The Vice-Versa Climate Phases of South Florida during the Late Pleistocene

Drastic climatic fluctuations occurred during Ice Ages.  Sudden warm spikes in average annual temperatures followed by rapid onsets of much colder climate phases altered the quantitative composition of plant species in the environment.  In most of North America oaks and other broad-leafed trees increased in abundance during warm wet interstadials but waned during arid cold stadials when coniferous parkland forests consisting of pine and spruce expanded in response to the changing climate.  These fluctuations were part of a feedback loop.  The warmer temperature phases gradually caused giant ice dams to weaken and break.  Torrents of cold freshwater from glacial lakes with floating icebergs flooded the North Atlantic, shutting down the Gulf Stream which during present day conditions moderate temperatures.  These deluges of iceberg studded meltwater are known as Heinrich Events, and they caused average annual temperatures to drop by 15-20 degrees F within decades.  In response to the drop in temperatures the Ice Sheets expanded for thousands of years until the next warm phase.  Climate also became drier because moisture for potential precipitation became locked in glacial ice.  However, evidence from a lake in south Florida suggest climate there was out of sync with the rest of North America north of the Rio Grande.

Graph depicting Heinrich Events and the subsequent fall in temperature.  Pollen evidence from sediment in Lake Tulane, Florida reveals the floral response to these climatic shifts, including the most recent 6 Heinrich Events.

Lake Tulane is located near Avon Park, Florida.

Scientists have taken numerous cores of sediment from Lake Tulane, located in south central Florida.  Lake Tulane is a very old body of water and has probably existed for almost 5 million years since this region emerged above sea level.  The sedimentary record goes beyond the limits of radio-carbon dating (50,000 years). The pollen composition in these cores shows the local environment’s response to Heinrich Events, but curiously it is the opposite from that of the rest of North America.  When the rest of North America experienced a warm wet interstadial, south Florida became cooler and drier (though mostly not sub-freezing).  The landscape transformed into an environment dominated by scrub oak, Florida hickory, red cedar, ragweed, grass, asters, staggerbush (Lyonia sp.), and rosemary (Ceratiola).  Scrub oak thickets surrounded by open spaces prevailed.  These are all drought-tolerant but shade-intolerant species, and red cedar is fire-intolerant, indicating the rarity of lightning-induced fires.  Conversely, when the rest of North America suffered cold dry stadials south Florida was warmer and wetter and pine forests spread across the land because lightning-induced fire frequency increased, and pine is fire tolerant.  Scientists find macrofossils of aquatic plants in the sediment representing dry phases because the lake was shallow enough to support emergent marsh vegetation, but these are absent during the wetter phases.

A brand new study introduced a new line of evidence that supports assumptions based on the earlier pollen evidence.  The authors of this study looked at variations in carbon and oxygen isotopes from Lake Tulane plant leaf waxes.  Scientists can understand the historical precipitation characteristics by studying the isotopic composition of plant leaf waxes.  (Plants synthesize organic compounds by using hydrogen atoms they absorb from water molecules.  The isotopes vary depending upon their source.)  They discovered that during climatic phases when scrub oak and ragweed dominated, average annual precipitation dropped by 22%.  The source of precipitation was different too.  Oak phase precipitation mostly came from storm fronts, but pine phase precipitation originated from tropical oceanic storms.

The vice-versa climate of south Florida was tied to the shifting Gulf Stream.  Under non-Ice Age conditions the Gulf Stream carries tropically-heated water to the North Atlantic as far as the coast of Canada, moderating temperatures.  When it shut down following Heinrich Events, the warm water stayed near the coast of south Florida, keeping it warm and wet while the rest of North America suffered dry cold conditions.  The Gulf Stream eventually restarted, bringing warmer wetter temperatures north, but this caused cooling and aridity in south Florida.

I hypothesize the Gulf Stream periodically began to restart within stadials, then shut down with new influxes of meltwater.  These partial changes likely influenced temperatures near the coasts of Georgia and South Carolina.  The climate may have temporarily been warmer in this region even during colder climate stages, and the composition of species here may have varied as well, though it involved different species than the Florida endemics.  Eventually, when the Gulf Stream restarted for longer periods, coastal Georgia may have experienced warmer climates centuries before northern latitudes of North America did.

