Going, Going… Not gone yet...
Ash trees are in trouble.!
Asset managers might be too without an effective response plan…
Ash trees are a familiar sight in our towns and countryside. Ash Dieback Disease (ADD) was first noted in the UK in 2012 but has already become widespread. ADD kills Ash trees and in continental Europe has led to the loss of around 90% of mature specimens. This is the first in a two-part series discussing issues surrounding ADD and the implications for asset managers. In this article, I’ll describe the disease and how to spot symptoms.
What is dieback?
Dieback is a progressive loss of health and vitality in the crown of a tree resulting in the emergence of successively larger diameter deadwood. Dieback starts at branch tips and moves backwards towards larger limbs and the stem, hence the name. Various patterns can emerge indicating different underlying processes. The living part of the crown gradually contracts and if severe enough, the tree may die. Dieback itself is non-specific and a specialist may be needed to determine the cause, prognosis and treatment.
What is Ash dieback disease?
Also known as Chalara Dieback of Ash, ADD is a fungal disease caused by Hymenoscyphus fraxineus (formerly Chalara fraxinea) resulting in leaf wilting, progressive dieback, shedding of deadwood and tree death. Death can occur within one growing season but progressive decline over three to four years is more common. The fungus spreads by wind dispersal but introduction on infected nursery stock was also significant during early establishment. Approximately 10-15% of trees appear to have some tolerance, while 1-5% may be fully resistant: nearly dead trees can be found next to apparently healthy trees!
How big is the problem?
Common Ash belongs to a large genus of 65 species known as Fraxinus. Many Fraxinus species important to our rural and urban landscapes appear to be susceptible though some, such as American White Ash (F. pennsylvanica), may be less so. Since its initial foothold in NE Europe, ADD has rapidly killed 75 – 90% of specimens across the Continent. The latest UK data suggests a 75-80% mortality but there is some hope that Scottish trees may have greater tolerance. The numbers involved are staggering: there are an estimated 125 million in woodlands and 27-60 million in non-woodland settings. Saplings are prolific: there may be 1-2 billion of these! The Woodland Trust has estimated that the financial cost to the UK may be as much as £15 billion while the ecological cost is likely to be severe.
ADD - The Symptoms
What are the symptoms of Ash Dieback Disease?
The early symptoms of ADD can be difficult to spot and a professional tree survey can be vital to making a timely response. There are some fantastic on-line resources available shown in Box 1. Symptoms are described in Box 2. Broadly speaking, diseased trees may be classified into four principal categories based on the proportion of healthy crown remaining and the Tree Council is recommending adoption of this approach nationwide.
Planning may most effectively be undertaken while trees are in Classes 1 and 2. By the time a tree has entered Class 3, it may be shedding significant quantities of significant deadwood, is likely to be unsafe to climb and to drop large (injurious) deadwood onto operatives while being felled. A tree in Class 4 may be considered as a major hazard requiring a fully mechanised takedown.
Symptoms to look out for:
- Leaf spotting and wilt
- Curled blackened leaves remain on the tree until autumn
- Jet black leaf litter
- Small white fungal fruiting bodies on leaf litter (June – October)
- Dead twigs, then successively larger dead branches
- Diamond shaped bark lesions at unions of infected limbs
- Epicormic shoots (similar to suckers or water sprouts)
- Formulation of ‘pom poms’ of foliage
- Crown develops a ‘hollowed out’ appearance
- Basal stem lesions may or may not be present
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