An angry and bitter row has erupted over plans by the Royal Society for the Protection of Birds (RSPB) to expand the remnants of Scotland's ancient Caledonian pine forest in the Cairngorms by planting more than 70,000 trees.

Three leading experts say that mass planting will destroy the unique naturalness of the forest and break its historic link to the woodlands originally established thousands of years ago after the last Ice Age. Claims by the RSPB that it is restoring the forest are "false and not credible", they allege.

But RSPB Scotland points out that its famed forest nature reserve at Abernethy, between Aviemore and Grantown-on-Spey, is not really a pristine wilderness as it's been grazed, ploughed, planted and fenced over the last 200 years. The criticisms are "wildly inaccurate" and the way they are being delivered is "extremely bad manners", it says.

Abernethy forest is the largest native Scots pinewood in the UK, and part of a 49-square mile national nature reserve in the Cairngorms National Park. Owned by the RSPB since 1988, it is known for the ospreys that nest at Loch Garten, as well as red squirrels, crested tits, capercaillie and Scottish crossbills.

Over the course of the next 10 years, the RSPB plans to extend the forest by planting 60,000 aspen, birch, juniper and willow trees over 590 hectares around the edge of the old Caledonian woodland. It also wants to start "pioneer" planting of 12,000 pine and other trees over 216 hectares away from the forest edge.

Concerned about the plans, Ramblers Scotland asked three respected authorities on Scottish pinewoods to comment.

The resulting report by Cairngorms research ecologist Dr Adam Watson, former Scottish Natural Heritage conservationist Dick Balharry, and Abernethy's former forest manager Basil Dunlop, is fiercely critical of the RSPB.

"The current RSPB approach at Abernethy is unacceptable," they say. They argue there has been agreement since the 1970s that the best way to extend the forest is to enable it to regenerate naturally by spreading its seedlings, not by artificially adding trees.

The planting plans "are a conspicuous departure from previously agreed policies in the Cairngorms and would damage the integrity of the Old Caledonian pinewoods owned by the RSPB", they say. "The proposals should be resisted."

The experts point out that parts of Abernethy forest are naturally descended from the ancient woodlands that covered large parts of Scotland after the last Ice Age. "Any planting would forever destroy the naturalness of the site, break the chain, and devalue scientific study," they argue.

"It is the amazing web of life that is the wonder of this living treasure. Claims by organisations that they are restoring or extending the Old Caledonian pinewoods by planting are therefore false and not credible."

One of the experts, Adam Watson, said he felt "betrayed by turncoats" in the RSPB.

"It is arrogant to claim that man can do better in a few decades than nature over thousands of years," he told the Sunday Herald.

RSPB Scotland director Stuart Housden accused the director of Ramblers Scotland, Dave Morris, of ignoring an offer to visit Abernethy and releasing his report to the media instead.

"This is extremely bad manners," he said. "Our immediate assessment is the report is wildly inaccurate and makes many unsubstantiated claims in the absence of facts, and this could have been clarified through dialogue."

Housden pointed out that Abernethy forest had been extensively altered by humankind over the last two centuries. The RSPB had consulted widely over its proposals in recent years, and had received no comments from Morris "or the individuals he has provoked".

RSPB monitoring over the last 20 years had revealed that broadleaved trees had been so damaged in the past that they were failing to naturally regenerate.

"Small areas will be planted, without any ploughing or other intrusive techniques, using saplings of local provenance," said Housden.

He was confident that the landscape the RSPB would pass on to its successors would be in a far better condition than it has been for centuries.

"We have a 200-year vision to restore this most precious of natural assets," he stated.

According to Morris, however, concerns about the planting plans had been brought to the attention of RSPB staff over the last few years, and now needed to be publically debated.

It was difficult to understand why the RSPB had rejected the experts' view that planting would "cause significant damage to this wonderful area", he said.