Common ragweed.  It was much more common in south Florida during the late Pleistocene than it is today.

There is no modern analogue for the abundance of ragweed (Ambrosia artemisifolia) in south Florida during the late Pleistocene.  Ragweed prefers cooler nights than occur in present day Florida for germination, and its abundant presence during the late Pleistocene indicates cooler nights prevailed, even during the out of phase warmer climate stages.  Ragweed is a tough plant that grows on disturbed sites such as abandoned agricultural fields and vacant lots.  It produces up to 32,000 seeds per plant, so it is able to survive heavy foraging by herbivores.  The seeds persist indefinitely and can wait for centuries before germinating when an environment transforms into a sunny one.  This means it can lay dormant through several stages of forest succession, and then colonize the habitat when conditions become favorable.  This may explain why it was so successful during the late Pleistocene when both sudden climatic fluctuations and megafauna foraging greatly disturbed and altered the landscape.  Many animals consume ragweed.  Rabbits and meadow voles eat the leaves while birds including juncos, cowbirds, quail, purple finch, mourning dove, goldfinch, and red-bellied woodpeckers eat the seeds.  Rabbits and meadow voles were common in south Florida during the Pleistocene, though the latter is presently restricted to 1 county in the state.  Farmers report livestock prefer giant ragweed (Ambrosia trifida) over clover.  It’s unclear if this species lived in south Florida during the late Pleistocene.  It is found in a couple counties, and this may represent relic populations, but they also may be invasive.  Livestock will also eat common ragweed, though they don’t like it as much.  Pleistocene megafauna such as horses, bison, and mammoth likely ate ragweed and grass, and they probably occurred in large herds during both climatic phases.  Other species probably common during the oak phase were Harlan’s ground sloth, northern pampathere, flat-headed peccary, and giant tortoise.  Flat-headed peccaries preferred thorny thickets, and the other 3 liked open environments.  Predators such as saber-tooths, lions, jaguars, and dire wolves fed upon the herbivores.  Ragweed survived megafauna foraging by producing large numbers of seeds, but 2 other plant species in this environment survived because their leaves were toxic–rosemary and staggerbush.  I think both climatic phases in Florida supported approximately similar populations of megafauna.

References:

Arnold, T. Elliott; et. al.

“Climate Response of the Florida Peninsula to Heinrich Events in the North Atlantic”

Quaternary Science Reviews 194 2018

Grimm, Eric; et. al.

“A 50,000 year old record of Climate Oscillation from Florida and its Temporal Correlation with Heinrich Events”

Science 9 July 1993

Grimm, Eric: et. al.

“Evidence for Warm Wet Heinrich Events in Florida”

Quaternary Science Review 25 Sept 2006

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Gulf Fritillary and Passion Flower Vine

Butterfly migration is even more amazing than bird migration.  Bird migration includes the same generation, but butterflies that begin migrating north never live long enough to return south.  Instead, butterflies gradually expand their range north as the weather warms; breeding, laying eggs, and dying.  The next generation advances farther north.  Then, several generations later, they begin moving south, retreating before killing frosts.  The gulf fritillary (Augraulis valinae) is an example of a migratory butterfly.  They winter in Florida, south Texas, and Mexico, but generations of them migrate as far north as Pennsylvania.  Gulf fritillaries were named because they are some times seen fluttering over the Gulf of Mexico.  Their larva feed upon passion flower vine (Passiflora incarnata) foliage.  The adults obtain their energy from nectar in flowers , and as the below photo represents, they often find some nutrition in animal feces.  Gulf fritillaries are particularly fond of lantana, a non-native shrub that rapidly colonized Florida during early Spanish occupation.

Gulf fritillary snacking on dog feces.

Passion flower.  Spanish conquistadors thought it symbolized the passion of Christ.

