Where I live in germany, there used to be extensive spruce monocultures forests everywhere.
They are mostly chopped down now and replaced with a mixed young forest, all without herbicides. (But with some planted trees, cleansing and fences to protect the young forest from deers) So after 15 years they surely are not comparible to old grown forests, but they are very diverse and alive. So I strongly question the assumption that herbicides are necessary or beneficial to create a diverse forest.
Most of the dominating species in the first years will be (were) replaced by something else eventually.
They are mostly chopped down now and replaced with a mixed young forest, all without herbicides. (But with some planted trees, cleansing and fences to protect the young forest from deers) So after 15 years they surely are not comparible to old grown forests, but they are very diverse and alive. So I strongly question the assumption that herbicides are necessary or beneficial to create a diverse forest.
Most of the dominating species in the first years will be (were) replaced by something else eventually.