Southeastern Grasslands are of Great Antiquity

I enjoy watching a summer thunderstorm.  Lightning strikes offer a natural fireworks show that sometimes surpasses the manmade kind.  The furious wind and roaring thunder show the excited side of mother nature.  It’s the dangerous side of nature–a human could be vacuumed into the sky by a tornado, electrocuted by lightning, or clobbered by a hailstone or wind-strewn debris.  Nobody, not a king nor a baby, is immune to these hazards.

Natural fireworks.

Some anthropologists and a few old school ecologists wrongly believe most of the grasslands that occurred in southeastern North America when Columbus accidentally sailed into the Caribbean were the result of manmade fires.  Reed Noss dispels this notion in his book, Forgotten Grasslands of the South. The map below shows the frequency of lightning strikes in North America.  The south, and especially Florida, has more lightning strikes than any other region of the continent.  Dr. Noss believes the high frequency of lightning strikes can spark enough wild fires to maintain abundant grasslands without any human activity.

Map of average annual lighting strikes in North America between 1989-1999.  Lighting strikes were naturally common enough to have sparked grassland-creating wild fires long before humans arrived in North America.

A longleaf pine savannah on fire.  Longleaf pines are one of the few species of tree whose seedlings can survive fire.

Several lines of evidence support Dr. Noss’s conclusion that anthropogenic activity was not necessary to maintain grasslands.  Formerly, longleaf pine savannah covered most of the coastal plain region of the south.  There were even patches of pine savannah in the piedmont region, though oak and hickory dominated that area.  There are over 900 species of plants endemic (meaning they live nowhere else) to longleaf pine savannah compared to just 80 endemic species found on the grasslands of the Great Plains.  A high number of endemic species suggests an ecosystem of great antiquity and stability.  Because evolution is usually a slow process, it’s not likely that all of these endemic fire-dependent species could have evolved in just the last 12,000 years.  Longleaf pines require fire intervals of 1-10 years or hardwoods will crowd them out.  Longleaf pines grow slowly, taking decades to reach reproductive age.  A species that reproduces this slowly would have never been able to adapt quickly enough to survive a sudden change in fire regime caused by man.  These fire dependent species must have already been present before man colonized the region.

Many species of animals are also endemic to pine savannahs.  The gopher tortoise and the red cockaded woodpecker are perhaps the 2 most well known vertebrates dependent on this fire-influenced environment. Gopher tortoise fossils have been found that date to millions of years ago, while red cockaded woodpecker fossils come from deposits in the vicinity of 200,000 years old.  The presence of these species and many others in the fossil record long predate man’s entrance into the region.  This is obvious evidence that southern grasslands preceded man.

There was a lower frequency of lightning strikes during the coldest stages of the Ice Ages.  Evidence from most fossil sedimentary sites show little, if any charcoal, indicating reduced fire activity.  However, less precipitation combined with megafaunal grazing created grasslands during the colder climatic phases.  Following the megafaunal extinctions, fire activity spiked because so much vegetation was no longer being eaten and instead it became dry tinder.

During the Last Glacial Maximum, longleaf pine savannahs still occurred in refugium located in Florida, Louisiana, Texas, and probably along the continental shelf where sea levels regressed.  Paradoxically, these areas were warmer and wetter during Ice Ages because the Gulf Stream shut down and warm water that normally circulated north pooled around these lower latitudes.  Oak scrub and prairie, the result of aridity and megafauna foraging, predominated in the upper coastal plain.  Longleaf pine savannah didn’t recolonize the upper coastal plain until about 6,000 years ago, but the pollen record suggests this type of environment has waxed and waned cyclically for millions of years, becoming common and widespread during warm and wet climatic phases.

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Fire Suppression = A Decline in Biodiversity (Part 2–The Animals of Longleaf Pine Savannahs)

Fox Squirrels come in several color phases–orange, black, and gray.  Some have white or gray masks as well.

