Ash Dieback in the Dales

Those of us close to the land will be aware of a noticeable change in our ash trees.  This summer in particular we saw a dramatic decline in older, stand-alone field trees, not helped by the late spring frost and drought. New woodland planting is affected, too, with ash being the predominant species in new broadleaved planting schemes up until 2012.

Ash dieback, the disease caused by the fungal pathogen Hymenoscyphus fraxineus, is now evident across the whole of the Yorkshire Dales National Park, although some areas are more severely affected than others. The disease is having a devastating impact on our landscape.

Ash trees are ubiquitous within our field-scape and historical field boundaries. Along with hawthorn, hazel and holly, ash is often the only obvious remnant feature of ancient hedgerows and wood pasture.

Upland ash woodlands are amongst our most precious, biodiverse habitats. These trees, as well as having great landscape importance, are irreplaceable in providing an ecological niche for rare lichens, fungi and invertebrates, and are also used by protected species such as bats.

While we expect to lose the majority, but not all, of our ash, a small proportion of trees are likely to show some higher genetic tolerance to the disease and these precious trees will, over time, slowly provide us with the next generation of ash trees. This, combined with ongoing trials to develop resistant ash trees, means that in the long term we can be optimistic that the landscape will recover.

Look at your trees

There is no doubt that landowners need to be assessing the impact of ash dieback on their holdings. The priority should be to look at trees which have targets. A target is something valuable that a falling branch or tree could hit – such as areas of high use, roads, public rights of way, homes and buildings.

While the summer gave us an indication of the physiological health of our ash trees, now that the leaves have fallen, this is the best time of year to look at trees structurally. It’s also the best time of year to see wood decay agents, such a honey fungus, which is more likely to gain a hold in trees with ash dieback, as they are more vulnerable to secondary infections.

It’s important that ash trees are not being felled unnecessarily. Trees which are otherwise considered safe and are not symptomatic of ash dieback, showing tolerance or reasonable levels of vitality, should be monitored. Landowners should consider whether trees with no target can be left to decline naturally.  These are ash trees which will cause no harm if they fail or drop branches, such as those in the open landscape or within woodlands.

Where trees are found to be posing a hazard, alternatives to felling such as canopy reduction will safeguard valuable habitat and allow retention of the trees for longer in the landscape. This is especially important for ancient and veteran trees.

Where trees are felled we would encourage landowners to replace them with species considered to be suitable replacements for ash. Sessile oak, alder, aspen, field maple, sycamore, beech, birch, rowan, disease-resistant elm, hazel and hawthorn are all trees supporting a high number of ash-associated species.

There may be an opportunity to take better care of existing ash woodlands by putting eligible woodland into a Forestry Commission woodland management plan (the YDNPA offers a service drafting such plans) and the enlightened souls amongst us are already planting trees on their holdings in anticipation of the loss of ash in the landscape.

In summary:

Trees showing no symptoms, or only mild symptoms, should be left but monitored closely.

Trees with targets (i.e. those where a falling branch or tree could hit something valuable) that are showing some or all of the following may need to be felled or reduced to manage any hazards:

• Extensive dead and brittle wood in the canopy, especially where the disease has spread to the larger branches of the tree.
• Cracks on the underside of large branches or in the main stem.
• Lesions or cankers in the bark, especially at the base. If present, these lesions can dry out, the bark can peel away and cracks in the wood fibre can become apparent.
• Signs of secondary infection which could render the tree dangerous – such as basal or root decay fungus like honey fungus.
• Around 40% or less normal leaf cover in summer particularly where the remaining leaf cover is patchy/ clumpy and concentrated in the central part of the tree canopy

Find out more

More information can be found in a great guidance document published by the Tree Council, DEFRA and the Forestry Commission

Further recent guidance from Forest Research includes much more detail on managing trees with ash dieback with links to further resources

And don’t forget…

Tree work should be undertaken by a qualified arborist who understands the risks to felling ash trees.

Remember to consider restrictions to tree works such as felling licence requirements (diseased trees are not exempt), and trees protected by preservation order or conservation area status. Restrictions also exist concerning the timing of works and the impact on wildlife – bird nesting season is generally March to August. Bats use the canopy of some trees in summer for maternity sites and use tree cavities for hibernation in winter so these times of year should be avoided for trees with bat potential.