A pile of woodchips was dumped in Milford Hollow, a conservation site in Salisbury managed by the Milford Area Preservation Group (MAPG).
The woodchips were left on top of fragile ferns and other native woodland plants.
Volunteers spent hours removing most of the mound, but many of the plants beneath had already been destroyed.
David Lovibond, Chair of the MAPG, said: “Our volunteers have spent years restoring and curating this vulnerable site and have created a haven for wildlife, and a place of peace for local people to enjoy.
“Unfortunately, not everyone respects this precious area, and the fly tipping is not the first time the Hollow has been vandalised.
“A couple of years ago newly planted saplings donated by the Woodland Trust were stolen, and there is an enduring problem with littering.”
Salisbury City Council, which supports MAPG, confirmed that the fly-tipper accessed the site using an unprotected path opposite St Martin’s School.
Mr Lovibond said the group has requested that the council consider installing a bollard at the top of the path leading to the site from Shady Bower to prevent further vehicle access.