Trees in the street: why we need them

Japanese zelkovas on 6th Ave in downtown Seattle


A question was posed at a recent urban forest symposium: if conditions are so challenging for survival of urban street trees, why do we bother planting them?  In fact, the benefits of street trees are greatest in dense urban areas surrounded by concrete.  The challenges, it is true, are numerous: limited soil volume, flooding or severely dry conditions in sidewalk tree pits, heat absorbed by surrounding pavement and building materials, soil compaction from walking and driving in the root zone, and injury caused by people or vehicles.  Benefits of trees are psychological, aesthetic, economic and environmental, but in this post I will focus on one environmental benefit: mitigation of urban heat island effect.    Heat absorbed by pavement and building materials contributes to the “urban heat island effect,” which means that cities maintain warmer temperatures than surrounding areas.  Warmer temperatures contribute to the formation of ozone and other air pollutants.  Trees provide shade, which reduces sunlight hitting the concrete and thereby diminishes the heat island effect.  Evapotranspiration, the process through which water vapor evaporates from leaves, also cools urban areas.  In short, the toughest environments for trees are the ones that need them most.