Trees and vegetation are most useful as a mitigation strategy when planted in strategic locations around buildings. Researchers have found that planting deciduous species to the west is typically most effective for cooling a building, especially if these trees shade windows and part of the building’s roof. Shading the east side of a structure also reduces air conditioning demand. Planting trees to the south generally lowers summertime energy demand for air conditioning.
Trees and vegetation absorb water through their roots and emit it through their leaves—this movement of water is called “transpiration.” A large oak tree, for example, can transpire 40,000 gallons of water per year; an acre of corn can transpire 3,000 to 4,000 gallons a day. Evaporation, the conversion of water from a liquid to a gas, also occurs from the soil around vegetation and from trees and vegetation as they intercept rainfall on leaves and other surfaces. Together, these plants take water from the ground through their roots and emit it through their leaves, a process known as transpiration. Water can also evaporate from tree surfaces, such as the stalk, or surrounding soil. processes are referred to as evapotranspiration. Evapotranspiration cools the air by using heat from the air to evaporate water. Evapotranspiration, alone or in combination with shading, can help reduce peak summer air temperatures. Various studies have measured the following reductions:
• Peak air temperatures in tree groves that are 5ºC cooler than over open terrain.
• Air temperatures over irrigated agricultural fields that are 3ºC cooler than air over bare ground.
• Suburban areas with mature trees that are 2 to 3ºC cooler than new suburbs without trees.
• Temperatures over grass sports fields that are 1 to 2ºC cooler than over bordering areas.
Trees and other large vegetation can also serve as windbreaks or wind shields to reduce the wind speed in the vicinity of buildings. In the summertime, the impacts can be positive and negative. In the wintertime, reducing wind speeds, particularly cold north winds, can provide substantial energy benefits. 

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