland and grassland during interstadials,  but about 30,000 years BP the situation deteriorated.

Ice sheets maintained a steady expansion from ~30,000 years BP-~21,000 years BP.  The initial drought that struck the region during this phase killed vegetation and caused a temporary spike of charcoal because the dead biomass was so flammable.  But after this initial spike, fire was rare to non-existent here.  Sand dunes rolled across the landscape because much of the region was sparsely vegetated.  I believe scrub oak thickets with thorny plants adapted to arid climates covered much of the landscape, but this type of environment doesn’t produce much pollen.  Thus, the amount of vegetation on the landscape then is understated in the pollen record.  For this reason I don’t believe the landscape was as bare as the authors of this study concluded when they wrote it was a “windswept sandy desert with riparian communities of pine and oak.”  Nevertheless, it was an harsh environment of thorny thickets interspersed with areas of bare soil and long distances between water and wetland environments where some trees and grass still grew.  Some tough species of mammals that could survive in this type of environment included horse, flat-headed peccary, helmeted musk-ox, and hog-nosed skunk.  Bison evolved into a smaller species more capable of living in a drier natural community. Overall, wildlife populations probably declined during this climatic phase.

About 21,000 years ago, the ice sheets began retreating and precipitation increased.  Oak and grass gradually increased in abundance, and eventually mesic species such as cypress, basswood, hemlock, and beech invaded the resulting wetter habitats.  ~12,000 years ago, man colonized the region and overhunted megafauna into extinction.  Human-set fires combined with an increase in biomass not being consumed by megaherbivores caused a great increase in fire frequency.

I’m skeptical of 1 claim made by this paper.  The authors estimated the average annual temperature and precipitation levels based on plant composition assumed from the pollen record.  During the Last Glacial Maximum they estimated the average January temperature at these sites was 20 degrees F, while the average July temperature was 68 degrees F.  However, they use 2 dubious assumptions.  They believe the pollen grains from northern species of pine can be distinguished from those of shortleaf pine, a southern species.  This is a doubtful assumption that I will examine more thoroughly in my next blog entry.  Moreover, the spruce pollen probably originated from an extinct species of temperate tree known as Critchfield’s spruce.  I don’t think they can estimate average annual temperatures based on pollen composition, unless the exact species are known with more certainty.

The outer coastal plain and the continental shelf, which was above sea level from ~80,000 years BP-~7,000 years BP, likely hosted richer environments than the inner coastal plain during stadials.  Sea breezes and weather fronts spawned in the Atlantic Ocean brought more moisture to the coast, allowing this region to maintain a mosaic of woodland, grassland, and wetland; while the inner coastal plain suffered greater aridity.  These fronts usually dissipated before they reached the inner coastal plain.  The coastal region likely served as a refuge for plants and animals that later re-colonized the inner coastal plain when climatic conditions improved.

Reference:

Spencer, Jessica; et. al.

“Late Quaternary Records of Vegetation and Fire in Southeastern North Carolina from Jones Lake and Singletary Lake”

Quaternary Science Review 174 October 2017

Arizona Sky Islands–Another Ecological Analogue for Pleistocene Georgia

Rapid climate oscillations, megafauna foraging, fire, and wind throws shaped the landscapes of southeastern North America during the Pleistocene.  The resulting environment in the piedmont region consisted of open oak and pine woodlands but with significant patches of closed canopy forests, savannah, prairie, scrub, and wetland.  This variety of habitats in close proximity supported a great diversity of wildlife.  The Pleistocene ecosystem in this region was unlike any extant environment.  Nevertheless, I’ve previously considered some regions as relatively close ecological analogues, resembling the Pleistocene piedmont.  Russian’s Far East was until recently a vast untracked wilderness of mixed forests with abundant game and apex predators.  (See: https://markgelbart.wordpress.com/2011/06/06/russias-far-east-the-modern-worlds-closest-ecological-match-to-pleistocene-georgia/ )  The Cross Timbers region of Texas and Oklahoma where the eastern deciduous forest gradually gives way to prairie may also be a vaguely similar analogue.  (See: https://markgelbart.wordpress.com/2012/06/13/the-cross-timbers-ecoregion-an-analogue-for-georgia-environments-during-some-stages-of-the-pleistocene/ ) I’ve come across a 3rd region that in some ways may resemble Pleistocene piedmont Georgia–the Sky Islands of Arizona, New Mexico, and northern Mexico.

