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  • The openness of an ash canopy is special.
  • Birch trees are shorter-lived than ash.
  • Symptoms of ash dieback.

Tree Disease

BY FIONA McWILLIAM
The appearance of ash dieback in this country last autumn prompted panic and recriminations. We look at the facts, at the alternatives available and at the other threats to the UK’s trees.

If you go down to the woods today, you are in for a nasty surprise.’ So remarked TV presenter, and (honorary) president of The Woodland Trust Clive Anderson, in a Guardian article last autumn. He was referring, of course, to the continuing spread of ash dieback, the disease caused by the Chalara fraxinea fungus.

Yet ash dieback is only one of many diseases and pests threatening the UK’s tree population. And while ash dieback’s full impact on the rural landscape will not be known until new tree growth emerges in the spring, the removal of ash from the landscape architect’s palette of trees looks set to have a major visual impact in the long term on our urban and suburban landscapes, and on vast areas of British parkland.

The Chalara fraxinea fungus has caused widespread damage to ash tree populations in continental Europe since it was first reported as an unknown new disease in Poland in 1992. It is especially destructive in common ash (Fraxinus excelsior), including its ‘Pendula’ ornamental variety. Narrow-leaved ash (Fraxinus angustifolia) is also susceptible. The disease is particularly damaging for young ash plants, killing them within one growing season of symptoms becoming visible. Older trees can survive initial attacks, but tend to succumb eventually after several seasons of infection.

In February 2012, the fungus was found in a consignment of infected ash plants sent from the Netherlands to a nursery in Buckinghamshire. In October, scientists confirmed a small number of cases in East Anglia, at sites including established woodland. The affected trees did not appear to have been in contact with imported stock, raising concerns that wild trees can be affected by wind-blown spores from mainland Europe.

The government eventually passed emergency legislation restricting imports of ash plants, seeds and trees at the end of October.

The Forestry Commission strongly advises tree and plant buyers ‘to be very careful to specify healthy stock from reputable suppliers, to practise good plant hygiene and bio-security... to prevent accidental spread of plant diseases, and to report any plant diseases’. Buyers and specifiers should also be aware that seed gathered from British trees is sometimes sent to nurseries in continental Europe to be cultivated before being re-imported as seedlings.

While this practice may make business sense in the short term, it certainly does not make biological sense, warns Dr Mark Spencer, senior curator, British and Irish herbarium, at the Natural History Museum’s Life Sciences Department. Nor, he adds, is it likely to make any sort of longer-term cost sense, when one considers the overall performance of non-native genetic stock of native plants which may have inherently reduced fitness for the British landscape and climate compared to locally-sourced genetic stock of native plants.

We should, he argues, use native trees wherever possible. This is a view shared by John Tucker, director of woodland creation at the Woodland Trust, although he admits this is not always practicable for urban planting.

Britain, has, however, been importing plant material and timber since the seventh century and in all probability will continue so to do. ‘There are so many million fungi in the world, and they are on every bean we import from Kenya, every cut flower we bring in from the Netherlands, every Christmas tree we order from Denmark or wherever,’ says Alan Simson, reader in urban forestry and landscape architecture at Leeds Metropolitan University, and a member of the Landscape Institute’s biosecurity sub-committee.

What turns a benign fungus into a rampant problem is unknown, he says. ‘It may be, in fact probably is, partially to do with climate change, but there may well be other things at work of which we know little.’ The usual palette of tree species used by landscape architects is woefully small, Simson says, and ‘we cannot really get away with planting native trees in the “serious urbs”, as most are not genetically programmed to survive in such environments’. Yet the unforeseen consequences of the global trade in plants are undoubtedly driving significant problems beyond that of ash dieback. Mark Spencer describes the unintended devastation of New Zealand’s insect population as a result of the successful spread of the fly agaric fungus, which arrived in the country with pine trees imported from Europe and North America. While vital to the health of many conifers and pines in the Northern Hemisphere, the imported fungus is pushing out the local fungal communities on which many insects depend.

