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)

 

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