Sky Islands are mountains that stand in the middle of the desert.  They host a variety of environments that change according to elevation.  A change of a few thousand feet in elevation equals the climatic difference of hundreds of miles in latitude.  In a day a man can ascend from an hot desert to temperate oak/pine woodland to boreal spruce/fir forests.  During Ice Ages the lowlands surrounding Sky Islands hosted continuous temperate forests, but now these forested environments are isolated on the mountains, surrounded by desert, hence the name Sky Island.

Mountains rise from the desert floor in Arizona, New Mexico, and northern Mexico.  They host diverse flora and fauna because the change in elevation supports a variety of environments adjacent to each other.

Sky Islands are rich in floral and faunal diversity because so many different natural communities are in such close proximity.  Sky Islands are home to 500 species of birds (over half of the species found in North America), 104 species of mammals, and 120 species of reptiles and amphibians.  Tree squirrels including Mexican fox squirrels, Arizona gray squirrels, and Mt. Graham red squirrels co-exist with rock squirrels (Spermophilus variegatus).  Rock squirrels live and nest in the ground, not trees.  13-lined ground squirrels, another species in the Spermophilus genus, also co-existed with tree squirrels in southeastern North America during the Pleistocene.  13-lined ground squirrels no longer occur in the region because they prefer open environments.  Their presence along with tree squirrels at some fossil sites suggest a more varied environment existed here during the Pleistocene.

Rock squirrel (Spermophilus variegatus).  Sky islands are home to 7 species of squirrels.  During the Pleistocene a squirrel in the spermophilus genus also co-existed with tree squirrels in southeastern North America, suggesting a more diverse variety of habitats within the region.

Arizona Sky Islands are also famous for a small subspecies of white tailed deer known as the Coues.  For some reason the Coues deer is a popular trophy among deer hunters.  Jaguars and coati-mundi roam the Sky Islands as well.

Coues deer–A small subspecies of white tail that lives on the Sky Islands of Arizona.

The oak savannahs and oak/pine woodlands of Sky Islands likely resemble natural communities that occurred in the piedmont region of Georgia during the Pleistocene, though they are composed of different species of trees.  Emory oak, Arizona white oak, Gambel’s oak, Canyon live oak, and blue oak grow with Arizona juniper, pinyon pine, yucca, bull grass, and bear grass.  Higher in elevation, silverleaf oak grows with ponderosa pine and Arizona pine.  Higher still, the forest may consist of ponderosa pine, Englemann Spruce, and Douglas Fir–trees of the northern Rocky Mountains.

Acorn woodpeckers are a communal species that hoards acorns.  They are a common species on Sky Islands.

The different types of forest attract many different species of birds.  Birds that prefer coniferous forests can be found with those that like oak forests. Tropical species including trogons, thick-billed parrots, buff-colored nightjars, and Arizona woodpeckers inhabit Sky Islands.  These species are found at few other sites north of the Rio Grande River.

 

The Chimney Top Fire

The Chimney Top is a series of dry rocky ridges located in the Great Smoky Mountains National Park where slate, schist, and phylite overlay erosion-resistant sandstone.  In some places precipitation has eroded away the top rocks, exposing the sandstone, and the formations resemble chimney tops, hence the name.  Last November, 2 unnamed juveniles set the surrounding forest on fire.  Drought conditions fed the fire, and it was fanned by 80 mph mountain wave winds.  Hot air from the fire rose up the mountain and when it met stable air, it ricocheted and accelerated downward in waves.  The fire burned over 15 square miles and spread into neighboring Gatlinburg, Tennessee, killing 16 people, 2 black bears, and uncounted small animals.  Yet, this forest will recover because many of the plant species growing on the ridge are well adapted to fire and in some cases even dependent upon it.

