Ash Dieback – how much should we worry?
It was big news in 2012 and warning notices appeared across the country. But should we still be concerned about ash dieback?
Centre for Ecology and Hydrology
Ash dieback, Chalara fraxinea, hit the headlines in the UK in 2012 following the discovery of infected nursery stock at various sites across the country in March, followed by reports of the disease in the wider countryside in November from woods in eastern and south-east England.
Chalara has been spreading westwards across Europe for the last 20 years (first recorded in Europe in Poland in the 1992) so it's arrival in Britain was no big surprise. Even in the absence of imported infected nursery stock (between 2003 and 2011 the UK imported more than 5 million young ash trees), the arrival of the disease, via airborne spores, was probably inevitable, and may well have occurred several years before 2012. By 31 March 2014, the disease had been confirmed at a total of 641 sites - 26 nursery sites, 349 recently planted sites and 266 locations in the environment in general e.g. woodland; see here for distribution map and other details.
The disease in ash was first identified as Chalara fraxinea in 2006, but this turned out to be the asexual stage of the fungus. The sexual stage, Hymenoscyphus pseudoalbidus, was subsequently identified in 2009 (although the fungus is still generally referred to as Chalara). The fungus appears to originate in eastern Asia, and has been found in ash in Japan without apparently causing disease. The infective agent of Chalara is the spores produced by the sexual stage of the fungus. Fruiting bodies, tiny white mushrooms, grow out of the old leaf petioles (the stalk attaching the leaf blade to the stem) on the woodland floor in summer and spores are released from about May to September, with peak production in July/August. Spore trapping suggests that most spores stay within 50 m of their source, but this probably says much more about our abilities to detect spores than their abilities to spread on the wind; with many millions being produced, it's not surprising that sufficient numbers to establish infection in the UK should have crossed the Channel (even in the absence of infected imports). Further bad news is that infected leaf petioles can support the production of fruiting bodies for several years. The spores enter trees through the leaves and the infection then travels down the petiole into the shoot causing lesions. Symptoms, typically dead leaves and shoots, take a while to become apparent - trees infected in the summer generally fail to show signs of infection until the following June. Young trees are most vulnerable, lacking the resources to repeatedly regenerate new growth, and may succumb rapidly to even a single infection. Older trees (and coppice regrowth) are killed by the build up of multiple infections, but whether or not mature trees can sustain chronic infection in the long-term is unknown. In a scheme in Sweden to protect veteran ash trees, of 330 trees, 62% were infected in 2009, 77% in 2011 and 84% in 2013 with mortality rates of 1.4% p.a. (2009-11) and 2.1% p.a. (201-13), with larger trees and pollards being less affected (so far). In addition to leaf/shoot dieback and lesions, infected trees can also develop trunk lesions (collar rot) at and below ground level, particularly in wet areas. Collar rot also occurs in trees lacking signs of dieback, raising the possibility that infection may also enter via the roots. Such trees, and those with overt symptoms of Chalara, are also prone to secondary infection (and mortality) by other diseases, especially honey fungus, and bark beetles.
Modelling studies have suggested that Chalara could spread across most of England by 2017 and on to Wales and southern Scotland by 2022, but there is obviously a great deal of uncertainty in this, and spread is likely to be sporadic rather than even. For example, 2013 turned out to be a good year for trees and a bad year for the fungus and hence spread appeared to be relatively slow. However, given the time lag in the appearance of symptoms, this could be a short-lived advantage. Experience of the disease across Europe, causing rapid and extensive infection/mortality, doesn't bode well for the UK. For example in Lithuania, the disease arrived in 1995-96, but the incidence of infection and tree death increased massively after 2000. Despite attempts at control by felling about 10% of ash is still dying each year. Initially, following clear-cutting, about 36% of regeneration occurred from stumps, but the regrowth was infected and died, and after 8-10 years all regeneration was from seed, most of which also dies (although there is some evidence of potential resistance - a rather severe natural selection experiment). Overall in Lithuania, ash has been reduced, so far, from 2.7% to 1.7% of forest and is being replaced by alder and birch, and to a lesser extent, by elm and aspen. In Latvia, sycamore is another common replacement species.
In much of Europe, ash is a relatively small part of the forest stock, but in Britain it is our second most common species, only a little less abundant than oak. Perhaps most importantly, although pure ash woods are relatively rare, ash is the defining tree species of the countryside in much of lowland England and Wales, being common and widespread in small woods, copses, hedgerows and as free standing trees - not to mention parks, gardens and road and rail corridors. In the 1960s, Dutch elm disease drastically altered the structure of the countryside, especially in eastern England, but curiously enough, knock-on effects of this on biodiversity appear to have been little studied. For birds, there was a temporary bonanza of invertebrates associated with dead wood, and a temporary increase in potential nest cavities, but by and large, bird populations just carried on. So, how much should we worry about the potential effects of Chalara on our woodlands, countryside and, ultimately, birds?