The fruit of passion flower is edible.  The seeds are covered in a gelatinous substance with a sweet-sour flavor and a tropical aroma.  Brazil produces and consumes the most passion fruit.  Imported passion fruit is occasionally available in the grocery store.

There are between 520-700 species of passion flower vine–taxonomists disagree about the number of species.  96% of them occur in the Americas, indicating this is where they originated.  Other species live in southeast Asia, Australia, and Pacific islands.  They probably colonized these regions by rafting on clumps of debris ripped from the land by  tropical storms.  P. incarnata and the crinkled passion flower (P. gracilis) are the only species that evolved to live in temperate climates.  P. gracilis  is restricted to 1 county in South Carolina, while P. incarnata ranges throughout eastern North America.  During the Miocene when most of North America was sub-tropical there were probably many species of passion flower native to North America, but just 2 evolved the ability to survive frosty seasons.

Passion flower vines are shade intolerant but drought tolerant.  They prefer disturbed areas, and I’ve found them growing on vacant lots in my neighborhood.  This species was well adapted to live during the Pleistocene when rapid climate change and megafauna foraging often drastically altered local landscapes.  Mammoths and other large animals girdled and uprooted trees, opening up the canopy so shade intolerant passion flower vines could thrive.  Many vertebrates, perhaps peccaries, fed on the fruit and distributed the still viable seeds in their dung.  Long Ice Age droughts also killed trees and let passion flower vine spread in the available sunshine, climbing over grass and tree saplings and across bare sandy soils.

When the Spanish conquistadors conquered the Americas, they found passion flower vine growing everywhere.  The soldiers were super religious, though they ignored 1 of the 10 commandments when they were butchering the Indians.  They thought passion flowers symbolized the crucifixion of Christ, known as the passion by religious zealots.  Supposedly, the 5 petals and 5 sepals represent the 10 apostles.  The 72 filaments = the number of thorns in Jesus’s crown.  The 3 stigmas = the cross.  The 3 stamens = the wounds in Jesus’s hands.  The leaf lobes resemble the spear wounds.  The dark spots under the leaves represent the 33 pieces of silver given to Judas to betray Jesus.  The flowers die after just 1 day, just like Jesus died after a day on the cross.  And the petals reclose like the tomb enclosed Jesus.  Some superstitious priest sure had an overactive imagination.

Did Large Carnivores Influence Dune Formation in Ice Age Georgia?

Over 100 years ago Australians built a 3480 mile long fence to keep dingoes away from livestock. For ecologists this provides a grand experiment of how the exclusion of a large predator influences ecosystems. However, there exists a considerable amount of conflicting scientific literature about this. Many studies report overgrazed regions on the dingo-less side of the fence that have poor soils as a result. The fence bisects a national park. One study confined to part of this park counted 85 dingoes and 8 kangaroos on the side of the fence with the dingoes, and 1 dingo and 3200 kangaroos in a comparably sized lot on the side that is supposed to be without dingoes. Tame livestock, feral goats and hogs, and rabbits along with the kangaroos contribute to these overgrazed landscapes. Parma wallabies, the greater bilby, and small rodents thrive on the side of the fence with the dingoes because the large canines suppress populations of smaller predators. Another study that claims to be more comprehensive than any other found no differences between either side of the fence. The authors of this study suggest there are no differences because dingoes have never been completely eliminated on the supposedly dingo-less side of the fence. They say other studies concluding there is a difference are local and anecdotal.

Dingo on a sand dune.

I think the most interesting study is a recent paper that found the presence of dingoes influenced sand dune formation in arid regions. On the dingo-less side of the fence sand dunes were larger and stabilized with shrubby plants growing on top. On the side of the fence with dingoes sand dunes were more shallow, bald, and dispersed by wind because plant growth was sparse. This seems counterintuitive. But this difference in dune formation is caused by the suppression of small carnivore populations. Dingoes reduce populations of foxes and feral cats (neither of which are native to Australia). In turn dusky hopping mice and rabbit populations increase, and they eat the seeds of plants and shrub saplings that keep dunes stabilized.