Needles and cone of the table mountain pine.  This species depends on fire to open its cones.

Fireweed also depends on fire.

The Chimney Tops.  Erosion resistant rock explains the chimney-like formations.

Photo of a burned ridge on Chimney Top.

The Chimney Top environment consists of rock chestnut oak (Quercus montana), table mountain pine (Pinus pungens), and heath balds.  Rock chestnut oak is fire resistant, and it thrives in the rocky shallow soils on the ridge.  Table mountain pine also grows well in the shallow soils, and it depends upon fire to open its seed cones.  Although long exposure to hot sun opens table mountain pine cones, the process is best facilitated by fire.  Park service employees noted a rain of pine seeds in the air a few days after the fire.  In 5 years the burned over ridges will be covered with pine saplings and fireweed.  Some heath balds completely burned to the ground–an unusual occurrence here because this region is the rainiest spot east of the Mississippi.  Heath balds are evergreen shrub communities consisting of mountain laurel (Kalmia latifolia), Catawba rhododendron (Rhododendron catawbiense), various species of blueberries (Vaccinium sp.) and huckleberries (Gayluccia sp.), and 1 deciduous tree–mountain ash (Sorbus aucuparia).  Heath balds are often adjacent to grassy balds and surrounded by forests of red spruce and hemlock.  Heath shrubs thrive on shallow acid soils located on mountain slopes.  Both heath and grassy balds are of ancient origin.  (See: https://markgelbart.wordpress.com/2016/05/16/the-extinct-helmeted-musk-ox-bootherium-bombifrons-and-appalachian-grassy-balds-during-the-pleistocene/ )  Scientists studied heath balds and discovered they grow on a layer of peat underlain by charcoal.  This suggests heath balds occasionally do burn completely, yet regrow in the same location.  This fire gives scientists the first chance to ever witness the rebirth of a heath bald.

Heath bald.

Forests are resilient.  The area in the photo below was clear cut during 1910.  The original forest consisted of chestnut, oak, and hemlock; many with trunks 5 feet in diameter.  The destruction of this locality spurred the creation of the Great Smoky Mountains National Park in 1926.  The 2nd growth forest that replaced the original tract is not as impressive but at least it is green.

This area was clear cut in 1910.  It has nicely recovered but is not as impressive as it was originally.

Bodark Swamps

Botanists believe the Osage orange (Maclura pomifera) was restricted to bottomlands along the Red River drainage when Europeans discovered North America.  Here, it grew in pure stands known as Bodark Swamps.  (A disjunct relic population lived in the Big Bend region.) This relative of the mulberry and fig is a shade-intolerant, early successional species capable of surviving flood events that kill competing trees, perhaps explaining why they grow in pure stands. Early settlers cultivated the trees as hedgerows used to confine livestock, and farmers spread this species all over North America.  Osage orange hedgerows were much cheaper than fencing, and they were widely planted until the introduction of barbed wire in 1875.

Range map of Osage orange.  There were probably additional disjunct relic populations located elsewhere on the continent that were never recorded by botanists.  This species was much more widespread during the Pleistocene.

Illustration of an Osage orange tree and fruit.

There is some indirect evidence the pre-Columbian distribution of Osage orange was wider than range maps indicate.  Hagen’s sphinx moth (Ceratomia hageni) feeds on Osage orange leaves and nothing else.  This species of moth is locally abundant in the Black Belt prairie region of Mississippi–evidence Osage orange grew on the margin of this natural community before European conquest.  Compact clay soils in the Black Belt Prairie favor grass over trees, and shade-intolerant Osage orange grows well in this environment where they have less competition from other trees. Hagen’s sphinx moth has an erratic distribution.  When agriculturalists were spreading Osage orange seeds it doesn’t seem likely they brought the moths with them.  Relic populations of Osage orange probably occurred wherever this moth is common.

Hagen’s sphinx moth, aka Osage orange sphinx moth.  Its only host plant is Osage orange.

