American chestnut
The American chestnut is a large, fast-growing deciduous tree of the beech family native to eastern North America. As is true of all species in the genus Castanea, the American chestnut produces burred fruit with edible nuts. The American chestnut was once common in the Appalachian Mountain range and was a dominant species in the oak-chestnut forest region of its central and southern range.
During the early to mid-20th century, American chestnut trees were devastated by chestnut blight, a fungal disease that came from Japanese chestnut trees that were introduced into North America from Japan. It is estimated that the blight killed between three and four billion American chestnut trees in the first half of the 20th century, beginning in 1904. Four mature American chestnuts exist within its former range, although many stumps and root systems continue to send up saplings. Most of these saplings get infected by chestnut blight, which girdles and kills them before they attain maturity. There are hundreds of large American chestnuts outside its historical range, some in areas where less virulent strains of the pathogen are more common, such as the 600 to 800 large trees in Northern Michigan. The species is listed as endangered in Canada under the Species at Risk Act. American chestnuts are also susceptible to ink disease, particularly in the southern part of its native range; this likely contributed to the devastation of the species.
Several groups are attempting to create blight-resistant American chestnuts. Scientists at the SUNY College of Environmental Science and Forestry created the Darling 58 cultivar by inserting the oxalate oxidase gene from wheat into the genome of an American chestnut. When expressed in the vascular cambium of the Darling 58 cultivar, the oxalate oxidase enzyme degrades the oxalic acid produced by the chestnut blight, reducing damage to the vascular cambium and resisting girdling of the trunk. As of 2021, the researchers who developed this cultivar are working toward applying for government permission to make these trees available to the public. If approved, these chestnut trees would be the first genetically modified forest trees released into the wild in the United States. Alternate approaches to developing a blight-resistant cultivar include cross-breeding among partially blight-resistant American chestnuts or crossbreeding with the moderately blight-resistant Chinese chestnut, then backcrossing with the American chestnut, with the goal of retaining most of its genes.
Description
Castanea dentata is a large, rapidly-growing, deciduous hardwood eudicot tree. A singular specimen manifest in Maine has attained a height of. Pre-blight sources give a maximum height of and a maximum circumference of. Post-blight sources erroneously report a greater maximum size of the species compared with pre-blight, likely the result of nostalgia, interpretations of pre-blight measurements of circumference as being measurements of diameter, and the misapprehension that pre-blight observations of maximum size represented observations of average size. It is considerably larger than the closely related Allegheny chinquapin.There are several other chestnut species, such as the European sweet chestnut, Chinese chestnut, and Japanese chestnut. Castanea dentata can be distinguished by a few morphological traits, such as petiole length, nut size and number of nuts per burr, leaf shape, and leaf size, with leaves being long and broad—slightly shorter and broader than the sweet chestnut. It has larger and more widely spaced saw-teeth on the edges of its leaves, as indicated by the scientific name dentata, Latin for "toothed".
The European sweet chestnut was introduced in the United States by Thomas Jefferson in 1773. The European sweet chestnut has hairy twig tips in contrast to the hairless twigs of the American chestnut. This species has been the chief source of commercial chestnuts in the United States. Japanese chestnut was inadvertently introduced into the United States by Thomas Hogg in 1876 and planted on the property of S. B. Parsons in Flushing, New York. The Japanese chestnut has narrow leaves, smaller than either American chestnut or sweet chestnut, with small, sharply-pointed teeth and many hairs on the underside of the leaf and is the most blight-resistant species.
The chestnut is monoecious, and usually protandrous producing many small, pale green male flowers found tightly occurring along 6 to 8 inch long catkins. The female parts are found near the base of the catkins and appear in late spring to early summer. Like all members of the family Fagaceae, American chestnut is self-incompatible and requires two trees for pollination, which can be with other members of the Castanea genus. The pollen is considered a mild allergen.
The American chestnut is a prolific bearer of nuts, with inflorescence and nut production in the wild beginning when a tree is 8 to 10 years old. Burrs often open while still attached to the tree, around the time of the first frost in autumn, with the nuts then falling to the ground. American chestnut typically have three nuts enclosed in a spiny, green burr, each lined in a tan velvet. In contrast, the Allegheny chinquapin produces one nut per burr.
Evolution and ecology
Chestnuts are in the Fagaceae family along with beech and oak. Chestnuts are not closely related to the horse chestnut, which is in the family Sapindaceae. Phylogenetic analysis indicates a westward migration of extant Castanea species from Asia to Europe to North America, with the American chestnut more closely related to the Allegheny chinquapin than to European or Asian clades. The genomic range of chestnuts can be roughly divided into a clinal pattern of northeast, central, and southwest populations, with southwest populations showing greatest diversity, reflecting an evolutionary bottleneck likely caused by Quaternary glaciation.Two lineages of American chestnut have been identified, one a hybrid between the American chestnut and the Allegheny chinquapin from the southern Appalachians. The other lineage shows a gradual loss of genetic diversity along a northward vector, indicating possible expansion of range following the most recent glacial maximum during the Wisconsin glaciation. Ozark chinkapin, which is typically considered either a distinct species or a subspecies of the Allegheny chinquapin, may be ancestral to both the American chestnut and the Allegheny chinquapin. A natural hybrid of C. dentata and C. pumila has been named Castanea × neglecta.
