Up 150,000 larch trees in Caerphilly’s Cwmcarn Forest are to be felled after becoming infected with a fungus-like disease.
Natural Resources Wales said the action was aimed to slow the spread of phytophthora ramorum, which kills the tree once infected.
Preparatory work has now begun in advance of major felling operations.
The Forest Drive will shut while the trees are felled but the visitor centre and other facilities will remain open.
About 6.7 million larch trees have been hit by the disease in Wales.