Sycamore summer symptoms
What's causing the damage?
This summer, surprisingly notable symptoms of poor health among individual sycamore trees have been observed by the team here at The Tree Council, and by others in the tree sector, so we headed out into the field and delved into the literature to investigate this trend.
An emerging trend
A few weeks ago, colleagues first started to notice sycamore trees with what appeared to be single dead branches. The branches were covered in dead, shrivelled, brown leaves on one or several branches, and the damage is currently very visible in sycamores and some acers, standing out starkly against the remaining healthy canopy. These symptoms seemed to be appearing quite suddenly and across numerous trees, in patches of woodland, roadsides and parkland. Over the next days and weeks, further reports came in of similar foliage wilt, from various parts of the UK. So, what could be causing this?
There were a few contenders.
Some fungi and tree diseases such as Ash Dieback or Dutch Elm Disease can impact stems and create the impression that a single branch has been killed - so could it be a fungus at work? This year, The Tree Council is working on a project focussed on Sooty Bark Disease, led by Forest Research and funded by Defra – so this was on our minds as a sycamore health concern. If Sooty Bark Disease was the cause of such geographically widespread symptoms, this could have significant implications for both tree and human health. But disease was not the only possibility – a very plausible alternative was that stems were being damaged by one of two possible suspects – deer and squirrels.
Different trees may be affected by different factors, and it is often difficult to conclusively identify a single cause – and that can require detailed inspection and diagnostics. Nonetheless, this did seem to be a distinct trend, occurring simultaneously across a wide geographic range – so likely caused by something quite common with a wide distribution.
An investigation
Needing to know more, our Director of Trees, Science and Research, Jon Stokes, with colleagues from the Defra Plant Health Team, carried out some further investigation, visually inspecting more than 50 affected trees in Kent, Hampshire and Oxfordshire. Looking at the trees, the damage often occurred 6+ metres from the ground and binoculars were needed to see what was going on. It soon became apparent that all the trees investigated showed signs of bark damage with often very visible teeth marks, examples of which can be seen in the Forestry Commission’s guide.
Whilst not conclusively the cause of all the damage around the country it did begin to look as though grey squirrels might be the cause of the problem – at least in these counties. So why could it have increased this year and suddenly become so visible?
A look at the literature
Previous research suggests there could be several reasons for squirrels carrying out this behaviour, including stress relief, scent-marking (Koprowski 1991), a calcium deficiency (Nichols et al. 2016), and food shortages (Kenward 1982). A particularly wet spring may also have influenced squirrel behaviour this year. But there does not appear to be a conclusive answer, with further research needed.
We consulted PhD student, Alexandra Ash, who is carrying out research on squirrel damage to trees, at the University of Southampton and Centre for Forest Protection. Alexandra described observing squirrels bark-stripping beech and oak trees, peaking in July, with more fresh damage apparent in 2024 than last year.
Alexandra told Tree Talk: “We don’t definitively know what drives squirrels to bark strip, as there’s very limited behavioural observations in the literature.”
Alexandra highlighted several of the papers listed above, adding that squirrels could be bark stripping due to their fondness for sugar, and that studies have found sugar content to be slightly higher in trees damaged by squirrels, compared with those which aren’t.
So why might there be an increase in the behaviour this year? Again, there seem to be multiple possibilities.
Alexandra said: “This could be due to weather, an abundance of mast crop from last winter, and/or higher squirrel densities, but we’re not exactly sure. There is some research going on at Forest Research to unpick the relationship between mast crops, squirrel density and squirrel damage.”
A complicated picture
The grey squirrel was introduced to the UK from North America in the 1800s and became one of our most familiar mammals, particularly for those of us who live in cities, towns, and suburbs. As a species, it is often not welcomed – it’s thought to be the main reason for the decline in the native red squirrel, though it should be remembered that human-driven habitat loss is also likely to have played a role in that sad tale, among other factors. The UK Squirrel Accord is a partnership (of which The Tree Council is a member) focused on important red squirrel conservation and awareness-raising; it would be fantastic to see a recovery of this species.
The damage grey squirrels can cause to trees (when their population density is high) doesn’t help their reputation. We face increased threats to tree health, including from climate change impacts and pests and diseases, at a time when we need thriving trees more than ever. If tree species are suffering from multiple different pressures at once, there is naturally a concern about their ability to withstand them, and an urge to tackle the threat. But the reality is that, in many parts of the UK, to significantly dent the population of grey squirrels would require concerted efforts and the use of new techniques such as contraception to reduce breeding success. The re-introduction of pine martens could also reduce population numbers, but this has other potential ecological impacts both positive and negative.
Ecosystems are dynamic and complicated and there is rarely a simple answer to the challenges faced by land-managers, foresters, conservationists and tree people! In this case, many of the damaged sycamores may recover and thrive. Whatever has sparked the suspected increase in ring-barking may not recur for years. On the other hand, it could happen again next summer. And perhaps there are other factors having an impact, not just squirrels. We should be pragmatic and led by evidence – where there are knowledge gaps, we should fill them.
Squirrels are arboreal creatures, following their natural instincts to lead a life intrinsically linked with trees. As I’m writing this substack, a grey squirrel has been scampering up and down the hazel tree in my garden. The acrobatics and agility on display are impressive, as is the dedication to reaching every single hazelnut. The squirrel’s escapades stir the tree into movement; its flexible branches bend and spring in response; its leaves dance. The squirrel will spread some of the hazelnuts around the garden and further afield – I find seedlings dotted around which could grow into new trees and support other wildlife. The relationship between grey squirrels, trees and people is not black and white, but has shades of grey.
Alexandra added: “We’re looking to develop humane, non-lethal methods to mitigate the behaviour that would directly target the behaviour. Currently in my PhD, we’re hoping to identify a chemical antifeedant that might deter squirrels from bark stripping due to bad smell or taste. Our current research aims to mitigate the behaviour instead of providing explanation for why they do it. We will be expanding our research to look into drivers of the behaviour within the next year.”
We would be interested to hear from any readers noticing damage to sycamore trees – please let us know about your observations.
Please always report any suspected tree pests and diseases to TreeAlert
Jess Allan is The Tree Council’s Science & Action Research Manager
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Excellent article that has helped to confirm some suspicions I'd had regarding sycamore decline in parts of the West Mids.
Noticed this on sycamores in SE London last year as well. We have a lot of squirrels!