The National Trust stepped up its fight against the expansion of Stansted Airport on Sunday by encouraging the public to oppose the plans.
The trust is hoping to protect Hatfield Forest in Essex
The trust is hoping to protect nearby Hatfield Forest, Essex, from the proposed second runway at Stansted.
It is launching a leaflet, website and petition to highlight the importance of the forest and the threats it faces.
The trust said the forest is a Site of Special Scientific Interest where there are trees more than 1,000 years old.
On Sunday members of the trust were running Time Flies - a family event to highlight the history of Hatfield Forest and the "environmental threats" the airport poses.
Adrian Clarke, Hatfield Forest property manager, said: "The forest is a vital community resource not just because of its history, but also because of the space and tranquillity it offers.
"If plans to expand Stansted go ahead, increased noise and pollution would mean that this role is badly undermined.
"The environment of the forest may also be under threat.
"We have important wildlife, much of it associated with the 850 veteran trees, some over 1,000 years old.
"We are concerned emissions from aircraft and road traffic associated with the airport may already be damaging the forest, and this could be worsened by airport expansion."
The trust is asking supporters to sign its petition and to explain why the forest is special to them.