Osage orange was even more widespread during the Pleistocene.  Mastodon dung excavated from the Aucilla River in north Florida contained Osage orange.  Fossil evidence of Osage orange reportedly was found in Ontario, Canada where it grew during warm interglacial times.  (The oft-repeated source of this information (Peattie 1953) mentions this but doesn’t cite his source.  I consider it a dodgy fact.  Who identified this fossil wood and from what site was it excavated?)  Osage orange became a relic species following the extinction of the mastodon.  A recent experiment determined Osage orange seeds can survive transit through an elephant’s gut but not an horse’s.  (See:https://markgelbart.wordpress.com/2015/04/10/asian-elephants-elephas-maximus-and-horses-equus-ferus-caballus-refused-to-eat-pawpaws-in-a-controlled-experiment/)  Horses and probably tapirs, a relative of the horse, consumed Osage orange, but this large fruit depended on mastodons and maybe mammoths for distribution across the landscape.  Elephants are capable of carrying viable seeds in their guts up to 40 miles before depositing them in great piles of fertilizer.  Without mastodons Osage orange range became more restricted.  Perhaps the Red River drainage and the Black Belt Prairie were where mastodons made their last stand.

Several characteristics of Osage orange show it co-evolved with megafauna.  The large fruits attract big mammals able to efficiently hold and transport the seeds in their guts.  Although horses, deer, squirrels and birds eat the fruit, they either destroy the seeds during consumption or pick at the fruit without distributing the seed.  Osage orange evolved thorns to deter megafauna from chewing on the tree itself.  And if the plant does get eaten, it is able to re-sprout from sucker roots.

Osage orange, along with yew, is considered some of the best wood for making bows.  Some archaeologists believe certain Indian tribes monopolized trade in Osage orange wood.

Osage orange fruit is not toxic, but it is considered inedible for human consumption.  Connie Barlow, author of The Ghosts of Evolution, reports it tastes like air freshener.  Some people think Osage orange fruit can be used as an insect repellant.  However, 1 scientist found 20 insect species on Osage orange fruit littering the campus at Louisiana State University.  The fruit is more likely to attract critters than repel them.  .

References:

Burton, James

“Osage orange: An American Wood”

U.S. Department of Agriculture Bulletin 1973

Ferro, Michael

“The Cultural and Entomological Review of the Osage orange (Maclura pomifera) and the Origin and Early Spread of “Hedge Apple” Folklore”

Southeastern Naturalist (13) Monograph 7 2014

Peacock, Evan; and Timothy Schauwetum

Blackland Prairies of the Gulf Coastal Plain

University of Alabama Press 2003

 

The Sourwood-Lettered Sphinx Moth-Black Bear Food Web

There are many intricate relationships between different species of plants and animals yet to be discovered.  The interrelationship of sourwood (Oxydendrum arboreum), lettered sphinx moth (Deidami inscripton), and black bear (Ursus americanus) was first noted in the scientific literature just last year.  Sourwood is a small tree, seldom growing to over 6o feet in height, that lives in oak forests and woodlands with acidic soils.  It is the sole species in its genus and a member of the blueberry and azalea family.  The leaves have a sour taste and can be chewed but shouldn’t be swallowed because they are mildly toxic with a high amount of oxalates.  Scientists were studying the occurrence of a major defoliation event of sourwood trees near Unicoi, Tennessee a few years ago.  Here, sourwood trees along with dogwood, summer grape, Virginia creeper, and greenbrier form the understory of a forest composed of red maple, black gum, northern red oak, pitch pine, Virginia pine, chestnut oak, scarlet oak, and striped maple.  They found the sourwood trees were being defoliated by larva of the lettered sphinx moth.

A sourwood tree in fall foliage.

The lettered sphinx moth.

The larva of the lettered sphinx moth feeds upon grape, Virginia creeper, and peppervine; but just recently was discovered to have a preference for sourwood over all those plants in the Vitis family.

The lettered sphinx moth is the only species in its genus that lives north of Mexico.  Lettered sphinx moth larva were known to feed upon the leaves of plants in the grape family which also includes Virginia creeper and peppervine.   Lepidopterists refer to these plants as “host species.”  However, when scientists discovered sphinx moth larva defoliating sourwood they conducted an experiment–they put sphinx moth larva in terrariums and offered them grape leaves and sourwood leaves.  The sphinx moth larva preferred the sourwood leaves.  This suggests sphinx moth larva will choose sourwood leaves wherever the ranges of sourwood and species in the grape family overlap.

