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New research provides hope for British trees affected by ash dieback

Ash woods face the worst threat - Alamy
Ash woods face the worst threat - Alamy

Naturally enough, everyone’s attention is focused right now on Covid-19, but it’s worth remembering that other pandemics are available. You may be happy if your garden has the space for a big tree, but given the apocalyptic headlines about ash dieback, you may be less happy if that tree is an ash. However, research published in the Journal of Ecology offers cause for mild optimism.

From 2012 to 2018, researchers monitored the progress of the disease in an area of about 23 square kilometres in northern France, around the ­village of Champenoux near the city of Nancy. The area included a couple of large tracts of woodland, together with areas of agricultural land with scattered hedges and small woods, plus the village itself.

In the large woods, ash was usually present at low density, along with oak and hornbeam, while many of the small woods were pure ash. Ash dieback was first reported in France in 2008, and observed for the first time in the study area in 2010. By 2012, two years later, ash dieback was observed throughout the study area, with three quarters of trees showing at least limited symptoms.

Clearly, the disease had no trouble spreading among the many ash trees present, which is hardly surprising, given the extremely effective airborne dispersal of its spores. But the researchers were surprised by what happened next.

Many trees went on to develop severe symptoms, with large amounts of canopy dieback, but almost all of these badly affected trees were in woodlands, and serious stem cankers in particular were largely confined to woodland trees. Trees out in the open, many of them in hedgerows, generally did not develop serious symptoms.

The question is: why not? A small part of the answer was the presence of other ash trees; those close to many other ash trees tended to develop more serious symptoms than those mixed up with other trees. But a much more important factor appeared to be climate.

Telltale signs of ash dieback - Alamy 
Telltale signs of ash dieback - Alamy

The ash dieback fungus is known to prefer cool, damp conditions, and the researchers’ temperature measurements showed that canopies of isolated trees were warmer, and probably drier, than those in woodlands.

Humidity was also implicated in another thing they found: a small river runs through the study area and, other things being equal, trees nearer the river tended to develop more severe symptoms.

Maybe you’re thinking this is all very well, but isn’t France a bit warmer than here, so does any of this apply to Britain?

True enough, but other research in cooler climates points in the same direction. Studies in Estonia had already shown that woodland-edge trees suffered less severe symptoms than trees in the interior of woodlands.

Add all this to the well-known finding that small trees are more badly affected by ash dieback than large ones, and it looks like that big ash tree in your ­garden, or your local park, is still likely to be around for quite a while yet.

Ken Thompson is a plant biologist with a keen interest in the science of gardening. His most recent book is Notes From a Sceptical Gardener, the second collection of his Telegraph columns. Visit books.telegraph.co.uk.