This last study is most interesting to me because sand dunes rolled across parts of Georgia during the coldest driest stages of Ice Ages, and I wonder if large predators influenced their shape and pattern. The arid climate caused some small rivers in Georgia to run dry. Wind blew the riverine sand into big dunes that are still evident today, though scrubby vegetation has since stabilized them. (See: https://markgelbart.wordpress.com/2012/04/09/the-ohoopee-sand-dunes/ ) I’ve hypothesized overgrazing by megafauna alongside shrinking water holes located in the river bed may have contributed to the erosion leading to sand dune formation. But maybe the presence of large carnivores played a role as well. Dire wolves, jaguars, and cougars suppressed populations of bobcats and foxes; causing an increase in rodent and rabbit numbers. The small herbivores stripped the vegetation bare, allowing sand dunes to roll. On the other hand hawks, owls, and snakes probably always remained abundant, and they likely provided a check on rodent and rabbit populations. Nevertheless, the notion large carnivores may have influenced dune formation in Georgia is an intriguing idea.

References:

Glen, A.; and C. Dietman, M. Soule, and B. Mackey
“Evaluating the Role of the Dingo as a Trophic Regulator in Australian Ecosystems”
Australian Ecology August 2007

Harris, Emma
“Dingoes have Changed the Actual Shape of the Australian Desert”
The Atlantic July 6, 2018

A 9 Mile Long Dogwood and Magnolia Grove in Alabama (circa 1775)

When William Bartram traveled through the south from 1773-1776 he observed many environments that today are either extinct or very rare.  In southern Alabama just east of Mobile he journeyed through a grove of dogwoods and magnolias that was 9 miles long.  This is how he described it.

“We now enter a very remarkable grove of Dog wood trees (Cornus florida) which continuing nine or ten miles unalterable, except here and there a towering Magnolia grandifloria; the land on which they stand is an exact level; the surface a shallow, loose, black mould, on a stratum of stiff, yellowish clay; these trees were about twelve feet high, spreading horizontally; their limbs meeting and interlocking with each other, formed one vast, shady, cool grove, so dense and humid as to exclude the sun beams at noon-day.  This admirable grove by the way of eminence has acquired the name of the Dog woods.”

The existence of an almost pure stand of dogwoods this large has long puzzled me.  Dogwood is a common understory tree throughout the south but I’m unaware of any natural location where it largely dominates as a canopy species.  Recently, I reread the passage, and the next morning I had a eureka moment–I believe passenger pigeon flocks created this unusually large stand of dominant dogwood trees.  The dogwood grove Bartram observed was likely the site of a massive passenger pigeon roost 50-100 years before he traveled through it.  Flocks of migrating passenger pigeons (Ectopistes migratorius) formerly caused eclipses of the sun lasting for 6 hours, and when they roosted their colonies would so damage the forest it would appear as if a tornado had struck.  The weight of the roosting birds would bust limbs and even crack enormous tree trunks in half.  The dung overfertilized the trees, often killing all of them.  These enormous colonies covered many square miles.  This explains the extent of Bartram’s dogwood grove.

Dogwood trees were already common in the understory of the forest, and the fruit ripens in the fall…exactly when passenger pigeons migrated to the south after nesting in the midwestern states.  It seems likely passenger pigeons fed on the dogwood and magnolia berries in the surrounding forest, and deposited the still viable seeds under their roosts in their dung.  Dogwood trees sprouted in the nutrient rich soil and thrived in the open sunlight created when the overstory trees were destroyed by the passenger pigeons.

Bartram’s dogwood grove was probably located in Conecuh County, Alabama.

Passenger pigeon migrations eclipsed the sun.

Bartram describes adjacent open plains that also resemble a landscape recovering from a passenger pigeon invasion.  Most of the 70 mile forest surrounding the dogwood grove consisted of oak, hickory, black walnut, elm, sourwood, sweetgum, beech, scarlet maple, buckeye, and black locust with an understory of dogwood, crabapple, and plum.  (Chestnut and pine grew on rocky hills.)  But some pockets of treeless plains within the forest and alongside the dogwood grove were composed of shrubs covered in grape vines.  The shrubs included silver bud, buckeye, bignonia, azalea, and honeysuckle.