Scientists hypothesize the oxalates ingested from the sourwood accumulates in the caterpillar, and the toxicity discourages avian predators.  Nevertheless, bears are able to eat the caterpillars.  The authors of the below referenced study found evidence bears were consuming large quantities of sphinx moth caterpillars during the defoliation outbreak.  They saw stem breakage, claw marks on limbs, and bear scat filled with caterpillar remains all around the sourwood trees.  Moth larva provides lots of protein and fat, and the partially digested plant material in their guts likely contains beneficial vitamins for the bears.  The bear scat in turn helps fertilize the soil around the sourwood trees.

Black bear feeding on forest tent caterpillars.  Caterpillars are nice fatty snacks for the bruins.

The interrelationship between sourwood, sphinx moths, and bears probably began during the Pleistocene or perhaps earlier; but it wasn’t noticed or recorded by people until last year.  There are countless other examples like this, yet to be discovered.

Reference:

Levy, Foster; David Wagner and Elaine Walker

“Deidamia inscripton (Lettered Sphinx Moth) Caterpillars feeding on Oxydendrum arboretum (Sourwood) and their Predation by Black Bears in Northeastern Tennessee”

Southeastern Naturalist 15 (3) 2016

Pleistocene Oysters (Crassostrea virginica)

Before humans harvested them, oysters lived longer, grew larger, and produced denser quantities of offspring.  Scientists compared oyster shells from Pleistocene-age oyster reefs with those from Native-American archaeological sites and modern harvests.  Pre-human contact oysters lived as long as 30 years, while oysters since human colonization never live longer than 6 years.  Pleistocene oysters grew up to 10.2 inches, pre-historic oysters from Native-American middens grew to 7.4 inches, and modern oysters reach 6.1 inches.  Native-Americans harvested oysters in a sustainable way, but populations of oysters since European colonization have been reduced by over 99%, despite restoration efforts.  Pollution and overharvesting have destroyed oyster numbers.  This is unfortunate because oyster reefs are a productive natural community, providing habitat for at least 303 species that have co-evolved with oysters over the past 135 million years, ever since these bivalves first evolved. Scientists estimate the original oyster population of Chesapeake Bay was capable of filtering the entire contents of this estuary in just a few days, so they help clean the water as well.  Modern day estuaries are suffering without more abundant populations of oysters.

Ancient oyster midden.

Pelican in front of a Georgia oyster reef at low tide.

A representative of every species living in oyster reefs could fill a big city aquarium.  Barnacles, mussels, clams, and bryozoans attach themselves to the reefs and live out their lives filter feeding just like their hosts.  Mud crabs (Eurypanopeus depressus) graze on the algae and detritus that accumulates on the reefs and sometimes feed upon the smaller oysters.  Oyster pea crabs (Pinnotheres) depend upon reefs for their very survival. The seashore springtail (Anurida maratima), unusual salt water insects, prey on microorganisms living on the reefs.  Amphipods, worms (Polydora and Polychaetas), anemones, mites, and hydroids are commensal animals dependent upon the existence of oyster reefs.  Boring sponges (Cliona) and starfish directly prey on the oysters.

The seashore springtail is a true insect that lives on oyster reefs.

The depressed mud crab grazes on algae, detritus, and small oysters on oyster reefs.

Many small species of fish swim in and around oyster reefs during low tide because the structure affords protection from predators.  Species of fish commonly found in Georgia oyster reefs include in order of abundance naked goby (Gobiosoma bosci), feather blenny (Hypoblennius hentzi), skilletfish (Gobiosox strumosus), seaboard goby (Gobiosoma ginsbingi), striped blenny (Chasmodes bosguianus), oyster toad fish (Opsanus sp.), and the crested blenny (Hypleurochilus geminatus).  During high tide larger fish such as sheepshead, black drum, and croakers move in and feed upon the shellfish and smaller fish living on the reef.

