Their basic production characteristics make Christmas trees an extremely friendly farm crop, both to our environment and to people and animals. A broad network of roots holds the soil and a continuous ground cover prevents surface erosion by water and wind. As a long term crop, trees allow a natural buildup of bird and animal populations. Tree farms provide stable refuge and feeding areas for wildlife, often very near large urban centres, at no cost to the taxpayer. Some farms allow visits at different times of the year. Their accessibility and the quiet, park like surroundings make these farms very pleasing to people. Like all plants, the conifers growing for eventual harvest as Christmas trees remove carbon dioxide from the atmosphere as they grow and produce oxygen as a byproduct. One acre of Christmas trees produces enough oxygen for 18 people. Without this process of photosynthesis, life could not exist on earth.