What percentage of a forest should be in black walnuts is a hard question to answer as it really depends. Overall, diversity in a forest is desirable for a lot of reasons. So ideally, you would want this newly developing forest to grown into a stand with a lot of diversity in the overstory. Look around and see what other types of trees are growing in that area with the walnuts. You will likely see a lot of light-seeded species that disperse easily in the wind (like ashes, elms, and maples) or species with fleshy berries that are spread by birds (like hackberries and cherries) and maybe fewer hard-seeded species that rely more on mammals to spread them (like oaks and hickories). If you are seeing a good mix of species then I would not think you would need to thin your walnuts much. If you have a nearly solid stand of walnut with little else growing, then you might want to then some of the poorer form trees out, leaving the healthiest, straightest trees to continue to develop into canopy trees. You may even consider direct seeding or planting tree seedlings of other species in the stand if they are not coming in naturally.
