![[logo] US EPA](https://webarchive.library.unt.edu/eot2008/20081013171337im_/http://www.epa.gov/epafiles/images/logo_epaseal.gif){kind=logo}
Trees and Vegetation
- Denotes link to glossary
definition Related Links
Planting trees and vegetation is a simple and effective way to reduce heat islands. Widespread planting in a city can decrease local surface and air temperatures. Strategic planting around homes and buildings directly cools the interior of homes and buildings, decreasing air conditioning costs and peak energy demand.
Tree planting cools the air and brings communities together.
Besides heat island reduction and energy savings, trees and vegetation can improve air quality, reduce carbon dioxide (CO 2) emissions, decrease storm water runoff, improve community livability, and provide other benefits.
Planting and maintaining urban trees and vegetation requires planning and care. Choosing plants suitable for local climate conditions; selecting a location to minimize the potential for damage to buildings, electrical wires, and sidewalks; and watering, mulching, staking, and pruning are important factors to consider.
How Do Trees and Vegetation Mitigate Heat Islands?
Trees and vegetation cool the air by providing shade
and through evapotranspiration (the evaporation of
water from leaves). Shade reduces the amount of
solar
radiation
transmitted to underlying surfaces, keeping them cool.
Shaded walls may be 9 to 36°F (5° to 20°C)
cooler than the peak surface temperatures of unshaded
surfaces. These cooler walls decrease the quantity of
heat transmitted to buildings, thus lowering air
conditioning cooling costs. Cooler surfaces also lessen
the heat island effect by reducing heat transfer to the
surrounding air.
Transmittance
varies by tree or vegetation type, but for deciduous
species – which shed their leaves in winter –
transmittance ranges from 6 to 30% in the summer and 10
to 80% in the winter.
Another way trees and vegetation cool the air is by absorbing water through their roots and evaporating it through leaf pores. This process uses heat from the air to convert water contained in the vegetation into water vapor. A mature tree with a 30-foot crown transpires approximately 40 gallons of water per day. Evapotranspiration alone can result in peak summer temperature reductions of 2 to 9°F (1° to 5°C). While this process reduces air temperatures, it does add moisture to the air. The positive cooling effect of vegetation usually outweighs any undesirable gains in humidity.
The U.S. Department of Agriculture Forest Service
estimates that every 1% increase in
canopy
cover results in maximum mid-day air temperature
reductions of 0.07 to 0.36°F (0.04° to
0.2°C). However, trees and vegetation are one
factor among many that affect prevailing weather
conditions.
What are Some Guidelines for Saving Energy with Trees and Vegetation?
When planting for energy savings, particular care
should be given to location. The following guidelines
can help lower air conditioning costs and city-wide
energy demand:
- To shade from summer sun, plant deciduous trees
to the west, southwest, southeast, and east of a
building. Special care should be given to trees
planted directly to the south. These trees may not
provide much shade in the summer, when the sun is
high in the sky, and may block desired wintertime
sun, when the sun is low in the sky.
- Deciduous trees work well, as they balance energy
requirements over the course of a year. In summer,
foliage cools buildings by blocking solar radiation.
In winter, after the leaves have fallen, the sun's
energy passes through trees and helps to warm
buildings.
- If there is not enough space for trees, grow
vines on a vertical or horizontal trellis to shade
the west and east windows and walls of buildings.
- To block winter wind, plant a row of evergreens perpendicular to the main wind direction, usually to the north or northwest of a building. Ideally, the row should be about 50 feet away, be longer than the width of the building, and grow to twice the height of the building.
How Much Energy Can Shade Trees Save?
Researchers in a joint study by the Department of Energy's Lawrence Berkeley National Laboratory (LBNL) and the Sacramento Municipal Utility District (SMUD) placed varying numbers of trees in containers around homes to shade windows and walls. Cooling energy savings ranged between 7% and 40% and was greatest when trees were placed to the west and southwest of buildings.
Another LBNL study modeled the effects of shading homes with vegetation in seven U.S. cities. By providing 20% tree canopy – the equivalent of planting one tree to the west and another to the south of a home – buildings could achieve annual cooling savings of 8% to 18% and annual heating savings of 2% to 8%.
The effectiveness with which trees provide shade and
save energy depends on their tree density, shape, and
placement. The dimensions of the shaded building, the
position of the sun in the sky, and whether a tree
keeps its leaves year-round also determine overall
energy savings.
Visit HIRI's Energy Savings page for more information on how using a range of heat island reduction measures – installing cool roofs, planting trees and vegetation, and using cool paving – can save energy across a community by lowering ambient temperatures and reducing air conditioning demand.
The
Tree Benefit Estimator
on American Public Power Association's (APPA) Web site
presents a simplified and easy-to-use method for
estimating the energy savings and other benefits of
tree planting. It was developed by Sacramento Municipal
Utility District (SMUD) based on experience with their
successful Shade Tree program.
Other Benefits from Trees and Vegetation
Vegetating urban landscapes not only improves community livability and air quality by reducing summertime temperatures and air conditioning demand, it provides additional benefits:
-
Carbon Dioxide. When trees and vegetation
reduce energy use, they also reduce CO
2 emissions from power
plants. In addition, vegetation removes atmospheric
CO
2 by sequestration.
Trees sequester – or store – between 35
and 800 pounds annually depending on their size and
growth rate. The total quantity of carbon stored in
mature trees may be 1,000 times more than the storage
in small, young trees. The
U.S. Forest
Service
estimates that carbon storage byurban forests
is between 400 and 900 million metric tons
nationally.
- Stormwater Management. During rain events, the
ground can become saturated and turn excess rainfall
into runoff. Stormwater runoff problems, such as
flooding and polluting of open water bodies, are
worsened by the large amounts of water-resistant
surfaces in urban areas.Trees and vegetation can
help reduce the runoff problem by decreasing the
volume of runoff. Researchers found thatevergreens,
conifers, and trees in full leaf can intercept up to
36% of the rainfall that hits them.
- Quality of Life. Trees and vegetation can help reduce noise, which may be highly valued in urban areas. They also provide shade from harmful ultraviolet radiation, particularly in playgrounds, schoolyards, and picnic areas. (Visit EPA's Sunwise program for more information on protection from overexposure to ultraviolet radiation.) In addition, trees and vegetation may increase property values, as several studies have shown that homevalues are higher on tree-lined streets. Lastly, community gardens and neighborhood parks can help reduce physiological stress, aesthetically improve an area, and provide an urban habitat for birds, animals, and insects.
The
Urban Forest Effects (UFORE) Model
was developed by the U.S. Forest Service to
quantify variables such as total pollution removed by
the urban forest, volatile organic compound (VOC)
emissions from the forest, annual carbon sequestration,
and the effects of trees on building energy use and
consequent effects on carbon dioxide emissions from
powerplants.