Posts Tagged ‘American Chestnut x Chinese Chestnut hybrids’

Pleistocene Chestnut Woodlands

August 25, 2013

The chestnut (Castanea dentata) was the most valuable tree of eastern North America’s ecosystem.  From north Georgia to central New York it composed up to 25% of the forest.  It provided a heavy annual crop of nuts eaten by every animal from mice to bison.  The chestnut tree has a tendency to become hollow, making it an important den tree as well.  The chestnut tree equaled food and shelter for wildlife.  The spring flowers attracted untold numbers of insect pollinators, and modern studies show the presence of chestnut trees increases the fertility of sandy loam soils.  Chestnut trees were found on dry rocky ridges and moist slopes.  William Bartram, heading north through Georgia  during his travels just before the American Revolution, began encountering chestnut trees in the upper piedmont where he found them growing on rocky hilltops associated with chinkapins and chinkapin oaks.  The chinkapin is a shrubby relative of the chestnut tree.  Chestnut trees grew 100 feet tall with diameters of 5 feet or more as the below photos indicate.

Stand of chestnut trees dating to sometime in the 19th century.  Incredible!  I’ve never seen a forest with trees this big.  What hath man ruined?

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Chestnut trees frequently became hollow.  Potential home for bear, giant ground sloth, peccary, bats or human.

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Chestnut leaves.  The scientific name Castanea dentata means toothy leaf.

Disaster struck in 1904 when a fungus (Cyphonectric parasitica), accidentally introduced on imported Chinese chestnut trees, spread throughout North America.  Lumbermen began clear-cutting chestnut trees, ostensibly to stop the spread of the blight for which American chestnuts had no resistance.  Unfortunately, this misguided policy eliminated many chestnuts that may have been resistant to the blight.  The once dominant American chestnut was eliminated from its range, its place in the canopy taken by oak, maple, and other less productive trees that don’t support as much wildlife.  The blight doesn’t attack the roots of chestnuts, so remaining chestnut stumps do sprout, but then die back, usually before they produce a crop of nuts.  Ecologists claim that wildlife has recovered since the chestnut tree die off, but this claim defies common sense.  The eastern forest is undoubtedly more impoverished without it.

Man is trying to undo this ecological calamity.  Horticulturalists have successfully developed resistant strains of American chestnuts by backcrossing them with Chinese chestnuts and selecting hybrids that are resistant to the blight.  Since 2006, they have planted thousands of hybrids that are 15/16 American chestnut x 1/16 Chinese chestnut in secret locations in North Carolina, Tennessee, and Virginia.  These locations are on  national forest land where they will be protected from timber operations.  American chestnuts are reportedly a more attractive tree and produce sweeter nuts than Chinese chestnuts.

Some rare individual American chestnuts have been found that are apparently resistant to the blight.  There is a 30 year old American chestnut in Warm Springs, Georgia; a live 85 foot tree in Talladega, Alabama; and a few in Ohio, Kentucky, New Hampshire, and Missouri.  The best known live grove  of American chestnuts is in West Salem, Wisconsin.  In 1885 a farmer planted 9 American chestnut trees here, and they increased to 2500 trees where they grow with white oak, red oak, northern pin oak, hickory, birch, basswood, black cherry, and big-toothed aspen in a remarkable forest.  They survived the initial blight attack because they were planted over 250 miles from the chestnut’s original range.  The distance kept them isolated until 1987 when the blight finally found them, but by this time scientists learned how to defeat the blight by using a slow acting virus that kills the fungus.  Chestnuts may eventually become an important eastern tree again.  They produce nuts in 7 years and outproduce oaks.  But this will take centuries.

Presettlement range of the American chestnut.  It grew as far south as Florida at various times during the Pleistocene.  Note the disjunct populations in southwestern Georgia, southeastern Alabama, and Missisippi.  These were relic populations from when this species grew throughout the coastal plain to Florida.  Chestnut trees were evidentally common in north Florida during the mid-Wisconsinian interstadial, but disappeared there during the Last Glacial Maximum when climatic conditions deteriorated.

Location of human transplanted chestnut trees that avoided the blight by being 250 miles outside the natural range of the species.  It’s the only place in the world where a person can currently see a nice mature forest dominated by American chestnut.

The chestnut tree has an interesting biogeographical history.  It’s a fairly primitive angiosperm, and an extinct species (Castanea ungeri) is known from as early as the Eocene 50 million years ago.  Fossils of Castanea ungeri  have been found on Greenland, showing how much warmer climate was then.  Some scientists speculate a species of chestnut may have even occurred during the time of the dinosaurs.  Pollen studies show that chestnut was a common tree in north Florida between 40,000 BP-31,000 BP (See:  This time period is part of what is known as Marine Isotope Stage 3 (which includes the time period ~60,000 BP-~30,000 BP), and it is also referred to as the mid-Wisconsinian Interstadial.  Climate fluctuated greatly during MIS-3 with broad-leafed trees increasing at the expense of pine during warm wet stages and vice versa during cold arid stages.  MIS-3 was not as cold and arid as the Ice Age, but summers were generally much cooler than those of today, perhaps explaining why chestnuts grew in the southeastern coastal plain then but didn’t at the time of European settlement.  Evidence of chestnut trees  disappears from Florida’s pollen record about 29,000 BP when climate became cooler and drier with the onset of the most severe era of the Ice Age.  The diverse forest of MIS-3 was replaced with more monotonous pine and oak woodlands and an increase in grasslands.  The chestnut woodlands of MIS-3 that grew in north Florida also consisted of 40% oak.  Open woodlands consisting of chestnut and oak likely existed from north Florida throughout the rest of the south during the mid-Wisconsinian Interstadial.  I suspect chestnuts were more common in southwestern Georgia than in the southeastern part of the state, based on the pre-settlement range map.  There were relic populations of chestnut in the southwestern part of the state but not in the southeastern region.  A pollen study from sediment off the coast of Georgia found that chestnut only made up about 2% of the pollen in coastal Georgia.  The Gulf Stream kept the southeastern part of the state a little warmer, and open pine savannahs were likely more prevalent there. Chestnut trees are fire adapted, but not as fire adapted as longleaf pine.  Longleaf pine can survive annual fires that would kill chestnut saplings, but mature chestnut trees are fire resistant.  More oceanic-induced lightning storms caused more frequent fires that may have shifted the balance in this region to favor more longleaf pine savannah over chestnut and oak woodland during the mid-Wisconsinian Interstadial.  Longleaf pine savannahs grow best with fire intervals of 3 years, while chestnut-oak woodland do better with fire intervals of about 20 years.

It would have been marvelous for a naturalist to travel through Georgia during the mid-Wisconsinian Interstadial.  There were probably a mosaic of varied habitats, and a person could have wandered from oak-chestnut woodlands to open pine savannahs in less than a day’s journey on foot.  The megafauna congregated in the chestnut-oak woodlands during the fall to eat the nuts but moved to the savannahs in spring and summer to forage upon the bounty there.  I think it would be better than a trip to modern day Africa.