As I write this (October 2019), autumn is now in full swing – Beech and Horse Chestnut are turning their rich golden browns, Hawthorns are covered in haws and the countryside is transforming from its summer greens to much warmer tones. However, for one tree species, this autumn is not one we are used to. Ash trees on a large scale are experiencing the first really obvious symptoms of the chalara ash dieback introduced to the Society by Jane Hargreaves in the 2017 Bulletin. Many young ash are now no more than bare sticks, with twigs often showing the copper colour characteristic of affected trees (See Figure 1). Such twigs will be found to be completely dead and unexpectedly brittle. Even some
large and mature trees have become prematurely skeletal or are fringed by dead twigs. The spread of this disease is quite shockingly rapid and its tragic impact on a very significant tree in the Dales countryside so soon after chalara’s first arrival is far greater than was initially expected by many. The purpose of this article is to update Jane’s paper and to bring to members’ attention the current situation regarding chalara ash dieback with particular reference to the local impact, which will be significant.
The Disease and the National picture
For those unfamiliar with the chalara ash dieback; it is a fungal disease and kills trees in a similar way to the Dutch Elm disease of the last century; the flow of nutrients under the bark is disrupted by the fungal mycelium causing death of the tissue. However, unlike Dutch Elm disease, it is not spread by a beetle, but by wind dispersed spores. Initially it is young growth that is infected, but the fungus then grows back down the limbs of the tree to the trunk, killing growth above it. In Britain, it has been introduced across large areas of the country by the planting of affected, often imported, stock. It has then been wind-born from such plantations in to the wild tree population.
To remind you, chalara definitely arrived in only 2012, although there is some evidence of earlier, unnoticed, arrival. In 2013, there was no obvious sign of it having arrived in the Dales. In 2014, it was mostly in sites that had been planted, although it had been found in ‘wild’ trees in the Ribble Valley and confirmed as being in a local 10km square, SD96. More locally, volunteers with the Grass Wood Supporters Group were only being trained in chalara recognition at the end of 2014 in preparation for monitoring its presence in the Wood. By 2016, it was reported as being throughout the Wood. Now active management of affected trees is underway. More on that below. The Forestry Commission has an interactive map depicting the spread over time of confirmed chalara infections. It is now present in over half of the UK and is still spreading. Nationally there is considerable work being undertaken with a strategic approach to research and management. I do not intend to summarize all that here, but if you are interested in further detail, the Forest Research webpage on Ash dieback (Hymenoscyphus fraxineus) is a good place to start and The Tree Council has recently published an Action Plan Toolkit with much practical advice and information for landowners. However, it is worth repeating current understanding from the former webpage:
- the spores are unlikely to survive for more than a few days;
- spore dispersal on the wind is possible from mainland Europe;
- trees need a high dose of spores to become infected;
- spores are produced from infected dead leaves during June to September;
- there is a low probability of dispersal on clothing or animals and birds;
- the disease will attack any species of ash;
- the disease becomes obvious within months rather than years;
- wood products would not spread the disease if treated properly;
- once infected, trees cannot be cured; and
- not all trees die of the infection – some are likely to have genetic factors which give them tolerance of, or resistance to, the disease.
Research in to resistant trees is progressing, starting from trees observed to have some degree of resistance. Other strands of research are also underway. However, even if successful, the landscape benefits are unlikely to arrive for many years. One project hopes to have seeds for trees with some tolerance by 2030. We then have to wait decades for such trees to start to replicate the ecological and amenity value of current trees.
The local impact
Ash is the dominant tree of semi-natural woodland in the Dales and by far the commonest field tree. As such, its loss is going to have a very significant impact both in terms of the changed characteristic landscape appearance and also for its important effect on natural history. Figure 2 shows the contrast between two adjacent trees in Threshfield Quarry.
One may be resistant, let us hope so, or has been merely lucky. Ash can die from many causes and the bare one clinging to a few leaves on its lower branches may be dying through other causes, but it presages what we shall shortly be seeing much more. Strid Wood is another local wood that is already showing considerable effects amongst its younger trees (See Figure 3).
