Volunteers spent hours planting the seeds at Mengham Park last year, and it is estimated that at least 30 of them were driven over yesterday.
Richard Coates, 77, who planted them alongside a ‘very large group of people’, said a lot of repair work needs to be done.
He said: ‘It’s going to have to be a big replanting session.
‘The danger was always that this would happen, and exactly what we thought would happen did. They just trampled and drove all over them.’
It is the not the first time the saplings have been harmed, with ‘mindless vandals’ damaging some saplings 400 last year.
Plants were provided by the Woodland Trust, and planted by Heart of Hayling Boxing Academy and Hayling Tree Wardens, last year.
Mr Coates added the section which borders a footpath, and near a children’s playground, were destroyed by the travellers as they mowed over them.
Supporting canes and see through plastic cones have also been ‘smashed’.
He said: ‘That whole one side is probably going to have to be redone when the travellers have gone.’
Mr Coates wants concrete blocks in the park as barriers.
The 77-year-old said he saw the remnants of a fire in the park, and beer cans strewn across the field.
He added: ‘It’s disappointing, but we’ll press on and replant as often as we have to.’
Leader of Havant Borough Council, Cllr Alex Rennie, was furious.
In an email to Mr Coates he wrote: ‘This is absolutely disgusting.
‘In addition to the defensive work, I will be writing to the Chief Constable to ask that the police use the full range of powers to tackle illegal encampments. We cannot let these people get away with criminal damage.’
A police spokeswoman said: ‘We are aware of an unauthorised encampment at Mengham Park on Hayling Island and our officers have attended the site to assess the situation.
‘We are working closely with the landowners, Havant Borough Council, to seek a suitable resolution.
‘Minor damage has been reported to four tree saplings at the location and officers have spoken to those involved and issued a community resolution for criminal damage.’