Shading – Trees shade buildings and reduce the amount of heat absorbed and stored by them. Planting shade trees on the west, south, and east sides of buildings provides the most energy savings in the summer. Shaded walls can be 9-36°F cooler than the peak surface temperatures of unshaded surfaces. A single large tree can save a homeowner $800 in energy costs over 40 years. Ideal shade trees have a broad crown with dense foliage and lose their leaves during the winter to allow the sun to warm the home. Some examples of good shade trees for Boston include:
Sugar Maple
Red Maple
Littleleaf Linden
Northern Red Oak
Japanese Zelkova
Tulip Tree
Pagoda Tree
Ginkgo
Trees also shade paved surfaces, reducing the "urban heat island" effect and ozone formation in the summer. In cities with populations of over 100,000, utility usage increases by as much as 2% for every 1°F increase in summertime temperature. This translates to the urban heat island being responsible for 3-8% of energy usage. Proper planting of trees can significantly mitigate the effects of the urban heat island, and save you money on energy costs.
Windbreaks - Trees planted on the north and west sides of buildings can reduce windspeed and block the infiltration of outside air into interior spaces, thereby reducing heat loss. According to U.S. Forest Service scientists, planting trees as windbreaks can reduce winter heating costs by as much as 10% in urban areas. Evergreens make the best windbreaks because they retain their foliage during the winter. Shrubs with dense foliage can serve as good windbreaks when planted near a foundation, creating dead air space.
Evapotranspiration – This is the process by which trees take water from the soil, and use heat from the air to convert water in the leaves to water vapor. This loss of water from the leaves occurs during photosynthesis when the leaf stomata, or pores, are opened and water is lost from the leaf while carbon dioxide is taken in. The process of evapotranspiration reduces nearby air temperatures and, when there are enough trees, can cool entire neighborhoods. Trees and shrubs planted around homes also provide evaporative cooling benefits. Trees placed near air-conditioning units will shade and cool the air around the units, so that less energy is required to cool your home, once again, saving you money.
Pollution Removal – Another benefit of planting more trees is to human health. Trees can be thought of as air filters for the air we breathe in the city, which is usually filled with dust, microscopic particles, and pollutants. Ailments such as asthma, and symptoms like burning eyes, coughing, and chest tightness are common with exposure to high levels of air pollution. Pollutants like sulfur dioxide, carbon monoxide, ozone, and nitrogen dioxide, and particulate materials at 10 microns (PM10), all of which cause adverse human reactions, are removed from the atmosphere by trees. According to US Forest Service research, a typical tree extracts almost 50 pounds of non-CO2 pollution over its lifetime. In fact, again based on Forest Service research, trees are approved by the EPA for use in reaching air quality goals in cities.
