Forests sometimes contain many tree species only within a small area  or relatively few species over large areas. Forests are often home to many animal and plant species, and biomass per unit area is high compared to other vegetation communities.Smaller areas of woodland in cities may be managed as Urban forestry, sometimes within public parks. These are often created for human benefits; Attention Restoration Theory argues that spending time in nature reduces stress and improves health, while forest schools and kindergartens help young people to develop social as well as scientific skills in forests.