Two of National Trust for Scotland's key properties - Torridon and Kintail & Morvich estates in Ross-shire - took the brunt of the fire caused by hot weather and strong wind.
A spokeswoman said out-of-control conflagrations caused an estimated £100,000 damage to a tree regeneration scheme, which would take 15 years to recover. "It's a devastating loss," she said. "We started to re-establish broadleaf trees like birch and hazel a decade ago."
Scorched earth in Torridon covered 13 sq km, while in Kintail 10 sq km of heath and trees were obliterated, she said. "The regeneration is part of a carefully orchestrated plan to join up existing patches of ancient woodland and is one of the trust's key conservation objectives."
Pete Selman, director of property and visitor services, said: "The loss of the mature trees is heartbreaking but no one was hurt. Our aim will be to begin re-establishment of the affected area once again."
Meanwhile, the trust is counting the cost of fire damage to over 500ha of land across Marsden Moor in Yorkshire.
Plants and wild animals perished during the blaze on the site of specific scientific interest, said countryside manager Gemma Wren. "A quarter of the trust's land at Marsden has been affected."