Fox Squirrel–Sciurus niger

I love these big colorful squirrels.  I lived in Niles, Ohio until 1975, and our home was bordered by oak woods on 2 sides.  Big orange fox squirrels were the playful denizens there.  But since I’ve lived in Georgia, I’ve only seen one–a black masked fox squirrel foraging with a group of gray squirrels (Sciurus carolinensis) in a pecan orchard in Burke County.

This range map is bullshit.  No statewide survey of fox squirrels has been done in at least 50 years, if ever.  It’s likely an accurate range map would show a much patchier distribution.

Southern fox squirrels differ in their habitat requirements from northern fox squirrels, despite being the same species.  The former prefer mature longleaf pine savannahs with fingers of oak forests, while the latter thrive in oak/hickory woods.  Fox squirrels are declining in Georgia because longleaf pine savannahs were largely replaced with shorter rotation loblolly pine tree farms.  Lumber companies harvest loblolly pines every 50 years which is not enough time for trees to develop snags.  The Trees are also planted closely and fire is suppressed.  Gray squirrels are more abundant today in state because they’re well adapted to the dense young forests that have sprouted on abandoned agricultural lands.  Gray squirrels escape predation by jumping from tree top to tree top, while fox squirrels prefer to dash on the ground as far as they can before retreating to a tree.  Though clumsy in trees compared to their smaller cousins, their larger size allows them to put up more of a fight, if a predator catches up to them.  This difference in behavior explains why gray squirrels occur in closed canopy forests, and fox squirrels prefer open parkland forests.  For this reason I think fox squirrels were more abundant in this region during the Pleistocene when open environments were common.  Areas managed for red-cockaded woodpeckers should benefit fox squirrels.  Forest managers used longer rotations and fire to maintain the bird’s required habitat.

Red Cockaded Woodpecker–(Picoides borealis)

Photo of a red cockaded woodpecker from google images.  All the photos in this entry are borrowed from there.

Thirty years ago, this bird was on the verge of extinction, despite having formerly been common throughout the south.  Fire suppression and short forest management rotation nearly caused the death of this species.  Young pine trees never develop the soft rot that red cockaded woodpeckers need for boring nesting cavities.  As a defense mechanism, red cockaded woodpeckers constantly peck wells below their nesting cavities from which pine sap continously flows.  The pine resin repels rat snakes–their number one predator.  For this defense mechanism to work, live trees are a must.  And without fire hardwood understory reaches the level of the nesting cavity allowing flying squirrels, and other predators easy access.  Flying squirrels will decimate red cockaded woodpecker nests.

In a successful effort to save the birds, scientists identified habitat requirements and some suitable land was set aside and managed using prescribed burns and longer tree harvest rotations.  Birds were relocated to the best habitat, artificial nesting boxes were installed to supplement the shortage of good nesting trees, and flying squirrel exclusion devices were used.  In many protected areas red cockaded woodpecker family groups (family groups consist of 2-10 individuals) have increased dramatically to the point where it’s no longer necessary to provide artificial nests or to protect them from flying squirrels.  At SRS for example the population grew from 1 family group in 1987 to 30 by 2003.

Sandhill Crane–(Grus canadensis)

These impressive birds grow to 5 feet tall.  They prefer to nest in grassy marshes adjacent to prairies or savannahs.  The real life version of Sesame Street’s Big Bird used to be common, but since grasslands and wetlands have declined so have the birds.  Georgia’s population includes a permanent one consisting of small family groups, and large congregations of winter migrants.  They’re omnivorous feeding on insects, crayfish, mice, snakes, frogs, worms, acorns, fruit, roots, and farmer’s crops.

Bachman’s Sparrow–(Peucaea aestivalis)

Another inhabitant of open pine savannahs that is declining in abundance.  I heard this bird’s song on a youtube video and recognized it as one I’ve heard.  Evidentally, the sparrow still occurs in Augusta.

