Author and University of Sheffield professor David Wood witnessed volunteers clearing vegetation in the Festival Wood area of the park in the suburb of Fulwood and expressed fears that they were removing tree and shrub cover vital to a range of nesting bird species.
The work is being carried out as part of a £300,000 restoration project with the support of site owners Sheffield City Council, which will also see a nearby lake de-silted.
Professor Wood says that previously, walkers were restricted to two narrow paths across the woodland and did not disturb wildlife because of the denseness of the vegetation.
However, the Friends of the Porter Valley said that they had been forced to provide more space for visitors because of pressure on walking routes during lockdown.
Professor Wood said: "The clearing of the undergrowth from a fairly extensive area of woodland has been going on for some weeks now. I saw a working party there recently and asked why they were clearing the area, through which I regularly walk from home nearby. I was told that it was being 'tidied up' because 'some people like to be able to see the lake from the path'. I pointed out that they were destroying habitat used by a range of breeding birds and was told that they were doing the clearance outside the breeding season and that there is plenty of other woodland for birds to breed in.
"The relatively dense nature of the undergrwoth meant that human traffic was almost entirely limited to the two surfaced paths that cross the area. The increased disturbance that will inevitably arise as a result of removal means that the breeding habitat (and success) of tree species is also certain to be negatively impacted."
Friends of Porter Valley chair Ann le Sage said: "Festival Woodland is a relatively small and very specific area of woodland above the Forge Dam in Sheffield and was planted 69 years ago, so next year it celebrates 70 years since its special creation.
"The Friends are the local parks group (all volunteers) and we work in partnership with Sheffield Council in the Porter Valley. This is a very beautiful linear parkland running upstream from Hunters Bar to the Porter Clough on the Peak Park fringe. We don’t do work that has not been approved by the council, which is the landowner.
"The whole length of the valley contains swathes of woodland that has been unmanaged for years and there is ample scrub, bramble, ivy and self-set tree habitat.
"Many passers through have been pleased to see the restoration of original sight lines in the Festival Woodland, but it is not always possible to please everyone. The valley is hugely popular at the moment because it is such a wonderful green lung for Covid constrained people. We do understand that there may be varying opinions and interests.
"Many of our volunteers are keen birders and walkers. Many are RSPB and National Trust members. We want to improve the habitat for wildlife and flora. Indeed we are about to expand our existing environmental activities in partnership with Sheffield and Rotherham Wildlife Trust with a formal nature monitoring project. Our other objectives are to restore the man-made heritage and enable people to understand and enjoy all that the valley offers.
"I am really concerned that the habitat in the valley is at risk for another reason; its huge popularity during lockdown has resulted in a massive increase in walkers, cyclists, runners and dogs, many off the lead. The paths, formal and informal, are thronged. Desire lines are being trampled through the emerging ground flora. Clearance and maintenance of the formal routes will, we hope, help to channel visitors."