Chestnuts featured in #AdventBotany

Last year, Alastair Culham (@BotanyRNG) and Jonathan Mitchley (@Drmgoewild) from the University of Reading teamed up to create a series of blog posts called Advent Botany (advent is the season leading up to Christmas and is traditionally a period of waiting or counting days).  This year Advent Botany 2015 features guest contributions. Today’s post, written by me and  crossposted here, is about chestnuts, one of my favorite seasonal plants.

My December buzzes along to the tune of “Chestnuts roasting on an open fire…,” otherwise known as The Christmas Song; Nat King Cole’s beautiful rendition will get you in the chestnut mood.

Caption: Street vendor roasting chestnuts (USDA).
Street vendor roasting chestnuts (USDA).

Throughout the Northern Hemisphere, freshly roasted chestnuts are the ultimate winter street food. Not only are they delicious but they warm your fingers as well as your belly. Chestnuts have a long association with Christmas and are commonly served in winter feasts (recipes here and here).

There are several species of chestnut found throughout the Northern Hemisphere, all producing edible nuts of differing sizes and qualities. [Note that the abundant horse chestnut trees (conkers) found throughout the UK are in the genus Aesculus and are unrelated to edible chestnuts, and that water chestnuts, Eleocharis dulcis, also are unrelated and not nuts at all.] Historical records show that European sweet chestnuts (Castanea sativa) have been cultivated in the Mediterranean region for more than 3000 years.

European sweet chestnuts Castanae sativa (Fir0002/Flagstaffotos: CC BY-NC).
European sweet chestnuts Castanae sativa (Fir0002/Flagstaffotos: CC BY-NC).

For the past 100 years, Americans have switched their consumption from native American chestnuts (Castanea dentata) to European chestnuts, not because they prefer them but because the American chestnut was essentially wiped out by chestnut blight disease in one of the worst environmental disasters of the 20th century.

1949 map showing the rate of American chestnut’s destruction  (Gravett, 1949).
1949 map showing the rate of American chestnut’s destruction (Gravett, 1949).

Until 1900, American chestnuts were abundant and widely distributed in eastern forests, where they were important sources of timber and food. Chestnuts accounted for about a quarter of the trees in a range that stretched from Maine to northern Mississippi, and they were big; often growing 30 meters or more in height. Towards the end of the 1800s, the fungus Cryphonectria parasitica was introduced into the United States from Asia, probably by a well-intentioned horticulturalist. Unlike its Asian counterparts, the American chestnut is highly susceptible to the chestnut blight fungus.

In just over 50 years approximately three and a half billion trees died: a number equivalent to half of today’s human population. The loss of the American chestnut has profoundly changed the nature of the eastern forests. You can learn more about the history of American chestnut blight, including a 30 minute video, here and here and here. Today all that remains of this once abundant species are a few widely spaced survivors and many root systems that throw up the occasional shoot. Because the fungus persists and the shoots remain susceptible, they rarely survive to reproductive maturity.

Dead American chestnuts, 1943 (USDA Forest Service).
Dead American chestnuts, 1943 (USDA Forest Service).

Other members of the Castenea genus have fared better. The European chestnuts are also susceptible to chestnut blight, although somewhat less so than the American chestnut. Since the 1930s the blight fungus has been found in chestnut-growing regions in Europe, but it has not unleashed the wide scale destruction that it did in America. Recently, chestnut blight was found in European chestnut trees growing in the UK. Chinese chestnuts (Castanea mollissima) and other Asian species are more resistant to blight.

Several efforts are underway to introduce resistance to American chestnuts and reintroduce them into forests. One strategy involves cross-breeding with resistant Chinese chestnuts followed by backcrossing to add back the more desired American growth form (see more at the American Chestnut Foundation). Another approach involves the introduction of a gene, oxalate oxidase from wheat into the American chestnut using a transgenic approach. The fungus produces the organic acid oxalate as part of its infection strategy, and studies have shown that production of an enzyme that breaks down oxalate confers protection from the pathogen. The benefit of this transgenic approach is that the tree retains all of its American characteristics, but it and its progeny are blight resistant. The downside is that the use of transgenic technology means that the trees are subject to extensive regulatory processes as well as residual public resistance. William Powell, a professor of plant pathology at the State University of New York, has led this approach which you can read about here and also see a TedX talk about.

Both strategies are ongoing and show promise, so the next tasks are to demonstrate resistance in the field and to repopulate forests with resistant trees. One participant describes this as a “hundred year project”. But to paraphrase Nat King Cole, American chestnuts are Unforgettable, though near or far.

What lessons can we learn from the American chestnut? First, there is the crucial reminder to adhere to plant quarantine regulations. We are no longer as naïve as our 19th century predecessors and cannot afford to take risks with the health of our forests. Second, the ease with which we accidently destroyed a species in less than a human life span reminds us that our world is fragile. Finally, as reflected in Charles Dickens’ great novel A Christmas Carol, being confronted with mortality, whether of man or tree, reminds us to take a moment to appreciate those connections that give our lives meaning.

Want more Advent Botany? Here’s Day 10, Oryza sativa (as in rice pudding), and Day 12, Paperwhites. See the full list here.

References

Anagnostakis, S.L. (2000). Revitalization of the Majestic Chestnut: Chestnut Blight Disease. APSnet Features.

FAO (2002). Non-wood forest products from temperate broad-leaved trees.

Forestry Commission UK (2015). Sweet chestnut blight (Cryphonectria parasitica).

Gravatt, F. (1949): Chestnut blight in Asia and North America. Unasylva 3:2–7.

Horton, T. (2010). Revival of the American Chestnut. American Forests.

Newhouse, A.E., et al., (2014). Transgenic American chestnuts show enhanced blight resistance and transmit the trait to T1 progeny. Plant Science. 228: 88–97

Powell, W. (2014). The American Chestnut’s Genetic Rebirth. Scientific American, March 2014: 68-73

Rupp, R. (2015). Can we engineer an American chestnut revival? National Geographic, 2 June 2015.

Zimmer, C. (2013). Resurrecting a forest. National Geographic, 11 March 2013.

Image sources: USDA, Fir0002/Flagstaffotos: CC BY-NC, Gravett, 1949, USDA Forest Service

 

 

 

 

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