The ambitious project aims to expand the largest single remnant of ancient Caledonian pine forest to its natural altitude limit within 200 years.
It will almost double the total size of the woodland at RSPB's Abernethy Forest, near Grantown-on-Spey, joining it up with the fragmented surrounding remnants.
The RSPB says it will "give nature a bigger home in which to flourish".
Abernethy already hosts some of the rarest species in the UK, with about 12% of the population of capercaillie as well as Scottish crossbills, crested tits, wildcats, pine martens, black grouse, golden eagles and many rare mosses, fungi and plants including twinflower.
According to the wildlife charity, managing and reducing the grazing pressure on the reserve from deer over 25 years has already enabled the Scots pine trees of Abernethy forest to expand by self-seeded natural regeneration, with more than 2000 acres of new pine saplings now established.
However, although the main component of Caledonian pine forest is the native Scots pine, a critical element of ancient pine forests is the inclusion of a broader range of native shrub and broad-leaved tree species - such as juniper, birch, rowan, alder and willow - and while recovery of the pine element at Abernethy has been successful, some of these other species remain extremely scarce.
To establish important seed sources, the conservation charity will plant close to 100,000 trees at the reserve over the next 10 years, including birch, aspen, two species of willow and alders.
It is hoped at least 40,000 of the saplings, which will be planted with the help of volunteers including schoolchildren in Strathspey, will survive grazing pressure from hares and other herbivores to reach maturity.
Jeremy Roberts, the senior site manager at Abernethy, said: "We have conducted some of the most comprehensive surveys of regeneration in Britain, and this has shown that the recovery of broadleaves has been extremely slow and localised compared to the pine element at Abernethy.
"Few broadleaves remain to provide the vital seed source, and of those that do are highly immobile and restricted.
"As these small groups mature they will themselves provide the seed source, inoculating the forest edge and providing a locus for these species to regenerate more widely, and restoring the forest to its diverse and species-rich former glory."
He said it might well be that the children and grandchildren of the school children assisting with the planting who will be the ones who see the difference.
The restoration project is one of a number across Scotland, with other bodies and private estates including Trees for Life, The National Trust of Scotland at Mar Lodge, Scottish Natural Heritage, Forestry Commission Scotland and estate managers at Alladale and Glen Feshie doing similar work.
Environment and Climate Change Minister Paul Wheelhouse said: "The RSPB should be congratulated for taking forward this pioneering project. It fits very well with the Scottish Government's vision for woodland expansion and community engagement. It is also great news that Scotland's national tree, the Scots Pine, is doing so well in Abernethy Forest.
"The move to plant more tree species in the forest is a positive one which will add great value to the environmental quality and biodiversity of the Cairngorms."