The naked goby is the most common fish living in Georgia oyster reefs.  They feed upon worms, crustaceans, and dead open oysters.

The skillet fish clings to oysters with its sucking mouth.

Land vertebrates forage oyster reefs during low tide.  Raccoons and wading birds find many a meal on the reefs.  Oyster catchers (Haematopus palliatus) specialize on feeding upon the oysters and other bivalves growing here.  Even boat-tailed grackles exploit oyster reefs–they eat the amphipods and pea crabs crawling over the reef.

The American oystercatcher thrives on oyster reefs.

Oysters have a complex life cycle.  They expel sperm and eggs into the ocean water, and when these sex cells meet by chance they form larva.  (Oysters change sexes, so that males become females and vice-versa.  Some individuals are hermaphroditic  and expel sperm and egg at the same time.)  The larva lives in the zooplankton until they develop a foot.  The oyster senses pheromones from other oysters on a reef and will attach its foot to the structure where it will remain for the rest of its life, filter feeding upon diatoms, dinoflagellates, inorganic particles, bacteria, and marsh plant detritus.

Oyster reefs also have life cycles.  When oysters begin colonizing an area it is known as the clustering phase.  Oysters attach to each other and on old dead oyster shells during the accretionary stage, building reefs.  Eventually, oysters reach a vertical limit and start building the reef horizontally during the senescent phase.  Large reefs block sediment and shell debris carried by tidal currents and this action can create islands.  Little Egg Island in the middle of the Altamaha River mouth is an example of an island created by an oyster reef.

References:

Bahr, Leonard, William Larsen

“The Ecology of Intertidal Oyster Reefs of the S. Atlantic Coast: A Community Profile”

U.S. Geological Survey 1981

Lockwood, R.; K. Kusperck, S. Bonanani, and Gratt, A.

“Reconstructing Population Demographics and Paleoenvironment of Pleistocene Oyster Assemblages: Establishing a Baseline for Chesapeake Bay Restoration”

North American Paleontological Convention 2014

Rick, Turbin; et. al.

“Millenial-scale Sustainablity of the Chesapeake Bay Native American Oyster Fishery”

PNAS 2016

Wharton, Charles

The Natural Environments of Georgia

Georgia Department of Natural Resources 1978

Pleistocene Tornadoes and Windthrow Ecology

Unstable weather conditions spawn outbreaks of tornadoes.  Cold fronts collide with warm air causing the chilled air from the upper layer of the atmosphere to plummet, creating swirling winds of great destructive force.  Tornado intensity is classified according to the Fujita scale or F scale for short.  Tornado wind speeds range from less than 73 mph (an F0 tornado) to estimated wind speeds of 261 mph-318 mph (an F5 tornado).  One of the largest outbreaks of tornadoes in recorded history occurred in early April, 1936.  At least 12 tornadoes struck the south from Tupelo, Mississippi to Anderson, South Carolina.  A tornado from this system that hit Tupelo left a path of destruction 15 miles long.  Another tornado from this storm traveled 50 miles from Alabama to Tennessee.  Two tornadoes merged in Gainesville, Georgia, killing 200 people in a factory and a department store.  Overall, this storm system wiped out 454 human lives.

F5 tornado in Oklahoma.  I hypothesize storms and tornadoes were much more frequent and severe during some Pleistocene climate phases than they are today, but they may have been less severe during others.

I hypothesize tornado frequency and intensity was greater during some climatic phases of the Pleistocene than it is today.  As far as I can determine, no scientist has ever published a study of paleotornado frequency, probably because there just isn’t any method to collect data about past transient phenomena. Incidentally, I invented the term, paleotornado, in case a scientist figures how to study them.  My hypothesis is conjecture, but I am confident it is correct.  I base it on 3 lines of indirect evidence.

