Ash and it's imminent decline

Ash dieback is one of the most discussed tree diseases in the UK. Following on from Dutch elm disease, ash dieback will also have a profound impact on the population of ash trees and their landscape. Found across the Great British countryside (except some of the Scottish highlands), ash is the 3rd most common tree in the UK. It is predicted that 80% of these trees will be lost to ash dieback. 


A seemingly healthy ash tree in a hedgerow, UEA

This disease is caused by a fungus, Hymenoscyphus fraxineus. The fungal spores are carried in the air and land on ash leaves; the fungus then penetrates into the leaves and grows inside the tree, blocking the xylem vessels. This causes dark patches on the leaves in summer, which then fully discolour to black, wilt and shed early. Shoots and leaves die back in summer and diamond shaped lesions appear where the branches meet the trunk. Eventually most trees die from the disease, with younger trees dying sooner after infection than mature trees. Another symptom of ash dieback is epicormic growth - new growth from previously dormant buds lower down the trunk, induced by stress. The airborne nature of the spores allows the fungus to travel long distances and colonise new areas. 


Ash can be identified in winter by its velvety black buds

As ash dieback has only been present in the British Isles for around 30 years, it's full effects are yet to be seen. The loss of ash trees is predicted to cost the British economy £15 billion through managing the disease and clearing infected trees, as well as the loss of ecosystem services. The mortality rate of ash dieback is particularly high because the disease originated in Asia and was introduced to Europe via imported saplings. In it's native range, Hymenoscyphus fraxineus has a much lower impact on it's sympatric ash species, Manchurian and Chinese ash, but European ash has very little natural resistance. 

Because ash is so common, the ecological impacts of ash dieback will be considerable. The loss of trees in woodland and across open landscapes will reduce the connectivity of habitats. The ecological value of ash is high, with 953 associated species, including 44 obligate associated species. Although pedunculate and sessile oak can support 69% of ash associated species, so may mitigate losses, no other tree can host all associated species. Predominantly ash woodland has distinct environmental processes; ash has low efficiency of nutrient reabsorption from senescent leaves, and it's leaves and roots degrade relatively quickly, meaning the nutrient flows in ash woodland are more rapid and complete than in other woodland. The open canopy and early leaf fall of ash means that light levels on the ground are higher than in other woodland, creating more opportunities for wildflowers such as dog violet and wild garlic. In turn, these flowers support a variety of insects and birds. High light levels also allow an understory of hazel to develop, an important habitat for species like dormice. The longevity of some ash trees makes them important for deadwood specialists such as the lesser stag beetle. 


Ash leaves are pinnately compound, with 3-6 opposite
pairs of leaflets and one terminal leaflet

Conservation surrounding ash dieback can have multiple approaches, though the continued spread of this disease cannot be prevented. Burning, deep composting or burying fallen ash leaves from areas known to have outbreaks can help slow the spread, as the fungus overwinters in the leaf litter. Encouraging individuals to wash shoes and bike tyres before and after visiting woodland could also help slow the spread. Members of the public can help through citizen science, by reporting new cases of ash dieback on Tree Alert. It has been found that the best woodland management strategy for biodiversity is to leave infected trees to naturally decline, which will ultimately select for tolerant trees. Research has discovered resistance in some individuals, meaning a recovery should be possible, in around 50 years time. Campaigns to educate and promote better biosecurity may not have a large impact on the ash dieback epidemic, but could prevent future outbreaks of other diseases.


An educational sign detailing the management plan of a woodland infected with ash dieback - excluding the public to reduce the need to fell infected trees.

https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/british-trees/a-z-of-british-trees/ash/
https://www.woodlandtrust.org.uk/trees-woods-and-wildlife/tree-pests-and-diseases/key-tree-pests-and-diseases/ash-dieback/
Ash dieback in the UK: A review of the ecological and conservation implications and potential management options
Estimating mortality rates of European ash (Fraxinus excelsior) under the ash dieback (Hymenoscyphus fraxineus) epidemic

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