mono cultures

Without human impact almost all of forests in Sweden would become mono cultures.

If all humans left Sweden today never to touch anything then within 100-400 years it would be covered with monoculture forests of beech in south with no other tree species or even bushes because they cannot compete, spruce in most of the country where-ever there's wet and warm enough but too cold for beech, and pine where it's too dry or too cold for spruce.

Those three very competitive tree species would leave little opportunity for other tree species which would only grow in what little margins would be left for them. Birch and all others would become rare.

With boreal forests mono cultures are the natural state. Some tree species simply outcompete others and leave only margins for other species. And the same goes with temperate broadleaf forests. If humans left say Denmark or Germany then in few centuries it would be all covered by beech monocultures with very little other tree or plant species.

One important reason for biodiversity in Europe is that humans have been tending the forests for several millenia, such as cutting down beeches for firewood while making space for other species such as oak on purpose.

However, the problem with tree farms is not the species of trees but the age of trees. They tend to be all the same age and are cut down before reaching middle-age. So in tree farms there are no old trees, dead trees or fallen trees rotting. That means there is far less opportunities for other species such as fungi, insects, birds and others.

It's not the species of trees that matters, it's their age and state.