a) Data from ice cores in Greenland shows average annual temperatures fluctuated dramatically during Ice Ages.  There was an alternating cycle of sudden warm spikes in temperature that melted ice dams which in turn released glacial meltwater and icebergs into the ocean, shutting down the gulf stream.  This caused an equally sudden reversal in temperatures.   By comparison today’s climate is relatively stable, yet even with a stable climate, tornadoes form with regularity. When climate changed more rapidly in the past, it seems logical to assume there was an increased frequency of colliding warm and cold weather fronts.  I believe the middle south was an Ice Age tornado alley.  Temperatures in south Florida and the Gulf Coast were warmer than they are today because oceanic circulation ceased and warm water stayed in the Caribbean, but the upper south was only a few hundred miles from the Laurentide Ice Sheet that covered Canada and New England.  Cold fronts blowing off the Ice Sheet met warm fronts originating from the Gulf of Mexico in what must have been an exceptionally stormy transition zone.

b) An unusually cold phase of climate, known as The Little Ice Age, occurred between 1310-1850.  Anecdotal historical references suggest storms were more frequent and intense during this time period.  In Europe several storms killed hundreds of thousands of people.  The Little Ice Age is a tiny blip compared to the climate fluctuations of the Wisconsinian Ice Age as recorded from Greenland ice core data.

c) Geological evidence suggests river flooding in southeastern North America was much more severe during the early Holocene (11,000 BP-6,000 BP).  These massive floods caused supermeandering river patterns.  An increase in river flooding indicates an increase in storm activity and hence tornadoes.

Windthrows open up the forest canopy and dramatically change the local ecology.

Tornadoes, thunderstorm downbursts, and hurricanes have a profound impact on forest ecosystems and may be a primary driver of evolutionary relationships.  Areas of forest felled by wind are known as windthrows among ecologists.  Tornadoes can travel for many miles, and they leave long scars of fallen and splintered trees that can be seen in satellite and aerial photographs.  These long windthrows create gaps in the canopy where shade intolerant species can thrive.  In southeastern North America canopy gap formation is beneficial for oak, pine, persimmon, sumac, grapevine, blackberry, composites, and grasses.  Windthrows can become tangles of luxuriant vegetation that provide forage and cover for forest edge species such as whitetail deer, cottontail rabbits, and ruffed grouse.  Fallen rotting timber attracts beetles, food for woodpeckers and other birds.  The extinct ivory-billed woodpecker formerly relied on vast tracts of timber with freshly created windthrows from annual storms.  Unlike extant woodpeckers, they depended upon early colonizing, shallow burrowing beetles.  Snakes and lizards lay their eggs in rotting timber.  Bears tear up these logs, looking for beetle larva, termites, and reptile eggs.  The pits created when trees are uprooted fill with water following heavy rains, and they serve as breeding pools for amphibians.  Most of the organisms that live in southeastern North America evolved to thrive in canopy gaps resulting from wind storms.  Plants able to resprout after sustaining wind damage have a competitive advantage over those species easily uprooted and killed, and the animals that browse and can digest those plants also enjoy a competitive advantage.

One study estimated wind felled 20 square miles of forest per year in pre-settlement forests of Wisconsin.  They also estimated the recovery time for northern hardwood-hemlock forests to erase the windthrow scar is 1210 years.  The recovery time in southeastern forests is probably quicker due to the longer growing season. A tornado can leave a long-lasting impact on the landscape, and wind may be a critical element, along with megafauna foraging and fire, that may explain why Ice Age environments were so much more open than they were in late Holocene environments.

Reference:

Canham, Charles; and Orie Loucks

“Catastrophic Windthrow in the Presettlement Forests of Wisconsin”

Ecology 1988

Pleistocene Pecans (Carya illinoinensis)

The pecan tree is 1 of 17 species of hickory trees.  Hickories are native to North America and Asia and formerly occurred in Europe, but Ice Ages, beginning about 2.5 million years ago, wiped them out there.  European mountains have an east to west orientation, while American mountains are oriented north to south.  Hickories prefer temperate climates, and the east-west mountains blocked their retreat in Europe during glacial expansions.  This explains why hickories and so many other tree species survived Ice Ages in North America but not in Europe.

