A forest is a complete ecosystem consisting of many species of plants and animals. Immature forests usually consist of pine trees and small shubs since the soil is shallow. The Pines contribute organic matter to the soil and making it possible for hardwood trees to finally take root. The ecosystem gets more complex over time as more and more species find a niche in the complex system. Ultimately it reaches a level referred to as a climax community.
Logging companies love to harvest old-growth forests because the girth of the trees is much greater and the financial rewards much higher than when harvesting more immature stands. However the highly evolved ecosystem that is the old-growth forest is completely dismantled by the clearcutting technique favored by the loggers.