Disease threat leaves UK oaks' future 'at crossroads'

15th Sep 2011

The future well-being of the UK's oak trees could be at a crossroads, says one of the UK's chief tree experts.

Keith Kirby from Natural England said that oak deaths from pests and diseases in some areas was a cause for concern.

In particular, a disease known as Acute Oak Decline in the Midlands has led experts to ask whether it would have a similar impact as Dutch Elm disease.

Dr Kirby made the comments ahead of a presentation to the British Ecological Society's annual meeting in Sheffield. 

"There has been a concern about increasing pests and diseases over a number of years," Dr Kirby said.

"There are the obvious Phytophthora diseases, particularly in south-west England, which are a large threat to commercial timber.

"It is also attacking bilberry as well so it is a concern to us in conservation terms of ground flora.

Phytophthora ramorum and Phytophthora kernoviae are strains of a deadly fungal plant disease that quickly kills trees it infects and can spread over a large area in a short space of time unless strict quarantine measures are put in place.

In recent years, the main concern about this disease has been its spread among commercial stands of Japanese larch, first in south-west England, then in Wales and Northern Ireland.

But, as Dr Kirby explained, despite the disease's common name - sudden oak death - there have been no recorded cases of an English oak (Quercus robur) being infected.

The misnomer is a result of the name travelling across the Atlantic from the US, where the disease was believed to have been brought into Europe and the UK, and where it does affect a number of native oak species.

Oak killer

"There are also other diseases, particularly something called Acute Oak Decline (AOD), which seems to be caused by a bacterium but a bit more research is needed to confirm this," Dr Kirby told BBC News ahead of his presentation.

"This is causing the death of oaks in woodlands throughout the Midlands and South.

"At the moment, it is at relatively low levels but at the back of the minds of everyone is whether we have another Dutch elm disease on our hands."

Tree professionals and conservation groups have also voiced concerns that if the disease got a foothold in the nation's woodlands, then the landscape would be changed forever.

AOD symptoms include "extensive stem bleeding" in which dark fluid seeps from small cracks in the bark and runs down the tree trunk.

In early stages of the disease, the health of a tree's canopy does not appear to be affected, but it may become thinner as the tree succumbs to AOD.

Research suggests that the disease becomes established in oak trees more than 50 years old.

The UK's two native species of oak - sessile and pedunculate - are both susceptible to AOD.

"So there is a background concern over the pattern of deaths of oaks from a number of different sources," Dr Kirby observed.

Key species

The risk of losing oaks could have a serious knock-on effect on woodland ecosystems.

"The oaks, as a genus, do seem to have a very large associated number of specialists insects, they provide a good habitat for a good range of lichen and fungi," explained Dr Kirby.

"Another thing is that it grows big and lasts a long time, so if it is given the time an oak tree will get big, have dead branches on it and start to rot on the inside but it will still live for a very long time.

"That means that you get an incredible range of different micro-habitats on a single tree - from the cracks in the bark to holes in the trunk. So old oaks are the most important of our veteran trees.

He added that although oaks' canopies provided shade, it was not as dense as some other species, such as beech.

"You will get quite a lot of shrub and ground flora surviving underneath it. And perhaps another thing that contributes to the richness associated with oak is that has been in Britain for a very long time - it was one of the early colonizers, but it has also been abundant for a long time because people found it so valuable.

"It is the tree that almost everyone wants to encourage their own woods."

Dr Kirby was making the presentation to fellow ecologists at the BES annual meeting at the University of Sheffield to update them on the findings of a study at Wytham Woods, Oxfordshire - one of the most studied areas of woodland in the world.

He told BBC News: "I had been doing some work on the fact that if we were going to get another Dutch elm disease, what was the sort of thing that would frighten us the most, and it would be something that attacked oak."

This work coincided with a survey of oak trees in Wytham woods for bird researchers, as oaks provided an important source of food for many bird species, especially during winter months.

"It then just struck me that it was a good set of data to start to explore the wider issue of what was the dynamics of the oak population in Wytham Woods," Dr Kirby recalled.

"That would then give us a baseline to start to look to see if the deaths of trees in Wytham was a symptom of one of the new diseases, or if there had always been dead oak trees through the woods and we had just become aware of them, or if there were some other factors causing them to decline - for example increased competition from an abundance of ash throughout the woods."

The study highlighted that within the study area, there was no immediate concern to the health of the woodland's oak populations.

"It is certainly not a species that is immediately under threat provided that no pests and diseases prove to be as damaging as Dutch elm disease," he said.

He concluded by saying that there was no need to panic yet but it was uncertain times as far as England's national tree was concerned.

"All was well in Wytham, however it is a watching brief as we may be at a crossroads."

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