Smardale Nature Reserve is set to be partially closed due to a major effort to counter tree disease in the area.
The Cumbria Wildlife Trust is carrying out major management of woodlands at the reserve near Kirkby Stephen.
The decisive action is in response to ash dieback, a disease affecting ash trees that is now prevalent across the United Kingdom.
The nature reserve will be closed from the Smardale end from Monday, October 23. It will still be accessible from the Newbiggin end, but the viaduct will be closed.
As part of this programme, the trust aims to remove ash trees that present a safety risk to the public due to their proximate location to walking routes.
The operation is in two stages, taking place between October and November 2023, and following a month hiatus, the project will resume from January to February 2024.
Reserves Officer at Cumbria Wildlife Trust, Andrew Walter, said: "The loss of large numbers of ash trees from Smardale Nature Reserve is a real shame, but it will give us the chance to restructure these wonderful woodlands and encourage other species to grow."
"When the railway was open, the track sides were kept clear of trees. However some of these areas of the nature reserve have had little management since the closure of the railway in 1960!
"If the trees aren’t near footpaths or the main track, we’ll leave them alone. This will give the best chance for ash seedlings that may be resistant to ash dieback to grow, and any trees with resistance to live on.
"Other trees will become standing deadwood, which is a wonderful resource for wildlife and fuels woodland ecosystems. Up to 40 per cent of woodland species depend on deadwood at some stage of their life, including many birds, mammals, fungi, plants and insects.”
The trust plans to utilise some of the fallen timber to offset part of the estimated £60,000 cost of the ongoing safety efforts.
Mr Walter said: “The loss of ash from Smardale and our other nature reserves will be devastating, but other wildlife will benefit.
"Ground flora will be given a boost, until the canopy closes over again.
"Other tree species will take advantage of the canopy gaps and in some cases the woodland age structure could become more varied, which is great for wildlife.
“We hope that some natural resistant regeneration of the ash will be possible too. We’ll keep an eye on those trees that appear to show resistance.
"Once we identify these disease-tolerant trees and look after them, we should be able to restock woodlands and the wider landscape with ash.”
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