Indigo Snake–(Drymarchon corais)

This snake grows to 9 feet long, making it the longest serpent in North America.  They’re rare because their habitat has been fragmented, and they need large ranges.  They hunt during the day and retreat into gopher tortoise burrows at night.  A wide variety of prey is taken–other snakes including venemous ones, small mammals, birds, frogs, and fish.  Indigo snakes don’t kill by constriction or envenomation, but instead bite the head of their prey and thrash, breaking the spines of the small creatures.  Their metabolism is faster then that of most other snakes.

Gopher Tortoise–(Gopherus polyphenus)

 

Gopher tortoises depend on a frequent fire regime to spark the growth of the kinds of plant they eat.  They also like sandy soil that makes it easy for them to dig their elaborate tunnel systems.  They’re a keystone species–over 60 vertebrates and invertebrates depend on their burrows for shelter.  (See also my article–“The Giant Extinct Tortoise, Hesperotestudo crassicutata, must have been able to survive light frosts” from my April or March archives)

Popular game animals such as white tail deer, turkey, and quail thrive in longleaf pine savannahs.  Savannahs were a favored habitat of many extinct Pleistocene species as well including mammoth, long horned bison, horses, llamas, Harlan’s ground sloth, hog nosed skunks, giant tortoises, and others.

Fire Suppression = A Decline in Biodiversity (part 1)

The title of this week’s blog entry seems to be a recurring theme in my essays about Georgia’s natural history.  Productive natural landscapes such as longleaf pine savannah, oak forests, and canebrakes are disappearing or nearly extinct due to fire suppression; and the same goes for “strawberry plains,” chinkapins, and many species of animals which will be the focus of part 2 of this series.

Prescribed burn at the Savannah River Site in South Carolina.  After decades of a Smokey the Bear fire suppression policy, the U.S. forest service finally realized controlled fires were beneficial to the ecology.

Fire has always been an important component of Georgia’s ecology.  Before people colonized the region, wildfires occurred irregularly but were unchecked and often burned on a large scale.  Fires were more common during interstadials because of an increase in thunderstorm frequency.  Oaks increased in abundance in correlation with wetter climate cycles, thanks to lightning-sparked fires that opened up the forest canopy for these fire tolerant/shade intolerant trees.  Mammoths, mastodons, and ground sloths littered the forest floor with tree branches and killed trees by eating the cambium under the bark.  During droughts, this litter dried into tinder ready to explode when struck by lightning.  I hypothesize that windstorms and tornadoes were common in the south during much of the Pleistocene.  Frigid katabatic winds blowing off the Laurentide Glacier met warm tropical fronts from the Gulf of Mexico and the Atlantic Ocean in the atmosphere directly over the southeast.  Because cold air sinks, when cold fronts hit warm fronts, great downdrafts of cold air microbursts snapped trees in half and leveled forests.  All this deadwood added fuel to fires as well.  Pollen records show grass was abundant prior to human colonization of the southeast–evidence that natural fires and other disturbances must have been common, though lower CO2 levels in the atmosphere, drier climate phases, and megafauna foraging and trampling contributed to the expansion of grasslands as well.

Following the extinction of the megafauna, forests grew densely for a few thousand years but Indians changed this situation.  Indians began burning the woods to improve habitat for game, and to eliminate shrubby growth that might provide refuge for ticks, stinging insects, large predators, and hostile human warriors interested in carrying off attractive women.  Burning the woods created favorable habitat for bison, deer, and turkey–staples of the Indian diet.  Most fires would not kill healthy mature trees but would eliminate saplings and unwanted brush.  (See the appendix below for tree species fire tolerance.)  Fire created open woodlands which were pleasing to the eye.  Open woodlands allowed more sunlight to reach ground level and in combination with fire sparked the growth of nutritious grasses, flowering herbs, and berry bushes.  Much of the southern uplands consisted of open pine savannah in the coastal plain, and pyrophitic oak forests in the piedmont.  Canebrakes that stretched for miles grew along the piedmont riversides and creeks.  C.C. Frost reconstructed a map of the natural environments on the Savannah River Site based on descriptions of original land acquisition surveys.  This probably is a good representation of a typical section of southeastern coastal plain.

