A single tree can be an entire ecosystem in itself from all the fungi, lichens, insects, mammals, birds, etc. etc. A loss of tree species means a loss of others too. A lot of migrants birds and animals depend on seasonal fruits and nuts as they move south so the effects of species loss can be very far ranging indeed.

While the idea that planting more trees to ameliorate climate change is a good one there is a risk that simple, ignorant, solutions such as planting fast growing eucalyptus or pine species could be seen as the fast path to success. Instead focussing on native species, which are slow growing in comparison, would actually solve the much greater problem that comes with climate change - habitat degradation and generate ecosystems that are generally most robust and able to withstand stresses better.

While it probably isn't the main concern, different species produce lumber with different properties. The loss of Elm trees in the uk removed a timber that was very useful for boatbuilding, for example (thought that is a bit of a niche concern).

We don't, and probably can't, know the ripple effects from each individual lost species. Some will have an important place in the incredibly complex food web. So we'll lose other species in other parts of the food chain. Some may turn out to be critical to something we find important. Once they're lost, no matter how critical they turned out to be, it's too late.

At the other extreme, we do know the problems associated with extensive monocropping. We've seen the effects of monocrop agriculture and forestry through the 20th century. The more healthy biodiversity we can keep in all species, the better for us.

It's not predictable what will happen - https://www.youtube.com/watch?v=cVOHgztZ3XI