Updated: May 2016
Ash dieback is a serious disease of ash trees, caused by the fungus Chalara fraxinea. It has already devastated ash woodlands in other parts of northern Europe, and is found in Britain. The fungus infects 60-90% of the trees in its path, causing leaf loss, bark lesions and crown dieback. Young ash trees are killed very rapidly by the disease. Older trees often resist the disease for longer periods but succumb with prolonged exposure. The disease is spread in the leaf litter, and the wind is believed to play a role in transmitting fungal spores.
The Forestry Commission has produced a pictorial guide highlighting the symptoms.
As of May 2016 the Fera map shows outbreaks of Chlara (ash dieback) in twelve of the fifty 10km squares either in or bordering Somerset. Six of the squares lie wholly within Somerset and we are aware of an outbreak in a further square at Ham that is not yet on the Fera map.
We have just had confirmation of 2 new outbreaks and a further 4 unconfirmed outbreaks in 6 of our nature reserves around the Cheddar area, one in the Mells Valley and one on the Polden Hills. This will make fourteen 10km squares with infection. Currently these new outbreaks appear to be confined to ash saplings. We won’t be able to assess whether the disease has spread into the larger woodland ash trees until leaves have opened.
On Mendip ash trees make up a very high percentage of woodland tree cover. In some woods this can exceed 90%. Heavy losses could result in the effective loss of many woodlands on Mendip, including the woodland flowers, birds and invertebrates and lichens that live in them.
Due to the rapid national spread of ash die-back we will not be limiting access to visitors, as this is not now thought likely to reduce the rate of spread of the infection. We and the Forestry Commission are still encouraging land owners and the public to be vigilant and report further suspected infections to Fera.
As the infection spreads there are increasing indications that some Ash trees are resistant. http://www.theguardian.com/environment/2016/apr/22/betty-the-ash-tree-offers-hope-against-deadly-dieback-disease-resistance. It is vitally important that any resistant or unaffected ash trees are identified and protected and SWT will look to identify such trees. These may provide the best hope for the future.
What SWT are doing to try and mitigate the impact of ash dieback:
- Where tree planting is required in our woodlands, SWT have been planting a range of alternative tree species to ash that, although they cannot ecologically replace ash, nevertheless provide similar conditions, in terms of light shade of the woodland floor.
- SWT volunteers have been collecting and growing on native woodland tree species to replace ash, where there is no likely natural regeneration (of other tree species).
SWT have been encouraging (through thinning) healthy and vigorous stands of ash, better able to withstand the secondary infections caused by Chalara.