Most upland areas were wooded until humans appeared in large numbers and introduced land maggots [sheep to the rest of you] in large numbers. They don’t kill mature trees but eat seedlings, so the long term effect is that the trees don’t regenerate and eventually disappear. In Cwm Idwal, Snowdonia there are some experimental plots that have been fenced off, but there aren’t many tree seed sources nearby so the regeneration is slow. Any climbers can tell you that there are species like holly and oak growing at surprisingly high altitudes on crags where the sheep can’t get them.
The most important timber for wooden ships was the big curved branches of mature oaks which made the main ribs, but a massive amount of smaller and lower quality material was used as well for all the other parts. The demand was massive and there was a lot of pressure to keep planting trees to maintain supplies. Even after we started building iron and steel ships it didn’t change – the Forestry Commission was set up after WW1 in order to maintain a strategic reserve of timber. When I trained back in the 1970s, being a forester meant farming trees, their present renown as proprietors of mountain bike trails is a recent thing.
