Dr Terry Mabbett reflects on the history of sweet chestnut – from ancient Greece to the Roman Empire, and its establishment in the UK, to the host of pests and diseases that threaten the species today.
CASTANEA sativa is commonly called sweet chestnut to distinguish its edible nuts from the potentially poisonous ones of Aesculus hippocastanum (white flowering horse chestnut). Castanea sativa was certainly a sweet experience for UK foresters and woodsmen growing stools and coppicing poles, but turned increasingly sour after the Second World War when cleft chestnut fencing fell out of fashion.
Over the centuries, sweet chestnut had found a niche as coppice poles for fencing because young wood cleaves so well along the straight grain of 15 to 20-year-old poles. But UK-grown sweet chestnut had become a one-trick pony with nowhere to go, leaving thousands of hectares of chestnut coppice languishing across southern England. Stools and stems continued to grow but were no longer managed or cut on a strict rotation cycle, leaving landowners lumbered with increasingly over-stood coppice.
Some relief arrived during last decade of the 20th century with a revival in traditional use and as biomass chip for boilers. But it came too late, for this domiciled, naturalised tree was soon to be confronted by exotic pests and pathogens with the capacity for terminal damage.
HISTORY OF SWEET CHESTNUT
Long-held belief is that Castanea sativa was introduced by the Romans during the 400+ year period from Julius Caesar’s first invasion of Britain (43 AD) until the first decade of the fifth century when the Roman administration ended. However, a team of researchers from the University of Gloucestershire and Historic England recently claimed to have knocked the stuffing out of this particular chestnut theory.
They say archaeological excavations failed to find any evidence that sweet chestnut was actually grown in Britain during the Roman period. The single record linked with Roman Britain was peelings from approximately five sweet chestnuts at the excavation of a Roman farmstead in Essex. These were discovered in a deposit of food waste which also included olives, stone pine nuts and Mediterranean fish bones, therefore suggesting they may have been among exotic foods imported from continental Europe.
So, what is documented about the ancient history of sweet chestnut and how and when it most likely arrived to be cultivated in the British Isles?
By end of the last ice age, Castanea sativa was restricted to refuges along the southern coastline of the Black Sea, on the southern slopes of the Caucasus and into north-west Syria.
Evidence exists for the cultivation of chestnut by at least 2000 BC with the tree widely discussed in Ancient Greek literature. According to Pliny the Elder (famous Roman naturalist and author, 23–79 AD), the Greeks obtained chestnut from Asia Minor as early as 5000 BC. However, more recent scholars like Swiss botanist Augustin Pyramus de Candolle (1778–1841) say it was undoubtedly wild in ancient Greece, with Theophrastus (371–287 BC), ‘the father of botany’, and Aristotle’s successor in the Peripatetic School, speaking about sweet chestnut covering the slopes of Mount Olympus.
Ancient Rome is credited with spreading the species throughout southern and central Europe. The nuts were highly prized for making a high-starch, low-fat flour for rations to feed and fuel the Roman Empire’s legions.
Given that the sweet chestnut remains were found at a Roman settlement in Britain, this strongly suggests it was they who introduced sweet chestnuts. It’s hard to believe that at least one enterprising Roman did not plant the seeds in British soil, but whether the tree was planted and grown on significant scale during the Roman occupation is clearly another matter.
However, we do know that trees were being grown in Britain in the 16th and 17th centuries because today’s landscape is littered with ancient sweet chestnut trees, having their roots in these Tudor and Stuart times. Under the right conditions sweet chestnut trees are exceptionally long-lived. The oldest known tree, growing on Mount Etna, in Sicily has a circumference of 190 feet and is estimated to be between 2,000 and 4,000 years old.
Professor G.S. Boulger (1886) refers to the Tortworth chestnut in Lord Ducie’s Park at Tortworth Court in Gloucestershire, as portrayed in Jacob George Strutt’s Sylva Britannica (1826), and possessing a recorded circumference of 50 feet in 1766 to indicate an age of at least 600 years. What’s more, ‘father of English forestry’ John Evelyn referred to the tree in his Sylva (1666) and said how during the reign of King Stephen (1135–1154) the tree already bore the title ‘Great Chestnut of Tortworth’, suggesting that even then it was already centuries old.
CONTEMPORARY SWEET CHESTNUT
So much for sweet chestnut trees with their roots in the 12th and 13th centuries, but what about the contemporary condition, status and future for sweet chestnut in Britain? In 2010, the British and Irish Hardwood Trust (BIHT) gave a ball park figure of 18,000 hectares for all sweet chestnut woodland in Britain and there is no reason to suspect the figure has changed much since then.
