By Henry Shaw,
Agri-Environment Adviser, CAFRE
OVER the past few years ash trees on our farms (like elsewhere) have increasingly shown signs of being sick. It is not just drought – this damage is being caused by a highly transmissible, wind-borne fungal disease.
Ash dieback disease was first identified in Ireland in 2012 having been introduced in trees for planting at that time. Before it arrived in Ireland it had been spreading for 25 years across Europe.
This disease is commonly called Ash Dieback Disease. Death rates of affected trees are very high, and, therefore, this disease is expected to lead to the decline and possible death of the majority of ash trees in our countryside. I suspect we have all now seen trees on our farms that are impacted.
Unfortunately Ash Dieback Disease is usually fatal, although the time it takes to kill a tree depends on a range of factors.
Experience from mainland Europe is that once the first symptoms of the disease become apparent it kills ash trees in several years.
Ash trees of all ages are expected to succumb to this disease and as a consequence of the abundance of Ash in the landscape our farms will lose many of their hedgerow trees (as well as Ash in woodland).
This will change our farmed landscape massively and will also create a host of less obvious effects as well. The loss of so many trees will reduce available shelter and shade for livestock, water retention for soils stability and flood prevention and, of course, wildlife – in particular the birds, bats and multitudes of smaller animals and insects that inhabit these familiar trees.
Ash trees exhibiting disease show symptoms including blackening and wilting of leaves and shoots in mid to late summer (July to September). These months are the best time of year to survey ash trees for symptoms in the foliage and help to avoid autumn changes in the colour of the leaves which could be mistaken for symptoms of the disease.
As the disease progresses and the plant’s circulatory system is progressively blocked, whole branches die back and the tree becomes “stag-headed” (where the live canopy shrinks back leaving dead – and brittle – branches sticking out like antlers). There may also be bark lesions and once infected the tree is much more susceptible to other fatal infections and rots such as honey fungus.
Infection mostly occurs through fungal spores landing on leaves, but infection can also occur at the base of trunks.
As it grows, the fungus destroys the infected tree’s transport channels, which results in the tree being unable to move water and nutrients around its structure. This lack of water and nutrient movement will cause the branches of the tree to fail and the tree ‘dies back’, hence the name.
Repeated loss of nutrition and water, the depletion of energy reserves because of the lack of leaves, and the invasion of secondary root killing pathogens (eg, Armillaria – honey fungus), cause the tree to become brittle, lose branches and eventually succumb to the disease.
A very few Ash on your farm may have a natural genetic resistance and it is perhaps worth making a note of any healthy ash trees as their offspring potentially may be important.
Several studies have reported that a low proportion of ash trees (1-5 per cent of the population) may be resistant.
Within your woodlands it is recommended that the best trees are retained (ie, those with minimal crown damage and no root collar lesions) to facilitate possible long-term adaptation of ash populations to ash dieback disease through potentially tolerant genotypes. This would seem to be a reasonable approach on farms as well – but only if it is safe to do so.
Ash trees can rot quickly and even if not showing obvious canopy dieback, can be significantly weakened internally. This obviously complicates your dealing with the potential risk on your farm – one that could impact life or property depending on location.
The extensive presence of ash dieback within the UK, Ireland and the EU means that it is no-longer classified as a quarantine pest in plant health legislation. Therefore monitoring and surveying of ash trees for any change over time is a matter for individual landowners at a local level consistent with their duty of care responsibilities.
This means that all landowners need to take an active approach to managing risks – and taking appropriate action when you identify a particular hazard. Two equally sick trees present different risks – a tree alongside a busy road or near a house or building, or where there is public access is a much higher risk than one equally sick located in an isolated field – even if the hazard is similar.
Practically, you must inspect your trees and establish the risk they present – you are responsible.
You must be able to prove that you did so – perhaps even if it is just a diary note of a visual inspection and not a full formal professional inspection so that you demonstrate that you have taken due care.
If you feel that a tree represents an unacceptable risk and that you are going to have to fell it, be aware that a weakened tree is unpredictable to deal with and thus very much more dangerous to tackle. In all cases, tree felling and associated work should only be carried out by suitably trained and competent individuals.
Human welfare is not the only consideration – also be aware that birds and bats are protected under legislation and are to be found in many of our hedges and older woodlands.
If bats are present a survey will be required. Wild birds and their nests are also legally protected under the Wildlife (Northern Ireland) Order 1985 when the nests are in use or under construction.
Some birds such as the barn owl and red kite re-use their nests and therefore their nests are protected at all times.
If a lot of trees are involved you may also need a felling licence as set out in the Forestry Act (Northern Ireland) 2010 although there are exemptions – consult Forest Service if you have any doubt as to the requirements.
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