Sometimes this causes seedlings to die, or grow really slow. It is kind of like when your mom fixes dinner for your family, but 10 extra hungry guests show up without an invitation. There really is not enough food for everyone, so you are still hungry when you leave the table. Healthy seedlings take 5 to 10 years to grow above the shrubs and grasses, and within 10-20 years they grow high enough to become a young forest. By the time the forest is 20-30 years old you can walk around inside the forest because it is so shady that few shrubs or grasses live there.
Foresters often thin the forest for the same reason a gardener thins a carrot patch.
Some of the trees have to die or be removed before the others can grow larger. Most forests are thinned 1 to 3 times before the stand is clearcut at age 30-80 years. As long as we thin out the forest, trees can grow bigger and bigger. Thinning a forest is like your family dinner without the extra guests. Without them, everyone gets plenty to eat.