Over 90 per cent is in England, with two thirds concentrated in the South East and partitioned almost equally between coppice and coppice with standards. The counties of Kent and East Sussex have traditionally been the centre of sweet chestnut cultivation, with additional significant amounts in West Sussex and Surrey. However, two East Anglian counties (Essex and Suffolk) have a lot more than most people suspect, although the coppice there is generally well over-stood and much more so than in compartments south of the River Thames.
The last decade of the 20th century saw the species poised to take off with a renewed interest and renaissance in chestnut coppice, but it was subsequently hit by two new exotic plant pathogens and an alien insect pest.
NEW THREATS FROM PESTS AND PATHOGENS
First to strike was Phytophthora ramorum, originally dubbed sudden oak death because of what the fungus-like pathogen was doing to native North American oaks in California and Oregon. The first mature tree in the UK found infected with P. ramorum was a 100-year-old native North American red oak (Southern red oak – Quercus falcata) in Sussex, in November 2003.
Native English white oaks (Quercus robur and Quercus petraea) were spared but other members of the plant family Fagaceae including common beech (Fagus sylvatica) and sweet chestnut were not, although only trees exposed to heavy inoculum pressures from other sporulation hosts such as Rhododendron ponticum were infected. Sweet chestnut itself turned out to be a sporulation host for Phytophthora ramorum.
Phytophthora ramorum infection of Japanese larch in western England in 2009 and rapid destruction of this premium plantation conifer pushed P. ramorum’s infection of sweet chestnut off the radar. However, in 2015 a cluster of new outbreaks occurred in western England, mainly in Devon but also in Cornwall and Wiltshire. And this time they were isolated from the immediate presence of other sporulating hosts, therefore suggesting long-distance travel of P. ramorum spores.
SWEET CHESTNUT BLIGHTED
In 2011, the Forestry Commission announced how chestnut blight caused by the fungus Cryphonectria parasitica, regarded as the world’s worst disease of the Castanea genus, had been found at a Warwickshire farm on trees planted in 2007 using planting material sourced from a nursery in France. A following trace-forward exercise found the disease at sites across the country on planted trees sourced from the same French nursery. However, final destinations of many consignments imported from the same infected nursery in France could not be traced because the paperwork was either lost or never existed.
Since then, new outbreaks of disease have been reported with regularity. In 2019, the Forestry Commission finally published a distribution map showing SPHNs (Statutory Plant Health Notices) issued between 2011 and 2019 within 28 10-kilometre grid squares. All but one of the affected grid squares is entirely within England, the exception being one which stretches across the border from Herefordshire into Wales. None are located further north than Cheshire.
The number of 10-kilometre grid squares currently recording outbreaks now stands at 35. The actual number of outbreaks is many more than the number of affected grid squares. By 2018, the Forestry Commission had already identified 37 outbreaks associated with trees imported and planted prior to 2014.
The Forestry Commission continues to keep its cards very close to its chest on chestnut blight, especially around location and other facets including symptoms and damage, stage of disease development and spread. Most outbreaks, including at least one each in Norfolk, Cheshire, East Sussex and East Kent, have never been flagged up before and, even if announced, the location of an outbreak is very loosely described. As a consequence, virtually nothing is documented in detail on exactly how this disease is currently affecting sweet chestnut in the UK, thus making it difficult for landowners and foresters to make reasoned judgements about the future of sweet chestnut as a commercial hardwood species.
Suspicions remain that most new findings are related to consignments of diseased trees imported from a single infected nursery in France beginning in 2007 but not identified during the trace-forward exercise. This strongly suggests the disease is generally not spreading away from sites where infected trees were planted. Despite climate warming, UK-grown sweet chestnut remains outside its climate comfort zone, but so apparently does the blight pathogen which appears to be working in our favour. That no outbreaks of disease have been recorded further north than Cheshire would tend to support this contention.
In 2019, the re-emergence of chestnut blight was recorded on sites from which the disease was previously thought to have been eradicated.
ORIENTAL CHESTNUT GALL WASP
In summer 2015, a notifiable and quarantine insect pest of sweet chestnut was found in Britain for the first time. The Forestry Commission took action to eradicate oriental chestnut gall wasp – OCGW (Dryocosmus kuriphilus) – from chestnut coppice woodland in North Kent and on street trees in St Albans in Hertfordshire, but multiple findings were subsequently recorded in London and out into the Home Counties in the following year. At the time, the Forestry Commission said: “Now that OCGW has been found in other locations in 2016, it is recognised that eradication and containment are no longer achievable, and we should focus on developing a long-term strategy for minimising population, spread and impact.”
OCGW has more recently been found as far afield as Oxford, in West Berkshire, as a sizeable cluster in Hampshire, and now close to the coast in East Sussex.