Thankfully a tightening of bio-security measures is likely in the UK, says Dr Jon Heuch, an independent arboricultural consultant and a member of the Forestry Commission’s Biosecurity Programme Board. A new British Standard is now being developed, he adds, and ‘we may see a need for much better chain of custody information and even third party certification if purchasers demand it’. An EU directive on invasive alien plants is also in the pipeline.

In terms of what trees we should plant instead of ash, says Emma Hill, a policy consultant with charity Trees for Cities, this depends very much on location. ‘If we are talking about rural locations, woodlands or natural areas, we would substitute with other native species,’ she says. ‘Ash trees are tolerant of most soils except light, sandy soils so there are many species to choose from.’

There is a wide range of alternatives species for sites with brown-earth soils, says Hill, ‘including aspen, beech, birch, field maple, hornbeam, oak, lime, rowan, sweet chestnut and sycamore.’ The species range is more restricted for calcareous soils, she adds, particularly for shallow soils. It includes beech, birch, field maple, hawthorn, holly, lime, rowan, whitebeam and yew. Alder, aspen, willows and oaks are possible alternatives on moist to wet soils.

Common ash trees, which make up an estimated 30% of our wooded landscape, are large, and long-lived, and provide light open canopies allowing light through to the woodland floor. This creates a good habitat for important species such as bluebells and ramsons. No one tree species would replicate the ash’s light open canopy, says Hill, ‘but a managed oak woodland with variations in tree density would provide a similar habitat for bluebells’.

Trees for Cities would not plant common ash in urban settings, says Hill, because of its ‘leaderless, rangy habit’.

‘In street locations we might plant common ash cultivars such F. excelsior Westhofs Glorie or F. excelsior Altena, which keep a much more regular form well into maturity,’ he says. ‘We might also plant manna ash such as F. ornus Louisa Lady or F. ornus Obelisk, but it is not yet known whether ash dieback affects this species,’

These trees would usually be medium to large in size with an oval or pyramidal form. Trees with a similar form and size include small-leafed limes (Tilia cordata) and cultivars, Turkish hazel (Corylus colurna) and hornbeam (Carpinus betulus) and cultivars. However, the habit of these trees is much denser than that of the ash.

Birch (Betula) species have a similar light, open habit but are much shorter lived and do not, according to Hill, “have the same presence as ash”. London plane (Platanus x hispanica) also has a more open habit but can become very large.

We must keep striving to find a solution to Chalara dieback, she says. ‘Experience from Denmark [where 90% of trees are thought to have been infected] has shown that maybe 10% of the ash trees are immune to the fungus. If that is the case here then resistant strains can survive. We need to find these trees and cultivate them.’

Tree specifiers need to also be aware of the many other pests and diseases, threatening our trees. As the Forestry Commission warns, ‘Britain’s trees are facing unprecedented threats, while climate change will create the conditions for even more pest and disease activity’. Among those already present in Britain are: acute oak decline, a condition affecting oak trees in parts of parts of England and Wales, in which bacteria, ‘are believed to be involved’; the Asian longhorn beetle, a wood-boring insect that can cause extensive damage to a range of broadleaved trees; bleeding canker of horse chestnut trees; chestnut blight, a highly damaging disease confirmed in sweet chestnut trees in Warwickshire and East Sussex in 2011; horse chestnut leaf miner moth, which was first found in Britain in 2002 in London, but has since spread to much of England and Wales; oak pinhole borer, which took hold in southern Britain after the 1987 gales, when it took advantage of the glut of suitable breeding material; and oak processionary moth, which severely defoliates oak trees and can make them susceptible to other pests and diseases. Even the smog-resistant London plane is now suffering from a new disease, Massaria, which weakens branches so much that they spontaneously drop to the ground.

Fiona McWilliam is a former editor of Geographical magazine.

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