SYCAMORES AND HORSE POISONING

Sycamore (Acer pseudoplatanus) has long had a bad press, but its reputation has sunk to a new low as researchers focus on it as the prime suspect in increasing cases of a fatal disease of horses – Atypical Myopathy. In 2013, the UK Equine Veterinary Journal published a study linking the disease with a toxin, Hypoglycin A, that is found in Sycamore seeds and was also present in all the affected horses. This followed another study linking Seasonal Pasture Myopathy (the American equivalent) with the same toxin found in a close relative, the Box Elder – or Ash-leafed Maple (Acer negundo). The story eventually reached the national press and local authority tree officers soon reported a surge of applications to fell protected Sycamores – those with Tree Preservation Orders or those within Conservation Areas. This presented something of a dilemma, not just to the tree officers, but also to the likes of us, Dorset based Tree Consultants, surrounded by horse country.

So is felling the trees the best or only solution, and if so, should we care? There is no doubt that many regard the tree as a pernicious weed. It is a prolific seeder that thrives in almost all conditions, causing gardeners to bemoan the efforts expended in digging up the seedlings. Aphids gorge on the sap, causing honey-dew to rain down on our car windscreens, despoiling paintwork and making footpaths slippery. Conservationists see it as an foreign invader of our native habitats - shading out the native species.

This enmity towards Sycamore is not new: in 1664 John Evelyn complained “Sycomor...is much more in reputation for its shade than it deserves; for the honey-dew leaves, which fall early (like those of the ash) turn to mucilage and noxious insects, and putrifie with the first moisture of the season; so as they contaminate and mar our walks; and are therefore by my consent, to be banish’d from all curious gardens and avenues.” (The mucilage referred to later famously became ‘the wrong sort of leaves’ that caused British Rail such a head-ache). Even Oliver Rackham warned in 1986 that ‘it is a tree that no responsible person should plant without carefully considering the long-term consequences’.

With such a troublesome Jonny-foreigner tree now killing our horses, should we have any qualms about starting up the chainsaws? It was once thought that Sycamores were introduced by the Romans*, and as Monty Python once asked, what have the Romans ever done for us (apart from roads, aquaducts, sanitation, medicine, irrigation etc.) ? Well, Sycamores can in fact grow into fine trees. They can reach 40m in height with trunk diameters of over 2m. They have made very important landscape trees, particularly in the north and Scotland. There are some famous examples, the Martyrs’ Tree at Tolpuddle here in Dorset being one.  Sycamores are tolerant of pollution and salt spray, and are very useful as a first line of defence against sea winds. Despite their initial sliminess, the leaves decay very quickly and boost earthworm populations. The aphid populations they attract are important to airborne insect feeders such as house martins. And now that Ash is threatened by Ash Dieback Disease, Sycamore has been said to be the most likely alternative host to certain species of lichen that rely on Ash. There can be no doubt that wholesale removal of Sycamores in proximity to horses would be regrettable – but is there an alternative?

Atypical Myopathy is a horrendous disease. Its onset is rapid and it is reportedly fatal in 70% of cases. The disease is on the increase across Europe - the Telegraph reporting a predicted increase to 100 cases in the UK in 2014.  But hang on – Sycamores are everywhere and have been for centuries - so have horses. So why were only 100 cases expected? And why is it increasing now? These questions have yet to be answered but, while research continues, there are other factors that appear common to most cases. Certainly, in all instances, the horses have grazed near Sycamores and over 90% of have occurred in the autumn when the seeds were falling, but importantly, the vast majority were kept on sparse pastures with an accumulation of dead leaves, dead wood and often not fed any supplementary hay or feed. It is a lack of supplementary feed and licks when grazing is poor that seems to tempt horses into eating the seeds. Professor Celia Marr, Editor of Equine Veterinary Journal advises "Where horses are grazing in the vicinity of sycamore trees, it is imperative that they are provided with sufficient supplementary feed as this will minimize the risk that horses might be tempted to ingest seeds containing this toxin.” It seems there is little risk to well fed horses.

Of course horse owners will understandably have concerns if their paddocks are near Sycamores, and they may well wish to fell the trees for peace of mind. But remember, Sycamore seeds - or ‘helicopters’ - are aerodynamically designed to travel great distances (there are records of Sycamore seeds traveling 4km) and it may be that you cannot eliminate the risk. Without doubt, felling will frequently be the favoured option without any real harm to our landscape (let's face it, they are prolific and great efforts are already made by individuals and organisations to keep it at bay), but before felling the better specimens it is perhaps worth giving some thought to the actual risk and how it might otherwise be avoided. Horses might actually appreciate the shade.
 
 
 
* Some current thinking suggests Sycamore arrived with Celts

Bits & bobs gleaned from:
Bean, W J: Trees and Shrubs Hardy in the British Isles
bhsscotland.org.uk website: Advice on Atypical Myopthy
Equimed.com website: Seeds of Sycamore Likely Cause of Atypical Myopathy (2013)
Mabey, R: Flora Britannica
Monty Python: Life of Brian
More D & White J: Cassel’s Trees of Britain and Northern Europe
Rackham, O: The History of the Countryside
The Telegraph website: Sycamore seeds warning to horse owners after death (2014)
Thomas, P: Trees: Their Natural history