Ash comes in to leaf later than many other forest trees, casts a light shade and loses its leaves with the first frost, if not before, the leaves breaking down more quickly than some other trees. As a result, it supports a specific and uncommon plant community in the limestone uplands. Close to home, Grass Wood is classified as a Site of Special Scientific Interest (SSSI) in large part because of the ground flora arising from these conditions. Ash also directly supports a wide range of fauna from the caterpillars of Lepidoptera through to Bullfinches. Mature trees provide an assortment of nesting opportunities for our more interesting birds such as Redstart, Nuthatch, and Tawny Owl. A good picture of the seriousness of the impact of chalara ash dieback is conveyed in this article written by the Yorkshire Dales National Park Authority (YDNPA) Media Officer, Andrew Fagg, and informed by Geoff Garrett, the YDNPA Senior Trees and Woodlands Officer. The YDNPA is coming to the end of strategic action to increase woodland cover by 2000 hectares and much of that planting contains ash. Figure 4 shows the effect
of the disease on a young plantation and that much of this strategic effort is about to go to waste. Beyond its natural history and landscape importance, ash is a valuable timber tree and the demise of ash will be of considerable direct economic consequence in the timber and furniture industry.
Management of Ash in the Dales
As there is no cure for chalara ash dieback, management is restricted to mitigation of the affects. Given the extent and seriousness of chalara dieback, there are major management implications for landowners and these can be illustrated by the approach being taken by the Yorkshire Wildlife Trust in Grass Wood. The wood is bordered by a public highway, contains a Public Right of Way and numerous informal paths. Experience elsewhere shows that infected trees not only die, but also rapidly become infected by secondary pathogens such as Honey Fungus, resulting in trees becoming brittle and prone to unpredictable failure. This propensity poses a risk to woodland visitors of all kinds – forestry and conservation workers, the public and, when roads are involved, traffic. One group of workers particularly at risk will be those with the job of felling trees in order to protect other woodland users.
The unpredictability of infected trees means that they soon become dangerous to fell by traditional chainsaw methods. When that happens, landowners are faced with expensive specialist machine-based felling. Consequently, trees identified as potential risks to people are best felled sooner rather than later. Such precautionary felling has just started in Grass Wood and this has confirmed the fears that infected trees rapidly succumb to secondary infection and rot. A little further afield, the Royal Horticultural Society is taking similar precautionary
action at its Harlow Carr garden, as shown in Figures 5 and 6, where a large ash tree has been reduced to a monolith. This is part of a Nation-wide picture of preventative measures, e.g. the Bishop’s Park, Abergwili, is felling all its Ash.
Fortunately, this will not be happening in Grass Wood where infected trees that are not in danger of falling on to major paths will be left as standing dead timber. Complete clearance of dead ash is neither needed nor affordable. Simultaneously, trees will be monitored in order to try and identify any that have a natural resistance. Ash seeds prolifically and there is considerable regeneration in Grass Wood. It is to be hoped that some of this will have a genetic makeup conferring a degree of tolerance that could be used in a future breeding programme. You may have noticed that the Woodland Trust has also marked up its roadside trees in Lower Grass Wood for felling. The question yet remains as to exactly what species mix, if any, will be used in re-planting to make good the loss of ash. Left to its own devices a wood such as Grass Wood is likely to go through a period of intense regeneration by pioneer species such as birch and eventual transformation in to a beech/sycamore wood. This will have very different ecology to the present day with a considerably impoverished ground flora and dependent fauna.
It remains to be seen what actions other Dales landowners take, but roadside Ash pose a particular problem, either for the adjacent landowners or for the Highways authorities. An extended period of intermittent road closures may be about to start and we should all be more cautious in our driving, particularly in high winds. We can be sure to see, just as after the spread of Dutch elm disease and the Great Storm of 1987, a glut of firewood on to the market. This will lead to a sudden release of carbon that will take a long time to be recaptured by replacement trees.
Lessons to be learned
To me, one prime message from the chalara ash dieback tragedy is that of the risks associated with global trade in biological material. From the earliest episodes of exploration, humanity has been introducing species to new areas of the world with often-disastrous results. From the British exporting rabbits to Australia to the Victorians returning from abroad with such plants as Japanese Knotweed, the dangerous consequences of moving biological materials without great care have long been known about. And yet Britain continues to ignore the potential benefits of being an island and allows quite unnecessary imports. Ash grows prolifically in Britain and the only reason why so much of our planted ash came from abroad was short-term commercial pressure. Such ‘savings’ now look foolish in the face of the potential £15 billion cost of ash dieback. To reinforce this message; there are many other pests and diseases, usually introduced through trade, which threaten our trees, so that even replacement plantings for ash are not necessarily future proofed. The best we can do to build-in some robustness in the event of future problems is to replant with a mix of species grown from native British stock known to be disease free. It is my view that nationally we need to take biosecurity a good deal more seriously, follow a precautionary approach, and act much more swiftly in the event of suspected problems.
All images – Chris Alder. Text and images © UWFS
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