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Ash and Chalara fraxinea

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Ash and Chalara fraxinea

Britain’s woodlands are exceptional.  Our unique geographic position in the face of prevailing Atlantic weather, together with our varied geology and landscapes, has give rise to an outstanding diversity of wooded habitats for our relatively small land area. But today these native woodlands are under pressure from the impacts of disease. The latest is a fungal pathogen, Chalara fraxinea or Hymenoscyphus pseudoalbidus (depending on what phase of its life-cycle it's in), commonly known as ‘ash die-back’, which has been spreading across Europe since the 1990's. A key pathway for has been the international trade in ash trees. Despite a ban on the import and movement of ash since October 2012, including seeds, plants and trees in the UK, cases of the disease have been confirmed across the UK, indicating a more holistic approach to the problem is required.

Impacts on biodiversity:

The loss of ash trees will reduce woodland biodiversity. Certain British woodlands are of global importance – and that’s down to our lower plants – the epiphytic[1] lichens, mosses and liverworts that could be so threatened with the demise of ash. Ash is critically important for many epiphytic species. Of the 536 lichens that grow on ash in the UK, 220 are nationally rare or scarce; and 84 have been categorised as under threat in Britain1. A significant loss of ash from woodlands, hedgerows, and 'open-grown' situations e.g. parkland and ffridd2 will be particularly damaging for these plants, especially in the long-term. These include species of high conservation concern that are under threat from habitat loss, both internationally and within Britain. Examples of lichens include Fuscopannaria ignobilis (in Scotland this species is a conservation priority under the NERC Act 2006) and Wadeana dendrographa (in England, Scotland & Wales this species is a conservation priority under the NERC Act 2006), both these species are dependent on ash.

Fewer species of moss and liverwort are specifically associated with ash compared with lichens. However, ash does support a number of epiphytic species of high conservation concern, such as lesser squirrel-tail moss Habrodon perpusillus. This species is found at just 10 sites in the UK, and the majority of these are on ash trees. It is a priority species of conservation concern both international (IUCN Endangered) and within Britain. Prince-of-Wales feather-moss Leptodon smithii, listed on Section 42 of the NERC Act, is potentially vulnerable in Wales as its colonies are almost entirely restricted to ash.

The potential impact on mosses and liverworts growing in ash-dominated habitats is a further concern, where the loss of ash will change light and humidity levels. The endangered Atlantic Pouncewort Lejeunea mandonii3 is found at just 6 sites in the UK, 5 of which are in ash-dominated habitats or directly on ash.

Ironically, a number of species now under threat from Chalara fraxinea had been brought close to extinction by another tree disease - Dutch elm disease4. Ash has provided an alternative host tree to former 'elm specialists', such as the GB Red Listed orange-fruited elm lichen Caloplaca luteoalba and sap-groove lichen Bacidia incompta, as ash has a similar bark chemistry to elm. Thus, the loss of ash from woodlands has the potential to reduce the abundance of a whole group of epiphytes that cannot grow on the acidic barks of trees such as oak, hazel, rowan, alder and birch. The loss of ash from woodlands may lead to trees with denser canopies taking their place which would create darker woodlands and reduce diversity of the ground flora. Ash is also one of the most important trees for invertebrates in the UK, supporting over a 100 insects of mainly beetles, flies and moths including uncommon or declining ones. At least 40 species are wholly or largely dependent on ash in the UK5.

Therefore, the potential impacts of Chalara fraxinea reach beyond the decline of ash trees, threatening the populations of a range of plant species dependent on ash as well as profoundly altering the habitat of ash-dominated woodlands. Furthermore, the epiphytes themselves perform important ecosystem services. For example, bryophytes regulate water by absorbing rain, slowing its journey to the ground, and maintaining humidity in the atmosphere. They also have a role in nutrient cycling by changing the chemical composition of rainwater that passes through the epiphytic bryophyte layer. However, the precise role of different moss and liverwort species in nutrient cycling still needs to be clarified. Finally, as plants act as both important refuges and food sources for invertebrates and animals, the loss and decline of species associated with ash could have damaging ramifications for other wildlife.

While it is feared that Chalara fraxinea will destroy all our ash trees, the more likely scenario is that a significant proportion of trees (probably between 60-90%) will get infected and the disease resistant trees will survive, producing the next generation that will have much greater resistance to this fungus. Older ash trees are unlikely to die from the disease; instead their new growth is likely to die back, stunting the tree’s overall growth and health. Those that do die will take years to do so. However, seedlings and young ash trees are more susceptible and are likely to die from the infection. Thus, young plantations and areas with natural regeneration could be badly affected. This loss of natural ash recruitment is likely to have a delayed impact on epiphytes of ancient ash but eventually the lack of suitably aged basic-barked trees will be felt.

