If you go down to the woods today, you are in for a big surprise – blue-spotted ash trees!
The Trust are currently assessing the extent of the dieback on ash trees known as chalara, which has been at Ashridge for some five years to a limited extent. First located in England in 2012 at a garden centre in Buckinghamshire on imported stock, it is now widespread throughout Britain. Trees on the estate which are subject to the fungus and pose a future risk to the public are marked with a blue spot ready for winter felling.
This fungus originates from north-east Asia where it lives in balance with its natural host the Manchurian ash without causing any significant issues. It was not until the host was introduced into Europe some thirty years ago that it was realised the pathogen could cause the rapid death of European ash. It is often the case that innocuous fungi, bacteria and insects go on to cause major problems once they get to new locations and find hosts that have not evolved defences against them.
Chalara has been ravaging European woods for over twenty years but DEFRA were unable to ban imported saplings because of EU rules at that time.
The woodland at Ashridge extends to some two thousand three hundred acres – some 50% of the estate – where ash trees represent 10% of the timber and oak and beech predominate at 20% each.
Ash trees have always been prominent at Ashridge as the medieval name of Asscherugge in the 13th century was descriptive of the place – it is alarming to think that they might not be around after seven hundred years. Ash was once one of the most widespread tree species in Europe but now it is on the red list of the International Union for Conservation of Nature. The Woodland Trust expects chalara to kill off 80% of ash trees over time – so serious an issue the government held a COBRA meeting in 2012.
Without a cure we must endure
Hopefully there may be some innate tolerance to ash dieback disease in the UK’s ash population.
With no chemical treatment available to kill off the fungus, it is hoped that a resistant strain will evolve to replace the lost trees. Studies have shown that some ash trees possess a genetic tolerance to the disease. Some trees that are fit and healthy and are not subjected to high levels of spores and deer predation may survive and pass on their fitness to the next generation – ash seeds, known as keys germinate readily.
The fungus overwinters in the leaf litter on the woodland floor producing white fruiting bodies between July and October, which then release their spores into the surrounding atmosphere. In ideal conditions spores can blow many miles. When spores land on developing ash leaves they adhere and penetrate the leaf and beyond. The fungus then grows inside the tree, eventually blocking its water-transport systems leading to its death. Though the tree can block the infection and fight back to some extent, repeated infections will eventually kill the vast majority of trees. Chalara attacks trees indiscriminately – saplings and young trees die quickly but older trees can take a number of years to succumb.
The National Trust policy at Ashridge is to “identify and manage woodland compartments of ash trees and to remove or mitigate infected trees and implement phased species restructuring of ash dominated areas”
The ash tree has a high biodiversity value with over one thousand species associated with the tree, of which forty four are totally dependent on Fraxinus.