 Forest, the forest is a large area dominated by trees. Hundreds of more precise definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing and ecological function. According to the widely used Food and Agriculture Organization definition, forests covered for billion hectares 9.9109 acres 15 million square miles or approximately 30 percent of the world's land area in 2006. Forests are the dominant terrestrial ecosystem of Earth, and are distributed around the globe. Forests account for 75 percent of the gross primary production of the Earth's biosphere, and contain 80 percent of the Earth's plant biomass. Net primary production is estimated at 21.9 gigatons carbon per year for tropical forests, 8.14 temperate forests, and 2.64 boreal forests. Forests at different latitudes and elevations form distinctly different ecosystems. Tropical forests near the poles. Tropical forests near the equator and temperate forests at mid-latitudes. Hired elevation areas tend to support forests similar to those at higher latitudes, and amount of precipitation also affects forest composition. Human society and forests influence each other in both positive and negative ways. State forests provide ecosystem services to humans and serve as tourist attractions. Forests can also affect people's health. Human activities, including harvesting forest resources, can negatively affect forest ecosystems.