A tree is a perennial woody plant. It is most often defined as a woody plant that has secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance.Trees are an important component of the natural landscape because of their prevention of erosion and the provision of a specific weather-sheltered ecosystem in and under their foliage. Trees have also been found to play an important role in producing oxygen and reducing carbon dioxide in the atmosphere, as well as moderating ground temperatures. They are also vital elements in landscaping and agriculture, both for their aesthetic appeal and their orchard crops (such as apples). Wood from trees is a common building material. Trees also play an intimate role in many of the world's mythologies.