Which trees feed other trees with nitrogen?

Plant a nitrogen fixer among young trees: sea buckthorn, alder, black locust, Siberian pea shrub, autumn olive (Elaeagnus) or bush clover. In a forest garden grown on a bare field, oaks beside black locust grew to 5–7 m in ten years, those far from it to 1–3 m. Prune the fixers every year and leave the branches as mulch: that's when their roots and leaves feed the neighbours. Fruit trees can be sown from stones and pips: they won't copy the parent and fruit later, but some will be hardier, deeper-rooted and drought-proof; keep those that survive late frosts and bear well, and graft or sow from them. Plant densely and let nature pick the strongest. A simple planting pattern is the «trio»: a fruit tree, another fruit tree, a nitrogen fixer (honey locust, black locust, sea buckthorn), and repeat; it solves most soil and pest problems.