The average height of the canopy trees in rainforests is 35 – 42 meters (115 – 137 feet) with a crown diameter of 13 – 22 meters (42 – 72 ft), but the tallest trees may reach 84 meters (275 feet). Trees apparently reach these heights because of competition for light; trees of the same species will be much shorter if they are kept in solitary conditions. The roots of tropical rainforest trees are superficial, since the topsoil tends to be thin, and so some of the largest trees are anchored by large horizontal roots (buttresses) which protrude above the ground. Leaves tend to be large and long, and much of the energy the plant takes in goes to making them, since a high rate of photosynthesis is required to survive competition for nutrients, light and space. These characteristics permit very high productivity of both plants and animals, and tropical rainforests produce more mass of both plants and animals than do other types of forests. Interestingly, although we regard rainforests as important sources of timber, tropical rainforest trees produce proportionally less wood and more leaves than do temperate forest trees.