wildlife expert chris packham is calling on the royal family to rewild their estates.
more than 100 children joined mr packham in delivering a petition signed by 100,000 people to buckingham palace.
Mr Packham told the BBC he wanted the royals to take "more dramatic action", including rewilding.
The children, their parents and other campaigners carried the Wild Card campaign petition calling on the Royal Family to rewild their estates to the palace on Saturday.
they said the royal family should lead by example before they appear as ambassadors at the cop26 climate summit in november.
"That's double the size of Greater London and 1.4 % of the UK's land surface.
But instead it is largely managed for grouse shooting and deer stalking.
If it were rewilded, he said it "really would be a temperate rainforest, filled with a much richer diversity of life".
Mr Packham said rewilding would help the rural economy, and, if the Royal Family supported his call, it would send an important message to the world.
Campaigners said while the average tree coverage is 37 % in the European Union, the Duchy of Cornwall estate owned by the Prince of Wales has only 6 % tree coverage.
If rewilded, the royal estates could reintroduce beavers, wolves, bison, wild boar, pine martens and white storks, ecologists believe.
Royal-owned territories include the Crown Estate and the Duchies of Lancaster and Cornwall.
"This is a time for action," he said.
"this is a very powerful family, they have a very powerful voice, and they could do something very powerful.
A 14-year-old, Simeon, was allowed behind the palace gates to deliver the petition to staff.
the queen and prince charles are due to attend cop26 in glasgow in november.
"that's a very large estate, 800,000 acres.
"So that land is not working to its greatest advantage to us at this critical point."
He also said that, overall, royal land had less forest cover than the national average.
Leading by example is the best way to lead and a lot of people follow their example," he added.
The submission of the petition, which was preceded by a march from Green Park tube station to the palace accompanied by a jazz band and the environmentalist choir SOS from the Kids, is the latest action by Wild Card.
A Royal Estates spokesperson said : "Members of the royal family have a longstanding commitment to conservation and biodiversity, and for over 50 years have championed the preservation and development of natural ecosystems.
"The royal estates are constantly evolving and looking for new ways to continue improving biodiversity, conservation and public access to green spaces, as well as being home to thriving communities and businesses which form part of the fabric of the local community."
Chris Packham has backed a popular petition, which has received more than 100,000 signatures, asking the royal family to rewild their land with more trees and fauna.
The campaign demands that 50 % of the UK be fully rewilded, but has called on the Royal Family as the UK's biggest landowner to act first.
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