More than 6,500 larches are to be felled in Belvoir Forest in a bid to save the oldest known trees in Ireland.
Around a quarter of the forest is to be cut down later this summer after trees were found to be infected with the virulent Phytopthora ramorum fungus, which has devastated larch plantations across Northern Ireland.
Some 220,000 trees have now been felled across the province after becoming infected, including forestry in the Glens of Antrim, Castlewellan and Slieve Gullion.
P. ramorum, which originated in China, can kill many tree species, but appears to attack Japanese larch first. It is a very aggressive disease that can spread from one wood to another on a moist wind or in streams, and even by birds, vehicles and people.
Forestry staff have stressed that Belvoir Forest will remain open to the public when felling takes place from the end of July.
Although there will be restricted access to areas where forestry operations are taking place for safety reasons.
Forest Service director of forest management Ian Irwin said: "We picked up the symptoms at the end of last year in the forest and from that time on we've been putting plans in place.
"Belvoir is a fairly important forest from an environmental point of view because of the ancient oaks and because of the level of public usage. We are going to have to fell the larch in the forest as the most effective way of preventing spread."
The plans to curb the disease are a delicate balancing act between completing the work before the fungus spores at the end of September and minimising the impact on wildlife and nesting birds, he said.
"We're working very closely with NIEA and the RSPB. We are planning to commence harvesting as late as possible so that the impact on the birds is reduced.
"Just over 20 hectares will be affected, which is about a quarter of the forest. The larch is a significant component of the forest and we have found symptoms through different parts of the forest."
Forest Service is finalising its plans, which include holding a public information event before it starts felling. It is working with NIE as one of the main electrical lines into Belfast passes through the forest.
Individual plans have also been drawn up to protect each of the 56 ancient trees that have been growing there for more than 400 years.
The public visiting the forest will be asked to observe good biosecurity by cleaning muddy boots before visiting other sites, and not taking branches away.
"The felling does allow other opportunities. Belvoir is a very important forest from an environmental point of view and we will be able to start to develop a new type of woodland with more native trees in the next rotation," Mr Irwin said.
Belvoir Forest is part of the former Belvoir demesne which was created in the early 18th century. It supports one of the largest concentrations of ancient trees in Northern Ireland and is home to the oldest known trees in Ireland. Tree ring studies reveal that some of the trees at Belvoir pre-date the demesne, with one tree stump dating as far back as 1642. Some trees may be older, as many of the older trees become hollow and unsuitable for tree-dating techniques.