More than 70 volunteers working on Epsom Common nature reserve in Surrey have clocked up more than 3,000 hours of work this year.
Epsom council said its "local army of volunteers" had taken on conservation tasks such as checking cattle, making charcoal and monitoring wildlife.
Councillor Clive Woodbridge said the "vital" volunteers helped the site win its fourth Green Flag award in 2010.
The Green Flag awards are to recognise the country's best green spaces.
Adding that volunteers provided a much-needed supplement to the work of paid staff, Mr Woodbridge said: "This award is a credit to the hard work and commitment of the many volunteers who dedicate their spare time to look after such an important area."
He said work by the volunteers amounted to more than 500 days during 2010.