Work has started to fell 5,000 trees infected with ash dieback at Westonbirt Arboretum.
The site's 200-acre Silk Wood has been closed for the removal of 2,000 ash trees over the next four weeks.
A further 3,000 trees infected with the serious fungal disease will be removed at a later date.
New species of other trees will be planted in the next five years to ensure the long-term future of the woodland near Tetbury, Gloucestershire.
Signs that ash dieback was infecting ash trees at Westonbirt were first seen in 2015 and some trees were removed in 2019.
The disease affects ash trees by blocking the water transport systems, causing leaf loss, lesions and leading to dieback of the crown of the tree, which makes it brittle and places it at risk of collapse.