In the great scheme of things, and over longer time-scales, it could be argued that diseases have come and gone, and things will always change - conifers were replaced by deciduous forest as climate warmed and then the forests were massively reduced/modified by human activity and so on - but, although long-term history may ultimately look after itself, we have to live in the here and now. A landscape with many fewer trees will be a sadder place, and could take a generation, or several, to recover - even assuming that the raft of other tree diseases and pests already present or imminent can be contained. The former include several species of Phytophthora killing a range of tree species including larch, beech, alder and juniper, Dothistroma septosporum attacking pine, acute oak decline and oak processionary moth, Thaumetopoea processionea). Potential future problems include the emerald ash borer Agrilus glabripennis, and pine pitch canker, Fusarium circinatum. It is possible that mature ash may be able to tolerate chronic infection for many years, but from experience across Europe, mortality in the region of 95%, especially in young trees, loss of regeneration and eventual replacement by other species is likely. Safety concerns, e.g. in urban areas and along transport corridors, will also result in felling of trees that might otherwise sustain an infection, although chronically infected trees will remain as a source of infection. So, the answer to the question - how much should we worry? - is probably 'quite a lot-. And what to do?
If the disease becomes rampant and widespread, then larger woodlands might be best left to die back and regenerate naturally, perhaps with some intervention to encourage or discourage certain replacement species and/or to adjust tree density. One defining feature of ash woodland is its relatively thin canopy so thinning to allow light penetration might be required to maintain understory structure and ground flora. Bird species typical of mature woodland, especially those like Marsh Tit that require a diverse vertical profile with a good shrub layer beneath a tall canopy, will probably be the biggest losers in this context, but as is usual in ecology, there will also be winners. In this case, species associated with scrub and early successional woodland, many of which have lost out in recent years to woodland maturation and reductions in management (e.g. Willow Warbler, Garden Warbler, Nightingale) could find themselves knee-deep in potential new habitat, at least for a few years. Ironically enough, increased availability of scrubby young woodland could be a lifeline for the Willow Tit and an increase in invertebrates associated with dead wood a similar boon for Lesser Spotted Woodpeckers.
In the wider countryside (and urban locations), to maintain stocks of mature trees in copses, spinneys, hedgerows and as individuals trees, it might be a good idea to start planting replacement trees now - not by removing the ash, but as additional trees. This might require some additional land given over to trees, and more careful management to avoid damaging young trees in hedgerows, verges and fields, but should be well worth the effort. If the disease takes 5 to 10 years (or more) to really take hold in the UK, then planting now would significantly reduce the time required to replace mature trees. Experiments are underway to find ash that is resistant to Chalara, but this will take time (and may have limited success in the short to medium term at least) so planting now with other species suitable for the locality in question seems sensible. Ash in the wider countryside supplies birds with food resources (invertebrates and seed), cover, nest sites, roost sites and song posts and increases landscape connectivity, increasing the ease (and safety) with which birds can move around the landscape. Mature ash may be particularly important as a provider of cavities for both nesting and roosting. Waiting for ash to die before planting will reduce these resources for years, and change the character of the countryside - possibly a classic case of 'you don't know what you've got 'til it's gone.-
I have worked in Monks Wood National Nature Reserve (and many other woods) in Cambridgeshire for 25 years. The wood has regenerated and matured since being largely clear-felled in the 1920s. Although apparently disease-free in 2013, it is 65% ash and in the front line for infection. Changes following infection will no doubt be fascinating and, at some point in a regeneration process, I could hope for Nightingales to return, but watching half the wood die will be immensely sad, so I shall do my best to enjoy it as is for the next few years (Red Kites, Marsh Tits, Nuthatches, bluebells and all). Similarly, I would encourage you to plant some trees if you get the chance and to make the most of your ash woods while you can - with imminent disease, pests and climate change, they may never be the same again.
Information in this blog was derived from notes taken at the FRAXBACK meeting (Living with ash dieback in continental Europe: present situation, long-term experience and future perspectives) held at the Linnean Society of London, 29 November 2013, and JNCC Report no. 483, The potential ecological impact of ash dieback in the UK (Mitchell, R.J. et al. 2014, https://jncc.defra.gov.uk/page-6459 ).
Ash canopy © Shelley Hinsley
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