Given the potential impacts of this disease on lichens, mosses and liverworts Plantlife has a number of recommendations:

  • No felling of mature ash trees other than for safety reasons. Inadvertently removing resistant varieties may hamper the ability to produce the next generation of Chalara fraxinea resistant ash.

  • Ancient and veteran ash trees be protected and retained even when infection has been confirmed as dead and decaying ash trees are valuable for epiphytes and other wildlife.

  • Optimal management of veteran and older ash trees and their environs to minimise stresses and increase their resilience to the disease.

  • A primary consideration is the provision of suitable 'replacement' trees for ash-supported lichens and bryophytes e.g. to provide a substrate with similar bark characteristics to ash. Different trees will do different jobs e.g. sallow and hazel field maple can provide an alternative host for some lichens, mosses and liverworts relatively quickly.

  • Guidance should be sought when planting/replanting in the vicinity of ancient or veteran ash trees known to support epiphytic floras. Small changes in microclimate can have harmful effects of epiphytes.

  • Sycamore should not be planted as a replacement. Sycamore is likely to be the main ‘beneficiary’ of Chalara fraxinea through natural regeneration and colonisation. In years to come it may form a larger component of British woodlands. Sycamore has a similar bark chemistry to ash and is an important host for epiphytic lichens e.g. Lobarion.

  • The widespread planting of disease-resistant strains of ash should be resisted until the full extent of natural resistance within British ash populations is known.

  • Where possible, natural regeneration should take precedence over tree planting. This is the genetic key to instilling natural resistance and resilience to disease in all woodland vegetation, but critical for trees in their role as woodland ‘creators’. In the case of ash, it will preserve its natural distribution across the UK as well as maintaining natural local provenance.

  • Models developed to prioritise mitigation efforts by region need to take full account of the biodiversity value of woodland ash, including priority (NERC Act 2006) and red-listed species, both directly and indirectly dependent on ash.

  • Plantlife does not recommend the use of fungicides to control Chalara fraxinea within natural habitats. Chemical treatments are being tested to determine their effectiveness against Chalara fraxinea, both in treating leaf litter and live trees. Fungicides may unavoidably harm native fungi and lichens that grow on, or in the vicinity, of ash trees. Such treatments could also damage natural fungus-plant interactions in the soil (mycorrhizae) that perform crucial ecosystem functions in woodlands.

  • Trees that are sourced for planting should be sourced locally or be of British provenance.

  • Nurseries stock needs regular monitoring to certify that it is free of disease and pest.

  • Increased understanding of the disease is urgently required and should be started without delay. Plantlife is calling for greater practical measures to be trialled by the Forestry Commission so as to learn the best actions for reducing, or at least delaying, the progression and intensity of the disease.

  • Chalara fraxinea surveillance and monitoring schemes should not confine themselves to woodland but include ash trees of open habitats such as ancient parkland, fields, wayside and hedgerows. Non-woodland habitats are important for ancient or veteran ash. Plantlife is concerned that DEFRA’s Chalara Management Plan will not address such habitats until as late as March 2014.

  • Surveillance schemes need be coordinated so that data can be readily shared and integrated.

Research questions that need answering include:

  • How is the risk and rate of Chalara fraxinea spread correlated within canopy density and age of stands?

  • What is the relationship between coppicing /pollarding and the spread of the disease?

  • Is there natural genetic resistance in our native ash, and how should resistant trees be identified, propagated and planted?

  • Is there a relationship between mycorrhizal associations and disease resistance?

  • Do woodlands that are actively managed suffer less from the disease because leaf litter is disturbed?

[1] Epiphytic plants grow on trunks and in the branches of other plants rather than rooting themselves to the ground. They are not parasitic. They gather nutrients from the air or water around them, relying on their host to provide physical support.

1 R, Worrell (2013) As assessment of the potential impacts of ash dieback in Scotland. Commissioned by Forestry Commission Scotland

2 Ffridd is used to describe the habitats present between the upland and lowland habitats of Wales. It is a diverse mixture of grass and heathland with bracken, scrub (often hawthorn and gorse) or rock exposures and may also include flushes, mires, streams and standing water.

3 Atlantic Pouncewort Lejeunea mandonii is a Species of Principal Importance in England (section 41) and Wales (section 42) of the NERC Act.

4 Dutch elms disease, also caused by a fungus known Ophiostoma novo-ulmi and is spread by the elm bark beetle, has resulted in very few mature elms, such as Ulmus procera, surviving in the UK. More than 25 million trees are thought to have died in the UK alone, with the species now only appearing to survive in hedgerows.

5 R, Worrell (2013) As assessment of the potential impacts of ash dieback in Scotland. Commissioned by Forestry Commission Scotland

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