According to the Forest Research website (18 July 2020), OCGW is a quarantine pest, giving national plant health authorities powers to take measures to contain or eradicate it. The UK has European Union Protected Zone status for this pest, which means that:
We have a legal status as being free from Oriental chestnut gall wasp.
Sweet chestnut plant imports must be certified as coming from an area declared to be free of OCGW.
The plant health authorities must do annual surveys to check that the UK remains free of OCGW.
It goes on to say that OCGW is ‘notifiable’ and that “If you think you have seen evidence, such as galls, of OCGW presence on sweet chestnut trees, you must report it immediately”.
WINDOW OF OPPORTUNITY
There was a potential boost for sweet chestnut in 2012 with appearance of Chalara ash dieback and a subsequent ban on the movement of common ash, which effectively stopped any planting of this native broadleaf tree. Forest nurseries promptly destroyed common ash stocks, amounting to hundreds of thousands of trees and without any compensation. The Forestry Commission recommended sweet chestnut as one of the alternatives to common ash for woodland planting and there was a predictable initial rush on sweet chestnut planting material.
Forest nurseries reacted to Forestry Commission recommendations by planting more sweet chestnut in spring 2013. However, it takes three years or more for seedlings to be at the saleable stage and by 2016 it was obvious demand would not reach anticipated levels, leaving forest nurseries with surplus, over-grown sweet chestnut trees suitable only for the bonfire. Whether or not the appearance of chestnut blight and OCGW had dampened demand is unclear, but chestnut blight subsequently made its mark, albeit indirectly, by contributing to a dearth of seed for planting.
Summer/autumn 2015 saw UK provenances and locations traditionally used for sweet chestnut seed collection and planting fail to produce seed due to drought. Coupled with issues around sweet chestnut blight restricting the import of traditional sources of seed from EU countries like France, Spain, and Italy, there was no commercial supply of sweet chestnut seed for planting in 2016.
Under UK government legislation already in place, sweet chestnut seed for planting could only be imported from the EU if grown in Republic of Ireland, Sweden, and several of the Greek islands. Neither the Republic of Ireland nor Sweden can supply sweet chestnut seed, while the Greek Islands are considered too far south to be considered as a source of seed. The net result was no UK seed harvested in autumn 2015 for planting in spring 2016 and no imports either.
CONTEMPORARY SWEET CHESTNUT
UK foresters can’t afford to lose sweet chestnut. Apart from traditional sweet chestnut coppice growers in Kent and East Sussex, many woodland owners and foresters farther afield in the south and east of England make good use of sweet chestnut coppice.
Southern England including East Anglia has seen conifer planting almost completely dry up over the last three decades, with no indication that the Forestry Commission has any other intention but to turn this whole region of England into an conifer-free zone. 30 years of conifer plantation thinnings have already been lost which is one good reason why sweet chestnut, with its continual and rapid regeneration of coppice growth, is vital to the dwindling numbers of foresters across these swathes of southern England.
However, nature has apparently not finished with sweet chestnut just yet. A resurgence of ink disease caused by Phytophthora cambivora and Phytophthora cinnamomi looks set to become a new blot on the landscape. These two exotic fungus-like pathogens were recorded and documented as causing disease in sweet chestnut as early as the 1930s. Scientists of the day commented how the disease could easily get out of hand if the UK climate warmed, and perhaps this is the reason why there is a current upsurge in reports of this potentially fatal disease affecting roots and collar and with the capacity to kill mature sweet chestnut trees.
REFERENCES:
Boulger, G.S. (1886) Some Familiar Trees (First Series), Cassell and Company Limited
Day, W.R. (1938) ‘Root-rot of sweet chestnut and beech caused by spread of Phytophthora. Cause and symptoms of disease: its relation to soil conditions’, Forestry 12(2): 101–116
Forest Research (2020a) ‘Sweet chestnut blight – Cryphonectria parasitica’ https://www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/sweet-chestnut-blight-cryphonectria-parasitica/.
Forest Research (2020b) ‘Oriental chestnut gall wasp (Dryocosmus kuriphilus)’ https://www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/oriental-chestnut-gall-wasp-dryocosmus-kuriphilus/.
Jarman, R., Moir, A.K., Webb, J., Chambers, F.M. (2019) ‘Sweet chestnut (Castanea sativa Mill.) in Britain: its dendrochronological potential’, Arboricultural Journal July 2017. DOI: 10.1080/03071375.2017.1339478.
Mabbett, T.H. (2016) ‘Sweet chestnut to the fore and in the firing line’, Woodland Heritage 2016.
Mabbett, T.H. (2017) ‘Sweet chestnut overtaken by events’, Forestry Journal August 2017.
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