9ee32 urban heat island

The term “Urban Heat Island Effect” refers to the phenomenon of city settings having temperatures anywhere from 2–10 degrees higher than the outlying countryside.

Signs / Symptoms

Symptoms of initial injury show up as leaf desiccation, caused by high temperatures combined with low moisture levels. Generally, symptoms are apparent throughout the entire canopy. Further decline results in twig dieback, progressing to limb dieback and eventual death of the tree.

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Heat Island

Cause / Conditions

Heat islands form as vegetation is replaced by pavement, buildings, and other structures. These surfaces absorb rather than reflect the sun’s heat, causing both surface and air temperatures to rise. Industrial growth and energy required to cool homes and buildings increase waste heat energy. The heat that is generated by these demands adds to increases in air temperatures. Similar conditions occur for the tree next to your driveway, in the middle of your deck, or next to your air conditioning unit.

Pavement is laid to create a cleaner parking area. As pavement is installed, heat island conditions are created. Trees that are included in such settings suffer significant damage, as the fine absorbing roots once free to grow and expand in a natural environment are smothered under a heat absorbing sea of asphalt. Many, many of these absorbing roots will die, depleting the root mass necessary to support the canopy of the tree. When given the option to park a car in the open sun or under the shade of a tree, most people choose to park near the tree. The heat emitted from parked cars causes further heat damage to the tree.

Government agencies recommend planting trees and shrubs to help reduce the “Urban Heat Island Effect” both by creating shade that cools temperatures and by their ability to transpire. One thirty foot tree can release enough water in one day to remove all the heat generated by a small space heater that has been running for four hours! Evapo-transpiration is the process in which the tree’s roots absorb water from the soil and transport it to the canopy where it is lost from the leaf into the atmosphere through evaporation. The tree uses heat energy from the air to evaporate water from its leaves, thus cooling the leaf surface as well as the air. This is just one of a tree’s many physiological processes.

Although the trees help reduce the “Urban Heat Island Effect”, the materials and demands that caused the effect to begin with are foreign to the natural tree environment. Many trees, if not allowed the space above and below ground necessary for survival, will succumb to the heat island effect.


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