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A worrying threat to our sycamores



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It was the UK’s second warmest September ever recorded and October has been the eighth warmest since 1884.

Something for the folks at COP26 in Glasgow to ponder, no doubt. The mild autumn meant that last week a lot of trees still had full, green canopies, although the frosts this week may yet give us some autumn colours.

I have had the chance to visit some different parts of the country in recent weeks and the thing that has been really notable this autumn is the continued decline of ash trees through dieback, caused by the fungus Hymenoscyphus fraxinea.

A maturing sycamore at the Sainsbury roundabout, Hardwick in Lynn. Picture: RICHARD MORRISH (52829599)
A maturing sycamore at the Sainsbury roundabout, Hardwick in Lynn. Picture: RICHARD MORRISH (52829599)

First noted in the UK in 2012, this disease has tended to kill young plantation trees but is clearly now killing big mature trees as well. Ash is one of our commonest native trees, so these losses will leave big gaps in our urban and rural tree cover over the next five to 10 years.

In Norfolk we also have two conditions killing oak trees – one thought to be caused by bacteria spread by beetles and another as yet only identified as a ‘complex mix of issues’. Climate change may well be a factor (where the effect of winter flooding and summer droughts may cause damage that is not evident until several years later).

What species of large canopy trees can we plant instead? Members of the maple family have until now shown strong resilience to diseases and pests that have affected other trees. Our native species are field maple, Acer Campestre, and sycamore, Acer pseudoplatanus.

Sycamores tend to be rather a maligned tree. Even the latin name, ‘false plane’, is a bit derogatory.

Their ‘helicopter’ seeds are prolific and can fly far from the mother tree which means they can give rise to hundreds of seedlings. In some woodlands this can give rise to a dense monocultural stand of trees which may shade out other understorey plants.

However, a single large sycamore is a stately tree, often the archetypical ‘tree shape’.

They can reach 35 metres tall, live for 400 years and support a large range of other species.

The timber is also useful. Sometimes the wood grows with an attractive ‘ripple’ grain pattern which is sought after for musical instruments and furniture.

They are probably not a native tree. The Romans are generally credited with introducing them from central Europe, but as they do not appear in much folklore, some people think introduction was much later.

Sycamore trees are tough, growing in open uplands, coastal areas and in polluted urban environments – but sadly I learnt recently, they may not be tough enough.

‘Sooty bark disease’ (caused by the fungus Cryptostroma corticale) is killing a range of maple species in Belgium and Holland and seems to be prevalent on sycamores.

The fungus invades through wounds (which of course can include pruning cuts) and then attacks the cambium under the bark, killing the tree.

It can produce vast amounts of tiny black spores that are then spread by the wind. The spores can also cause respiratory problems for arborists.

This is really very sad news and it seems only a matter of time before the disease is found in the UK.

In our push to plant more trees in the UK we obviously need to consider a warming climate when selecting species, but also this growing array of problematic pests and diseases.



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