It isn’t often that environmental issues make it into the news or prompt a meeting of the UK Government’s COBR crisis committee. The gradual decline of a rare species may worry environmentalists but it sadly doesn’t make for dramatic headlines. However over the last week the rapid spread in the UK of Ash Dieback, or Chalara fraxinus, has received extensive coverage in the media. A ban on imports was issued by UK Parliament on 30 October 2012.
One of the concerns is the potential impact on our landscape and how it might change the landscape character. Ash trees make up around 5% of UK woodland cover as well as being important street and parkland trees. If the UK experiences the same level of tree loss as Denmark, estimated to be between 60 and 90% of their ash population, the change to our landscape could be substantial.
Tree cover and tree species are two factors in landscape character – any changes to these can substantially change the character. If you compare a conifer plantation with a deciduous woodland in the same area the landscape character is quite different, even though the underlying landscape is the same. Landscape character is best described as what makes a landscape unique. The full definition is “a distinct, recognisable and consistent pattern of elements, be it natural (soil, landform) and/or human (for example settlement and development) in the landscape that makes one landscape different from another, rather than better or worse.”
The final phrase is the one to focus on. In purely landscape terms the character may change but the impact of this can be controlled with careful landscape management . This doesn’t take away from the tragedy of large scale tree loss, the impact on biodiversity, the loss of stock from plant nurseries or the fact that our sector should work hard to minimise the spread of the disease, but it is important to counter some of the apocalyptic predictions in the UK press.
So what do we do as professionals? Copenhagen has implemented the 10% or Santamour Rule to counter the impact of any future tree diseases. The Santamour Rule was developed by Frank Santamour of the US Arboretum and recommends that
Urban foresters and municipal arborists should use the following guidelines for tree diversity within their areas of jurisdiction: (1) plant no more than 10% of any species, (2) no more than 20 % of any genus, and (3) no more than 30 % of any family. Strips or blocks of uniformity (species, cultivars, or clones of proven adaptability) should be scattered throughout the city to achieve spatial as well as biological diversity.
In Copenhagen the landscape team follow this rule so when new street trees are planned the percentage of that tree in the immediate area is assessed so no one species exceeds 10%. They also vary the species used along each street rather than the traditional two rows of identical trees. In a way this simply mimics biodiversity, nature’s way of surviving rapid environmental change.
Moving away from single species schemes will be a challenge as there is real drama in the use of a small palette of trees and shrubs. Think of a street planted with London Planes, or a stunning avenue of Sweet Chestnut trees lining the drive of a stately home. If our planting schemes lack variation we need to assess the risk of losing the whole scheme if a new disease appears. The risk in a small private garden might be worth taking but if the scheme screens a nuclear power station a diverse mix would be needed.
Other roles for landscape architects, along with ecologists and other environment professionals, are to assess an area’s vulnerability to disease due to the dominance of one species and to continue to monitor for Chalara fraxinus and other diseases.
But we shouldn’t forget that trees are not static and that they die eventually, regardless of disease. The Long Walk , a broad avenue that leads up to Windsor Castle, was originally planted in the 1680s with a double row of elms. The majority were felled as they reached the end of their lives and replaced from the 1850s onwards with a mix of Horse Chestnut and London Plane, with the remaining elm removed in the 1940s due to Dutch Elm Disease. This iconic landscape, with careful landscape management, has survived over 300 years.
How to Report Chalara fraxinus (Ash Dieback)
Forest Research Tree Health Diagnostic and Advisory Service
T: 01420 23000;
Forestry Commission Plant Health Service
T: 0131 314 6414;
Fera Plant Health and Seeds Inspectorate
T: 01904 465625;
Useful Links and Further Reading
Forestry Commission guide to ash dieback, including a pictorial guide
Advice for foresters – Silvicultural Strategies for Forest Stands with Ash Dieback
Blog posts by Dr Gabriel Hemery with his ten point plan
NBS Landscape includes a section on checking for disease in Section Q35
How to identify Chalara fraxinus