The final tree is being planted in England's largest new native forest, the Woodland Trust said.
More than 600,000 trees have been planted over 10 years at the Trust's Heartwood Forest site in Sandridge, Hertfordshire, by 45,000 volunteers including more than 17,000 school children.
The forest has been created from 347 hectares (850 acre) of arable land, with new young woods linking up pockets of ancient woodland alongside wildflower meadows, hedgerows and open grassland.
The Woodland Trust said wildlife had blossomed at the site, with the number of bird species spotted doubling from 35 to 63 over the last five years, including hen harriers sighted for the first time.
Sightings of threatened linnets have increased by 250% and butterfly populations have increased by 160%, while water shrews, barbastelle bats, common lizards and grasshopper warblers have made their home there.
Lady Verulam, the Lord Lieutenant of Hertfordshire, who planted the very first new tree at the site, is returning to plant the last, along with a team of volunteers, the Woodland Trust's chief executive Beccy Speight, and chairwoman Baroness Barbara Young.
Ms Speight said: "This is a big moment for both the Woodland Trust and for nature in the UK.
"It's quite remarkable what has been achieved at the site - we've taken 347 hectares of arable land and, aided by planting from volunteers from all walks of life, turned it into England's largest new native woodland, open to all.
"So many people have been involved from the local community and further afield - school children, businesses, religious groups, the public and our very own volunteers.
"It has achieved something amazing, a green and natural place where everyone can find space, peace, wildlife and miles of beautiful young woodland to explore."
The site features a community orchard, with 600 fruit trees, and a Disney Magical Wood, which saw a visit from Mickey Mouse.
In 2009 the BBC's tree o'clock challenge saw 100 people plant more than 20,000 trees in an hour, and in 2010 Princess Beatrice planted trees with school children.
Visitors to the forest, which has swathes of bluebells in the ancient woodland areas in spring, have grown from 45,000 to 140,000 a year in the last 10 years.