Fields protecting ancient woodland will have to be sold if a conservation charity falls short of funding targets.
The Woodland Trust has bid for land at Bardney Limewoods near Horncastle in Lincs and has 12 months to raise the £1m needed to secure the sale.
The 200 acres will be planted with new trees to protect some of the oldest woodland in Britain.
The trust is hoping to raise £600,000 from a public appeal and the rest through grants and sponsorship.
Project manager Peter Lowe said: "The proposed Wragby site will help buffer and safeguard one of Europe's most valuable concentration of ancient woodland around Bardney Limewoods. "