The totality of trees and shrubs planted in our metropolitan community is known as our “community forest,” more commonly known as the “urban forest.”
In Portland, more than 50 percent of the urban forest canopy exists on private property. This forest-in-a-city provides a wide range of benefits to our community: it absorbs and slows down stormwater, provides wildlife habitat, reduces air pollution, extends the life of pavement, modifies the local climate, reduces noise pollution, and provides aesthetic beauty—to name just a few.
One of the more easily measured benefits that trees provide is energy conservation. On a community-wide scale, shade from trees helps reduce the heat island effect caused in urban centers by buildings and pavement, which absorb light energy and reflect it as heat.
Trees save energy in several ways. Leaves absorb light energy, thereby reducing reflected heat. They also absorb water through their root system and release moisture through the leaf surfaces by a process called transpiration, which cools the air in much the same way our skin cools us through the process of sweating. Trees can reduce the surrounding air temperature by as much as 10 degrees Fahrenheit and the temperature directly under the tree by as much as 25 degrees Fahrenheit. It only takes one hot summer day in Oregon to send us looking for the shade of a tree.
At your own home or business, trees can be planted strategically to take advantage of their energy-saving benefits. Properly placed trees can reduce the solar heat absorbed by buildings and reduce cooling costs by 30 percent. Large-growing deciduous trees (trees that lose their leaves in winter) are the best choice for shading and cooling. During summer, their leaves block the sun and provide shade, yet during winter months the sun passes through the leafless canopy.
In the northern hemisphere, placing trees on the south and west sides of your house can help take advantage of the shade, especially if they are planted to shade the building during the hottest part of the day. Trees with a broad canopy will provide the greatest shading benefits. A combination of large and smaller trees can be used on the west side of a building to reduce the late afternoon heat and glare when the sun is at a lower angle.
Shading other impervious surfaces such as driveways, sidewalks, streets, and parking lots also can improve the micro-climate around your home significantly by reducing glare and reflected heat. And if your air conditioner is not located in a shaded area, consider planting a small tree or large shrub nearby to shade the unit. Shading an air conditioning unit can increase its efficiency by 10 percent.
For winter savings, consider planting evergreens and large shrubs on the north and northwest side of your property to slow winter winds and reduce heating costs. Plant with diversity in mind; use more than one species when designing and planting. Adding large shrubs and even small and medium deciduous trees can improve diversity, aesthetic interest, and wildlife habitat.
Want to learn more about the benefits of trees?
Odom is a Neighborhood Trees Specialist for Friends of Trees.