Dogwood berries.  Passenger pigeons ate them.  They taste bittersweet to me.

Flowering dogwood.  Bartram’s dogwood grove must have been beautiful during early March when the tree blooms.

I’ve always wondered how forests recovered following an invasion of passenger pigeons.  It didn’t occur to me until just recently that Bartram had described just such a site, though he was unaware of how the landscape he described originated.  In summary I shall list the lines of evidence for my hypothesis that Bartram’s dogwood grove was the result of a massive passenger pigeon roost 50-100 years earlier.

1. The size of the site (9 miles in extent) is the same size as many passenger pigeon roosts described by colonists.
2. Heavily fertilized soils support monocultures.  The site, fertilized by pigeon dung, supports just 1 dominant species with 1 minor component.
3. From Bartram’s description all of the dogwood trees appear to be the same age, suggesting they all germinated during the same year.
4. Passenger pigeons arrived in the region when dogwood trees bear fruit.  This makes my hypothesis plausible because passenger pigeons are the only species that could have planted dogwood seeds on such a large scale.
5. Adjacent areas also appear to be recovering from a passenger pigeon invasion.  Bartram describes pockets of plains where there are no overstory trees, just shade intolerant shrubs covered in grape vines.
6. The complete absence of overstory trees indicates a sudden traumatic tree-killing event in the recent past

The Pliocene Marine Extinction Event

A major marine extinction event rubbed out at least 36% of the ocean’s vertebrate genera about 2.5 million years ago.  Scientists believe the extinctions were caused by a sea level fluctuation, resulting from glacial expansion.  Ice Ages increased in intensity during the late Pliocene and as more atmospheric moisture became locked in glaciers, sea level fell.  Habitat for many coastal species simply disappeared because their near shore environments rose above sea level.  A new study determined 55% of marine mammals, 43% of sea turtles, 35% of sea birds, and 9% of sharks and rays went extinct. I believe this estimate may undercount the actual loss because there are likely some extinct species yet to be discovered by paleontologists.  Many species of invertebrates became extinct as well.

Most of the genera lost were impressive and interesting.  Metaxytherium were a widespread genera of dugongs that grazed sea grass off coasts all across the world.  Thalassocrus were a group of aquatic sloths that evolved from giant ground sloths.  Giant predatory sperm whales (Livyatan) preyed on whales.  Psephopherus, giant sea turtles, laid their eggs on beaches.  The islands off the coast of South Africa, where several species of extinct penguins nested, became connected to land when sea level fell, and predators were able to invade and destroy their colonies. And of course the famous giant white shark, Megalodon, hunted the many species of now extinct whales that lived during the Pliocene.  Most species of baleen whales were smaller and more agile then because they had to avoid these large predators.

Metaxytherium floridanum swam near and over what today is Florida.

Thalassocrus, an aquatic genera of sloths.

2 of the largest predators that lived during the Pliocene–Livyatan melvillei and Carcharocles megalodon.  Both grew to 60 feet long.  Baleen whales were smaller and more agile then, enabling them to escape predation.  The extinction of these predators allowed baleen whales to evolve to a greater size, so they can gorge on food, then fast when they migrate to warmer calving grounds where killer whales, their only modern marine predator, are uncommon.

During the Pleistocene new marine species evolved that were better adapted to the fluctuating sea levels of alternating glacials and interglacials.  New genera increased by 21%.  However, this means there is still a deficit of -15% fewer marine vertebrates than there were during the Pliocene.  Sea life may reclaim the land though, if sea levels keep rising.

An octopus recently found its way into a Miami parking garage.  If sea levels keep rising, marine life may reclaim territory it lost during Ice Ages.

Reference:

Pimiento, C. et. al.

“The Pliocene Marine Megafauna Extinction and its Impact on Functional Diversity”

Nature Ecology and Evolution 1 June 2017

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

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.