I have been replanting a small pocket wood, for almost a decade. What was once a plantation (hybrid black poplars) is now a mixed woodland with, not just trees, but different layers - tree canopy, shrubby under-story and herbaceous ground covering. What I have learned; not only do trees fare better in mixed plantings, they also utilise 'nurse' trees as sort of tree guardians. I planted a number of fast growing 'pioneer' such as birch, hazel or willow... which protect smaller, slow-growing saplings (black poplar, hornbeam, oak, limes, field maple), with a mycorrhizal network, and encourage straight, vigorous growth from the saplings. Planting 1st year whips within the rootzone of the adolescent pioneers has been working well (for me). The fast growing (and short-lived) pioneers can be cut down or coppiced to start a new growth cycle. Blackberry is also a terrific natural tree protector, keeping the ground free from weeds, without overly competing with new saplings.