After Dutch elm disease and ash dieback, will oaks be next? – Dr Ruth Mitchell

A decline in the number of oak trees would have a cascading effect on biodiversity

Dutch elm disease arrived in the UK in the 1960s. Growing up as a child in southern England, I wasn’t even really aware of losing such a tree. I just accepted the fact that a tree called elm was dying out. To me it was “just one of those things that happened”, I didn’t really think about the consequences. Today, those living north of Inverness are only just seeing the impact of this disease and may still be lucky enough to see mature elm trees, however many people may never have seen one.

The profile of tree diseases has risen up the public consciousness in the last decade since the arrival of ash dieback, Hymenoscyphus fraxineus. More people are now aware of dead ash trees. However, we rarely stop and think about what the wider consequences of a decline in a particular tree species are. Does this drive declines in other species that utilise the tree? This is something that will be discussed in Edinburgh today at the annual Chartered Institute of Ecology and Environmental Management conference.

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At the James Hutton Institute, our work on ash has shown that 955 species are associated with this tree in the UK. Of these, 45 species were identified as being obligate on ash – unknown from other tree species – and a further 62 species as being highly associated, rarely using other species. This means that not only are we seeing a decline in one of our most common trees, but that ash dieback could drive declines in a whole range of other species. So, it’s not just a question of what trees we are losing but what other species too. Our baseline for biodiversity is getting lower and lower.

Not only will my grandson grow up with ash as a relatively uncommon tree but there might be a whole range of associated species that might be a lot rarer in his lifetime than in mine. Fortunately, of course, many species use multiple tree species and can use others than ash. However, those trees may also be under threat. There has been an exponential increase in the number of tree diseases establishing themselves over the last century.

Just thinking about oak, we now have the oak processionary moth, acute oak decline and chronic oak decline. A decline in oak would also have cascading biodiversity impacts. We need to consider the cumulative impacts of these diseases not just on trees but on our biodiversity. How resilient are our native woods and their associated biodiversity to multiple types of such diseases?

There are, of course, things we can do to make our woods more resilient. Generally, a more diverse community is thought to be more resilient because if one species is lost, then there are others in the community that can fill their functional role. This is what we are studying in a project called DiversiTree, which aims to increase resilience of current and future woodlands by working across a range of scales, from microbes to minds, to understand the methods to diversify tree species composition and the impacts of doing so.

DiversiTree addresses four knowledge gaps related to the diversification of woodlands, from how stakeholders understand forest diversity, their diversification strategies and visions for diverse future forests to assessing the diversity of the microbes found on leaves in different types of woodland and if that helps trees to better defend themselves against diseases. It’s also looking at how we implement and communicate management strategies to increase woodland resilience.

In addition to making our woodlands more resilient to diseases, we also need to ensure that, in our efforts to restore woodlands and create new woods, we don’t accidentally spread diseases. In all habitat restoration and creation activities, appropriate risk assessments and biosecurity protocols are critical. Recent research by the Hutton suggests that, in many cases, this is not being carried out.

So, although my grandson may grow up with fewer ash trees and their associated species in the landscape, I hope he will also grow up with a greater diversity of trees in our woods and forests and surrounded by a greater public awareness of the importance of reducing the risks of spreading plant diseases.

Dr Ruth Mitchell is the biodiversity and ecosystem group leader at the James Hutton Institute

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