Jon Ives explores a number of the consequences and potential interventions where ash dieback disease is present
The arrival of ash dieback disease three years ago alerted professionals and the wider public to yet another exotic disease threatening our native tree population.
Unfortunately, what distinguishes Chalara from many other imported pests and diseases is that it cannot be treated or contained.
The impact of the disease is likely to be on a par with Dutch elm disease both in terms of reduction of visual amenity and loss of biodiversity. Ironically, ash has benefited substantially from the loss of elm trees. In certain parts of the countryside it is the dominant hedgerow and woodland tree.
Over time sites vacated by ash will recruit other species to fill the void. This process will be very much down to site conditions but sycamore will probably be the main beneficiary, with oak, beech and birch also likely to benefit.
Aside from these species, there will be other short term winners with an increase in pathogenic as well as saprophytic fungi.
Mature ash trees which are weakened and go into decline over a number of years may well be killed by honey fungus (Armillaria spp.) rather than Chalara itself. Where ash is the dominant tree there will be a profoundly negative eff ect on biodiversity as it hosts scores of lichens, mosses and liverworts that are reliant upon it or the conditions it fosters. Some of these species once relied upon elm trees so it is possible they will find alternate hosts but for many the loss of ash may result in their extinction.
Non-specific invertebrates as well as small mammals such as the dormouse will inevitably suffer from loss of habitat but as they are more mobile there is potential for them to adapt more readily.
From a health and safety perspective the most important consideration is the potential for failure of the tree or parts thereof with subsequent injury or damage to people or property.
The Occupiers’ Liability Acts of 1957 and 1984 confer a duty on an occupier to take reasonable care to ensure that visitors to the property are safe from harm. The tree inspector charged with inspecting trees on an owner’s behalf should be aware of the potential for foreseeable harm to occur as a result of defects arising from Chalara.
When undertaking Visual Tree Assessments (VTAs), tree inspectors will need to make an informed judgement on the most appropriate course of action once the disease has been identified.
Clearly there will be variables to consider such as occupancy levels of the site etc, but for large specimens of high amenity value where there is a gradual dieback in the crown it may be appropriate to recommend removal of deadwood combined with a shorter inspection interval.
Where dieback is rapid and identified in young and early mature trees it may be more appropriate and cost effective to recommend removal at an earlier stage.
Similarly, when surveying ash trees within the context of a BS5837:2012 survey, the disease should be considered a material consideration even if the individual tree is showing no signs of the disease.
Jon Ives is senior arboricultural consultant at CGM Group in Norfolk, carrying out site survey and specialist consultancy services. He studied arboriculture and amenity forestry at the University of Aberdeen and retains links with its research departments.