Forest is a complex ecosystem and based on some books [1][2] my understanding is that fungi plays a huge role in them. They provide a lot of things that trees need in exchange for what they need, they also create kind of a market which allows trees even from different species exchange their resources. Different trees have different strong sides and can make use of different conditions. Perhaps this, thanks to the huge forestweb underneath allows them to thrive.

Also, trees like to grow slow and solid. Older trees from the same specie will feed small tree hidden in their shadow and provide it necessary resources so that when some bigger tree falls and it can take it place, it can also grow faster. It's possible that when there's competition between species they grow faster because there's a fight for sunlight. The year's growth will be bigger and wood would be less dense (but it is sold by volume).

1. The Hidden Life of Trees, Peter Wohlleben

2. Entangled Life, Merlin Sheldrake