Is this tree guild particularly promising or not: apple, hazelnut, and black locust? Or substitute apple for other fruits like pear or currants or mulberry, and black locust for some other kind of locusts like honey locust?
I envision them going together as a cluster of 3, planted just ~5' apart from one another so they form a miniature closed canopy in the center, then leave a ~10-15' gap between each cluster of 3, spreading over time to reforest part of a farm field.
Part of my motive asking this question is also just to learn more about guilds. For folks who think they have an answer to this question of if/how well it is likely to work - how do you know!? How can I know?! If you already tried such a guild, that's obviously a good tell. I wonder how experts go about thinking of these things to decide in advance if they're worth planting out and trying.
For one I can plant some smaller test runs, but even then if it works for a couple years I'd probably expand it before the test groups reach maturity.
For two, I can be careful about site selection. All 3 of those plants like well-drained soil, and I have pretty wet clay-ey soil but will be doing things like using sheet mulching with mounds and hugelkulture to help regulate hydrologic conditions. For sunlight I have full-sun and can orient the trees so the most sun-loving are south-facing. So at least with a cursory amateur assessment, it seems like it could work.
I'm in zone 4/5 and so far haven't had great luck with apples but am starting some promising local crabapple seeds now. Hazelnuts have been doing well and I want lots of them, thus making them the focus of the guild. Locust is as locust is - if anything it'll be doing too good for comfort! But we already have plenty of comfort as humans, so the locust can do its thing.