Although the benefits of diverse forest systems are well known, many countries' restoration commitments are focused on establishing monoculture plantations
This right here. It's a shitty strategy, and pretty much lip service in terms of efficacy. 70% difference in efficiency aside, a monoculture doesn't promote the use of that land by diverse wildlife. A monoculture also puts the entire stand at risk for disease or some other thing wiping everything out. Just look at the mountain pine beetle for instance. While the areas decimate by pine beetle had 'monoculture' canopies, that had the right species in the understory and seedbank for those area to return eventually, after the die off.
The reason behind the 70% figure is likely that diversity promotes better access to some soil nutrients or provides better growing conditions/microsites and thus the biomass of the systems increase as a result..
Chen (2022) talks about how soil P is bolstered in diverse stands because you get a mix of deeper rooting plants pulling P from subsoil horizons to shallower depths, making it much more accessible to the majority of the forest spp.
New trees also cannot support wildlife the same as old trees. Animals like owls need large, old, dead trees with large cavities, as they cannot make their own nests. Forests need to develop for almost 200 years for those conditions to exist. It's also needs to be speciea of trees that will support the lives of prey animals as well. Without time and balance, the forest is essentially dead for anything but logging. Save old trees, cut new trees!
I'm 100% in favor of preserving old-growth forests, but would it be effective to build habitats specificially for owls, distributed throughout both new and old-growth forests?
It's mainly the small and medium adaptable owls that are still doing well that will use nest boxes. Owl boxes are also decently large since owls stay in the nest until essentially they are full grown and 10 to 20 feet (3 to 6m) off the ground.
Large owls don't cavity nest and just need lots of tall dense trees with a thriving population of small mammals.
Birds of prey will also learn where nesting boxes are and take advantage of that. I was at a bird banding event the other day and they said they had to move their capture nets every season because raptors will learn where they are and pick birds out of it.
There are just many things that are hard or expensive to recreate by hand.
It's still there. It's just getting further and further away. It's very valuable to the lumber industry.
Many owls live very different lives than each other. Large owls don't migrate and will spend their lives a few miles from where they were born. If that home disappears, they don't know where else to live or find food.
Owls that migrate come from up north in the wilderness where they don't really even know what people are, like the Saw Whet Owls. They're used to moving and finding new food, and are much smaller.
Some like Barn Owls and Barred Owls are aggressive, highly adaptable, can live along side people, and can kill most other owls for territory. This along with human activity is what is driving the Spotted Owls in US and Canada to extinction, since multiple species are trying to kill them.
So we still have old growth forest and owls, but neither the amount or diversity of either we used to.