By Alistair Johnston
There are many environmental forces, both biotic (i.e. living), such as fungus, bacteria, and insects and abiotic (i.e. non-living), such as drought, salt, and mechanical injury), that have an impact on the successful establishment of trees in the urban setting.
Air, water, and soil pollution are prime stressors that constantly affect urban trees and impact the balance, which is important for overall health. Air pollutants, including sulphur dioxide (SO2), absorbed by the foliage, interfere with photosynthesis and respiration.
Salts, which mainly enter the urban environment because of winter de-icing, are extremely harmful to trees, and may be the most common and impactful abiotic disorder. Salt, such as sodium chloride (NaCl), impacts a tree through contact from salt spray and salt-contaminated soil. Salt spray damage is particularly noticeable adjacent to streets and highways where prevailing winds consistently batter exposed trees. The resulting appearance on conifers is a browning of the needles and on deciduous trees is sparse foliage cover, severely stunted growth or mass response growth commonly known as ‘witch’s broom.’ In many cases, extended or severe exposure results in mortality.
The negative forces of Mother Nature are enhanced by the urban environment because urban trees are not equipped to withstand severe weather conditions due to their diminished overall condition. For example, during the summer, the urban heat island effect, caused by the heating of hard surfaces, increases drought stress on trees. The effects of storm impact are often more severe because swirling winds accelerate as they pass by tall buildings. Also, flash flooding along tributaries receiving mass quantities of water from adjacent hard surfaces often uproots trees.
Read the full article: Growing Trees in the Urban Jungle