Overgrazing by large herbivores such as red deer and sheep has been the primary cause of the lack of tree regeneration in the Caledonian Forest for the past 150-200 years.
As a result, by the late 20th century, most of the remnants of the forest consisted only of old trees, because all the seedlings were eaten before they could grow to more than a few centimetres in height.
With the red deer population in the Highlands having increased from 150,000 to over 350,000 since 1965, and in the absence of any natural predators to control those numbers, the only tree seedlings to grow successfully have been those inside fenced areas or in remote locations inaccessible to the deer. See the image above for an example of the grazing level inside and outside of a fenced area.
Red deer are a forest dwelling species, and are an important part of the forest, but at present their numbers are totally out of balance with the small fragments of our native forests which remain. Although all trees are browsed by deer, broad-leaved trees are more palatable than Scots pine, and have likely been selectively overgrazed for centuries, so that they are disproportionately under-represented in many of the forest remnants today. Rowan and aspen are perhaps the most-preferred of all trees by the deer, so there is virtually no chance of these species regenerating in unprotected areas under present conditions.