This is a topic that has been studied and examined in many contexts, from urban planning and agricultural to psychology and politics. There is a general agreement that planting more trees and greenery leads to an improvement in public health. There was a movement in the 1970s, concurrent with the environmental awareness movement, to plant more trees in urban areas. Trees provide shade, which helps keep buildings and streets cooler during hot weather, and this can directly impact public health. During heat waves in cities, people flock to public parks to seek relief in the shade. In a more general sense, trees increase oxygen in the atmosphere via photosynthesis (this is the name for the biological process that occurs during the growth cycle of a tree), and this can help alleviate the effects of air pollution, as trees also consume carbon dioxide (which can cause health problems if there is too much of it in the atmosphere) during this process.
Trees also appear to have a beneficial impact upon psychological health and well being. Recent studies show that increased planting of trees in urban areas not only reduces the harmful impact of air pollution, but also helps reduce stress. Trees have been the subject of many poems throughout the years, and there is a national day in the US for celebrating trees called Arbor Day. The Arbor Day Foundation promotes and helps support the planting of trees across the country.