Ashes to ashes
The gap left by the devastation of Britain’s elm population by Dutch elm disease has largely been filled by the ash tree. However, the spread of ash dieback now poses a new threat to our woodlands. With a host of new diseases attacking our native treescape, Tony Kirkham, Head of Kew’s arboretum, explains how he's optimistic about managing the damage caused.
When I began my career in arboriculture in 1974, I could never have imagined what challenges the treescape of Britain would be facing almost 40 years later. In the 1960s and 70s, prior to me starting a forestry apprenticeship, Dutch elm disease was threatening to wipe out the elm population in southern England. As I completed my studies at Kew in 1981, I watched from the Jodrell Laboratory as one of the last of the mature, flamboyant English elms was felled in Kew Gardens, on the site of the Iris Garden (the Grass Garden today).
I don’t really remember the hedgerows being graced with these elegant, large-framed deciduous trees or their billowing cloud-like canopies filling the English landscape, and it’s rare to see an elm reach maturity today. I do however remember thinking, ‘Well, that’s the end of Dutch elm disease, now we can get on with life.’ Little did I know what the rest of the treescape of Britain would be facing over the coming four decades – such a short time in tree years.
In the last ten years, we have seen an increasing threat to both our native and introduced tree species from many newly arrived, aggressive diseases. So aggressive in fact that they make honey fungus (the disease that all gardeners feared) seem like a mere sniffle in the tree world. Almost all of our favourite tree species, from the oak to the horse chestnut, face infection from potentially life-threatening diseases, and the big question on all of our minds is: ‘What do we plant for the next generation?’
The latest introduced disease, first brought to our attention in spring 2012, is known as ash dieback. This fungus (Chalara fraxinea) will seriously affect our native common ash (Fraxinus excelsior), killing the leaves and bark tissue, causing shoot death, cankers, crown dieback and ultimately the demise of the entire tree.
Ash is common in native woodlands the entire length of Britain and, since the loss of the elm, now plays a significant role in rural areas as the principal hedgerow tree. It is also planted widely as an amenity tree throughout our urban areas. At Kew and Wakehurst Place we have more than 500 ash trees, comprising some 43 different species and cultivars.
At present we are uncertain as to which ash species are susceptible to the disease. It is thought that the North American species are more resistant than the European and Asian species. One of our main roles, when the disease finally reaches Kew, will be to monitor susceptibility and resistance, and to disseminate this information. This will help people select which ash species to plant for the future.
There is another positive outlook – on the Continent, where ash dieback has been prevalent for more than ten years, not all ash trees have been killed. This suggests that there is some genetic resistance to the disease. Most native ashtrees are propagated from seed, which leads to wide genetic diversity across populations, so hopefully some of our mature and veteran specimens will survive and can be bred from over time.
At Kew, we continue to look on the bright side. We have recently planted 13 young ash trees in the Gardens that had been raised in the Arboretum Nursery, and we will continue to manage and monitor our ash collection. At present there is no cure for ash dieback and no proven method of reducing its spread, which is primarily by spores produced from fruiting bodies on the previous year’s leaf litter. We will ensure that all leaves around our ash trees are quickly destroyed by turning them into mulch in situ, rather than taking them to Kew’s compost heap, as this could spread the spores further afield. When we find infected trees we will retain them until they become a potential safety hazard.
Kew’s seed scientists are working with the Forestry Commission to gather ash seeds, which are being screened for disease resistance. They will also be used in research, which, hopefully, will contribute to the survival of this species.
This article was originally published in Kew Magazine, Spring 2013