The American chestnut population was reduced to 1–10% of its original size as a result of the chestnut blight and has not recovered. The surviving trees are "frozen in time" with shoots re-sprouting from survivor rootstock but almost entirely undergoing blight-induced dieback without producing chestnuts. Unexpectedly, American chestnut appears to have retained substantial genetic diversity following the population bottleneck, which is at odds with the limited incidence of blight resistance/tolerance in extant populations.
The pre-blight distribution was restricted to moist, well-drained, steep slopes with acidic loam soils. According to analysis of old forest dust data, the tree was rare or absent in New England prior to 2,500 years before present but rapidly established dominance in these forests, becoming a common tree over a range from Maine and southern Ontario to Mississippi, and from the Atlantic coast to the Appalachian Mountains and the Ohio Valley. Within its range, the American chestnut was the dominant timber of mountain ridges and sandstone soils. Along the Blue Ridge Mountains of North Carolina, it dominated the area above the range of the Eastern hemlock and below 1,500 meters. In Western Maryland, it comprised 50% of ridge timber and 36% of forested slopes.
The tree's abundance was the result of a combination of rapid growth, relative fire resistance, and a large annual nut crop, in comparison to oaks, which do not reliably produce sizable numbers of acorns every year. Historically, the mean fire return interval was 20 years or less in chestnut-predominant ecologies, with a forest stand pattern that was more open than is currently the case.
The American chestnut was an important tree for wildlife, providing much of the fall mast for species such as white-tailed deer, wild turkey, Allegheny woodrat and the passenger pigeon. Black bears were also known to eat the nuts to fatten up for the winter. The tree contains more nitrogen, phosphorus, potassium, and magnesium in its leaves than other trees that share its habitat, so they return more nutrients to the soil which helps with the growth of other plants, animals, and microorganisms. The American chestnut is preferred by some avian seed hoarders and was particularly important as a food source during years where the oak mast failed. The functional extinction of the American chestnut was thought to have resulted in the extinction of the tree's host-specialist insect, the Greater Chestnut Weevil, until the beetle was rediscovered in 2025.
Parasites
The appearance of invasive pathogens of the American Chestnut into the eastern deciduous forest ecosystem is just one instance of the Columbian exchange of pathogens. While the Columbian exchange moved valuable crops between the Americas, Europe and Asia, there was also a downside, as the rapid introduction of invasive and unfamiliar pathogens resulted in serious damage or extinction of some host species.Chestnut blight
Prior to the chestnut blight, the American chestnut was a dominant tree in the ecosystem of the eastern deciduous forest. It was said that a squirrel could walk from New England to Georgia solely on the branches of American chestnuts. Once an important hardwood timber tree, the American chestnut suffered a catastrophic population collapse due to the chestnut blight, a vascular disease caused by an Asian bark fungus. The fungus was introduced when infected Japanese chestnut trees were brought to North America in the late 19th century.Chestnut blight was first noticed on American chestnut trees in what was then the New York Zoological Park, now known as the Bronx Zoo, in the borough of The Bronx, New York City, in 1904, by chief forester Hermann Merkel. Merkel estimated that by 1906 blight had infected 98 percent of the chestnut trees in the borough. While Asian chestnut species evolved with the blight and developed a strong resistance, the American chestnut and Allegheny chinquapin have little resistance.
The airborne bark fungus spread per year and in a few decades girdled and killed more than three billion American chestnut trees. Salvage logging during the early years of the blight may have unwittingly destroyed trees that had high levels of resistance to the disease and thus aggravated the calamity. New shoots often sprout from the roots when the main stem dies, so the species has not yet become extinct. However, the stump sprouts rarely reach more than in height before blight infection returns, so the species is classified as functionally extinct since the chestnut blight only actively kills the above ground portion of the American chestnut tree, leaving behind the below-ground components such as the root systems.
In the 1900s, it was recorded that the chestnut blight would commonly reinfect any novel stems that grew from the stumps, therefore maintaining a cycle that would prevent the American chestnut tree from re-establishing. However, some American chestnut trees have survived because of a small natural resistance to the chestnut blight.
The high density of American chestnuts within its range and the lack of natural immunity allowed the blight to spread quickly and cause infection and die-off in nearly every tree exposed. Early attempts to treat chestnut blight were both chemical, such as the use of fungicides, and physical, such as removing infected limbs through tree surgery and the removal of infected trees from cultivated and wild stands. Quarantine measures were also put into place, with the later support of the Plant Quarantine Act, which was an attempt to prevent the importation of other potential plant pathogens.
These attempts to contain the spread of chestnut blight were unsuccessful. The devastation of the species was worsened because the chestnut blight resulted in isolation of remaining specimens, resulting in asexual propagation of many isolated American chestnuts, low genetic diversity of stands of American chestnuts, and consequent vulnerability to extirpation.
Chestnut blight is not to be confused with sun scald, where winter sun reflects off of snow, warming the bark on the sun-facing trunk. This snow-reflected sunlight repeatedly warms and thaws the trunk during the day, resulting in vulnerability of the bark and cambium to freezing cold temperatures during the subsequent night, eventually resulting in bark cankers that resemble chestnut blight. Also, sun scald makes the damaged bark vulnerable to invasion by pathogens.