The study looked at how different types of trees planted together affect the growth and variety of new trees and shrubs underneath. They planted oak trees with maple and nettle trees in different combinations and measured the diversity of plants growing below. They found that more diverse mixtures of trees led to more varied and abundant new plants growing underneath. The amount of tree cover and depth of leaves on the ground also affected the growth of new plants. This shows that planting different types of trees together can help natural forests grow better.