It depends. For forestry stock there's no real way to avoid monoculture if you need a lot and you need it soon (40-60 years). There's much more to this answer though because Phytopthora is hammering larch, Ips is hammering spruce and red band needle blight is hammering pines. That's another topic on itself. Broadly though, there's nothing wrong with a monoculture per-se ([Pando][1], Boreal woodland) it just depends on how it is managed and how well the ecology does in response to it.

That being said, personally, I favour the continuous cover approach of mixing up natives broadleaves with non-native conifers as long as the site isn't ancient or semi-ancient natural woodland (ASNW) or plantation on ancient woodland site (PAWS). For those sites, they're too important for use as a commercial forestry site and arguably the ecology needs to be restored, maintained and managed. Those sites are precious and should be managed properly in-line with their identified [NVC identifier][2]. The one exception to this is coppicing. Having a coppice on ancient sites where coppicing was practised is one of the few woodland management techniques that adds to ecology over all four woodland layers over all time frames.

I never thought I'd answer that question on HN. I appreciate you asking. What's your take on forestry monocultures?

[1]: https://en.wikipedia.org/wiki/Pando_(tree) [2]: https://jncc.gov.uk/our-work/nvc/