Assessing the impacts of ash dieback

Spring is almost upon us, and early signs of budburst mean that trees will soon be in leaf. Warmer temperatures and leafing trees will both have implications for ash dieback, the disease found last year in ash trees in the UK. The fungus that causes the disease is dormant throughout the winter, and sporulates in the spring. Leaves present vital signs of the disease, and they will help indicate the incidence of disease across the country. Effective measures to deal with both these issues are therefore necessary. A report released by Confor last month outlines the potential impacts of dieback, and follows on from work by Richard Worrell assessing the potential impacts of the disease in Scotland. The report also shows the extent of ash woodland in private ownership, highlighting the need for any actions from government to take this into account.

After oak and birch, ash is the third most common broadleaved tree species in the UK. Confor’s report estimates that woodland ash trees cover over 140 000 hectares of land in UK. In addition to this, 12 million ash trees are present outside of woods and forests, covering urban and recreational areas. The majority of forest volume in the UK is not publicly owned – out of a total forest area of 3 million hectares in the UK, only 28% is managed by the Forestry Commission. For ash, this figure is much lower, with only 3% of ash woodlands not owned by the private sector.

Ash dieback has been present in the natural environment of the UK since October 2012. Although the majority of cases are still confined to nursery sapling stock, cases have been found ‘in the wild’ in eastern England and south-east Scotland (see this Forestry Commission map for details of all reported cases). The disease is caused by the fungus Chalara fraxinea, which causes leaf loss and crown dieback in infected trees, potentially leading to the death of trees. Common ash (Fraxinus excelsior), as found in the UK, is highly affected. Saplings are particularly vulnerable, succumbing rapidly to the disease.

The direct effects of ash dieback on tree populations are clear. Infection in young trees is likely to lead to death within 10 years. High rates of infection and tree death are seen in continental Europe, where the disease is established. One of the worst affected areas is Denmark, where 90% of ash trees are infected. The loss of these trees from the landscape will alter woodland and urban environments alike. As highlighted by Professor Alan Downie, Dr Erik Kjaer and Dr Joan Webber at an online Q&A event hosted by OpenAshDieBack earlier this week, the loss of ash trees will likely lead to negative impacts in the environment. Professor Downie pointed out that ash is the sole foodplant for at least 27 species of invertebrate, and Dr Kjaer highlighted that ash has many associated species, including orchids. The loss of ash would therefore change ecosystems and biodiversity levels. In addition to these species-specific effects, Professor Downie also highlighted the general effects of losing tree cover – impact of the loss of canopy cover on ground flora and fauna, the release of nutrients from the soil, and changes in carbon sequestration.

In addition to these ecological effects, the loss of ash trees in the UK could have an impact on wider economic processes, as highlighted in the Confor report. The loss of ash as a timber tree, and the changes that would have to be made in the management of woodlands that currently have ash present would be economically detrimental. For private owners, the costs of surveying, felling, and replacing ash trees are likely to be high, and the effects of this could be long-lasting. An increase in the amount of timber in the market could also drive prices down, affecting landowners even further.

The government outlined its current actions for ash dieback in December in Defra’s Interim Chalara Control Plan. In this, four key objectives were highlighted:
• Reducing the rate of spread of the disease
• Developing resistance to the disease in the native ash tree population
• Encouraging landowner, citizen and industry engagement in surveillance, monitoring and action in tackling the problem
• Building economic and environmental resilience in woodlands and in associated industries

Given the high rates of private ownership for ash woodland areas, action on the third objective is vital. Dissemination of information about the disease, its spread and how land owners can react best are all necessary for an effective response across the total woodland area in the UK. For landowners to engage in monitoring ash dieback, resources must also be available for them to do so. The number of inquiries sent to the Forestry Commission’s Tree Health Diagnostic and Advisory Service has increased by 1000% over the past six months. As diseased trees come into leaf over spring, and more trees become infected when the Chalara fungus sporulates again in summer, this high workload could even increase.

The report compiled for Confor also covers the potential impacts of red band needle blight, Dothistroma, on pine populations in the UK. This highlights that it is not just ash dieback that could cause ecological and economic problems. The number of tree diseases present in the UK has risen exponentially over the past 20 years, and now, almost all tree species are under threat from at least one disease or pest. Red band needle blight and ash dieback threaten up to 18% of woodland in the UK. Combined with other diseases and pests, their effects can be greatly exacerbated. 30 million elms died of Dutch Elm disease in 1985, and the report compiled by Confor highlights that the extent of private ownership of ash woodlands needs to be taken in to account to prevent damage on this scale occurring again.

What do you think? We’d love to hear your views on this subject, whether you’re a landowner with ash, or you have an interest in policy in this area.