Presettlement vegetation map of the Savannah River Site  reconstructed by C.C. Frost and published in the below referenced book.  Longleaf pine savannahs made up 80% of the landscape.  Bottomland hardwoods, pyrophitic oak forests, longleaf pine-turkey oak sandhills, canebrakes, and Carolina bay wetlands were also common.  The area also includes a bluff forest with northern species of plants but there was less than 300 acres of this.

SRS lies within the coastal plain so longleaf pine savannah dominated the region until 1820.  Note all the yellow in the above map which represents pine savannah.  The pink represents canebrakes, now a completely extinct ecotone on the site, having been replaced by bottomland forests.  Pyrophitic oak forest (the brown) were well represented as well.  Early European settlers copied the burning practices of the Indians, but about 1820 an improved version of the cotton gin was invented, making cotton farming highly profitable.  This is when the burning stopped, and much of the natural forest was converted to farmland.

Vegetation map (also from the below referenced book) of the SRS in 1952 after the government purchased the land.  The federal government replanted much of the land in slash and loblolly pine because longleaf pine was hard to obtain, the trees grew more slowly, and seedling quality was poor.

By 1952 the remaining forests were of poor quality.  Fire suppression ruined the upland forests, and logging the best trees degraded the bottomland forests.  Most of the forests had been replaced by horizon-to-horizon cotton farming.  Note on the map that the cleared land closely corresponds to the former range of the pine savannah.  Note also how canebrakes completely vanished, replaced by bottomland forest.  Canebrakes required specific flooding and firing regimes to persist.  Bamboo cane colonized areas near rivers where the trees were either destroyed by prolonged floods or by the nesting of enormous flocks of passenger pigeons which would kill the trees via overfertilization as their dung accumulated beneath their nests.  Bamboo cane continued to dominate only if the canebrakes were burned about once every 10 years.  Fires more frequent than that created grassland; fires of lesser frequency than that led to the growth of bottomland hardwood forests which shaded out the cane.  Canebrakes provided lots of fodder for bison, denning sites for large carnivores, and habitat for many small species from swamp rabbits to rare types of butterflies.  The burning of canebrakes took place in winter, and it sounded like combat because the hollow stems exploded upon ignition.

Canebrakes were formerly very common in the piedmont.  In 1773William Bartram travelled through central Georgia on his way to the Gulf Coast.  His party traversed on high, dry, gravely ridges but were always in sight of extensive cane meadows which flourished at the bottom of the hills alongside creeks.  On the ridges themselves pyrophitic oaks forests grew.  He found plenty of fire adapted plants growing between the oak trees including goldenrod, asters, rosinweeds, cone flowers, milkweed, false aloe, and spurges.  Bartram noted that Indians set fires annually, and normally clear rivers turned black with ash.

All southern pines are fire tolerant and adapted to survive in environments with frequent fires.  Most oak species also are fire tolerant, though to a lesser degree than pines.  However, longleaf pine is adapted to fires as frequent as 1-4 years, a higher fire frequency than most oaks and pines can survive in the long term.  The long pine needles form a sheath that protects the main stem of a sprout.  Mature loblolly pines, shortleaf pines, and many oak species survive light to medium fires, but their saplings can not.  Longleaf pine saplings can survive fire, explaining why longleaf pine savannahs became the dominant ecotone on the coastal plain.  Fires were less frequent  in the piedmont and mountains because rugged terrain and myriad creeks formed natural firebreaks.

Reference:

Kilgo, John; and John Blake

Ecology and Management of a Forested Landscape: Fifty years on the Savannah River Site

Island Press 2006

Appendix I: Ranking fire tolerance among species common in the southeast

Longleaf Pine

Loblolly and Shortleaf Pine

Black oak

White oak

Red oak

Appendix II: Best fire regimes

Grasslands–annually

Longleaf pine Savannah–1-4 years

Canebrake–10 years

Oak forest–10-30 years (sources differ wildly on this)