Evidence of fossil pollen grains suggests hickory trees grew alongside dinosaurs during the late Cretaceous, though the oldest fossil hickory nut dates to about 34 million years ago.  Most early hickory species had thin shells, but they evolved thicker shells about 38 million years ago in response to the evolution of tree squirrels.  Squirrels love the nutrient rich nuts, so hickories evolved nuts with thicker shells, and the squirrels in turn evolved greater gnawing power.  Evolution is a constant struggle.  However, pecans retained the thinner shells of their early ancestors.  This puzzled me because it seems as if squirrels would have eliminated all hickory species with thinner shells because they were easier to exploit.  I wondered if pecans were a recent species, cultivated and spread by Native Americans.  I’ve concluded however, based on certain lines of evidence, that pecans are an ancient species of hickory, not a recently evolved species manipulated by man.

Wild pecan tree.

Native range of wild pecan trees.  Man has greatly expanded this range by planting pecan orchards.  Georgia is now the leading producer of pecans, though they are not native to this state.  Pecans need longer growing seasons than other species of hickory because their nuts mature later.

Genetic studies determined pecans have a large genetic diversity within populations.  If pecans descended from human cultivation, they would have low genetic diversity because they would descend from a small population initially cultivated by man.  I was also mistaken in considering squirrels the only major predator of hickory nuts.  The pecan weevil ( Curculio caryae ) infests all species of hickories, and pecan trees growing in mixed stands with other hickory species have an advantage over their cousins.  Pecans mature later in the season than other species of hickory.  The pecan weevil hatches and emerges in August and will infest whichever hickories have developed kernels.  Because thick shelled hickories mature before thin-shelled pecans, the pecan weevil will infest them first and go through their life cycle without ever infesting the thin-shelled pecan.  Weevils will wait for pecans to mature, if no other hickory trees are available.  So though thin shelled pecans may suffer heavier squirrel predation, they are less likely to have their nuts destroyed by weevils, if they grow near other species of hickory.

Pecan weevil larva.  Pecans mature later than hickories.  Though squirrels favor pecans over hickories, the later maturing pecans are less likely to be attacked by pecan weevils in mixed forests, giving pecans an advantage over other hickory species.

Pecans are native to river bottomland terraces where they grow in forests dominated by sycamore, sweetgum, and elm.  Other subdominants in these terrace forests include water oak, box elder, silver maple, cottonwood, green ash, hackberry, and other hickory trees. Pawpaw, bamboo cane, pokeweed, grape vine, poison ivy, and green brier make up the thick undergrowth of bottomland forests.

Pecans hybridize with 5 other species of hickory.  The nuts produced by wild pecans and hybrids vary in quality.  Most are smaller and have somewhat thicker shells than cultivated varieties of pecans, and some even have high amounts of bitter tannins–all part of their ongoing evolutionary war with squirrels and weevils.  Human cultivation of pecans on a large scale began circa 1900.  Though Georgia isn’t part of the pecan’s native range, the state is the leading producer, and there is a large demand in China and India, resulting in high prices at the grocery store for the nuts.

Most cultivated varieties of pecans don’t mature before the first killing frost occurs in Midwestern states.  A few small early maturing varieties can produce in the Midwest as can the hican–an hybrid cross between pecan and shellbark hickory.

 

 

 

 

 

 

 

 

 

 

 

Hican nuts–an hybrid cross between a pecan and a hickory.  They produce earlier maturing nuts  for northern locations that have frosts before pecans can fully develop.

The southern Mississippi River Valley has long served as a refuge for pecans and other hickory trees during glacial expansion cycles.  Here, they grew in mixed Ice Age forests with spruce, beech, walnut, and oak.  Pecans expanded their range north up the Mississippi River Valley following the end of the last Ice Age.

The oldest written recipe for pecan pie dates to 1886.  It was a custard pie made with sugar, eggs, and butter.  (Sugar/custard pies originated during the Middle Ages.)  Pecan pies became more popular in the 1930s when some unnamed employee of Karo syrup invented a recipe for a pecan pie using syrup as well as sugar.  I prefer my pecan pie made with maple-flavored corn syrup.

Pecan pie with whiskey maple cream sauce.  I like my pecan pies made with maple flavored corn syrup.