One autumn, a beech seed blows into a sunny meadow. Following a cold, wet winter, it sprouts. The seedling grows into a sapling. The sapling grows into a taller tree. After several years, it grows lateral branches. Each spring, these leaf, making a shady, cool space under the tree. Over the years, leaf litter decays, forming deep, rich soil. Nevertheless, over time the population of meadow grass under the tree declines. What limiting factor has affected the grass's growth?