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How trees improve cities

By Frances Waite, Coastal Region Urban Forester, S.C. Forestry Commission

How do trees help cities and towns conserve on air conditioning? How does shade from trees assist in energy conservation?

Trees properly placed around buildings can reduce air conditioning needs by 30 percent and can save 20 – 50 percent in energy used for heating according to the U.S. Department of Agriculture Forest Service. Cities can be 2 to 10 degrees warmer than rural surroundings due to what's often called the urban heat island effect. Planting more trees keeps urban spaces cooler and more livable.

What kinds of trees help reduce erosion and other flood concerns?
All mature trees can help reduce erosion due to their wide-reaching fibrous and woody root systems. In addition, tree canopy captures and slows rainfall, which reduces impact to the soil.

Baldcypress is an urban-friendly tree for flood-prone areas; black gum (Nyssa sylvatica) is also a good option. Live oaks are good for the coast, as they are tolerant of salt spray.

Evergreen species uptake the most water in the canopy annually, since the abundant needles capture more water and stay on the tree all year. Larger trees, whether deciduous or evergreen, hold water in the canopy until saturation is reached. Water also flows down the trunk and into the roots.

Having adequate soil volume in the planting area is important to also hold water and make sure water reaches roots. Examples of evergreen trees to consider that can also tolerate occasional 'wet feet' include American holly, Eastern red cedar, magnolia and live oak.

What trees would be appropriate for high-traffic areas?
Typically, high-traffic areas are urban sites with little to no pervious area. The most important thing for healthy, attractive urban trees is the adequate rooting space which is critical for health and survival. In general, small maturing trees need less below-ground space, therefore, may be better suited for high-traffic areas.

Examples include Eastern or Oklahoma redbud, Chinese fringetree, and Osmanthus species (tea olive), and also are trees that have a nice fragrance. Upright, narrow canopy trees, such as 'Slender silhouette' sweetgum and 'Fastigiata' European hornbeam, may also be an option for tight spaces.

Be mindful that the more impervious surface the greater the reflective heat from buildings, streets, and parking lots. Sun scald may become an issue during the summer. Trees to consider avoiding include: Bradford pear, due to poor branch structure and unpleasant smelling flowers, and female ginkos, which have a bad odor when fruiting.

Which trees have roots that won't break sidewalks and extend too far away from the tree?
All trees have roots that extend out a long way as they grow and mature, and sidewalk maintenance will be a factor when trees are planted adjacent to sidewalks. Just make sure to follow the minimum planting dimensions (see table). Vase-shaped trees like Princeton elm or Japanese zelkova would be better from an above ground aspect. It's important to match up the right tree characteristics with the site limitations.

Of course, some cities and towns have areas that are natural gathering spots or downtowns that are highly urbanized. So space can be at a premium with streets, sidewalks, parking, shops and restaurants.

Designers and arborists can help municipalities ensure there is enough room for tree canopy, roots, and soil to perform environmental functions by putting in infrastructure beneath the pavement. Trenches can be dug to create room for tree roots under pavement, and structural soils, suspended sidewalks, or structural cells are strategies when space is minimal. Since this is an expensive endeavor, priority areas must be chosen in cities and towns.

Which trees would require little time and labor to maintain?
All trees require time and labor to maintain, but the right trees in the right locations are the easiest to maintain. For example, if you are planting trees in a sidewalk setting, make sure you have at least minimum planting dimensions.

If you don't have adequate root space, eventually you will likely have a buckling sidewalk. In areas where space is tight, structural soils or plastic cells can be used to help create room for tree roots under paved areas. Also, know that over time, maintenance like root pruning will be needed even if the minimum planting dimensions are followed.

Low maintenance really begins with proper planting. Even if you purchase a well-formed tree, if it's planted too deep, too shallow, with the burlap and straps left on it, or without enough water, it will be an unhealthy tree prone to disease or pests problems because of stress and will be short-lived.

Choosing trees with good form from the nursery is also key for minimizing maintenance.

When looking at quality trees in a nursery setting, arborists are looking at trunk, branch structure, crown, leaf and root characteristics. Generally, the form that arborists like to choose in high-use areas are single trunk with a single leader. Trees with encircling roots (those that have been in containers too long) will not grow well and likely die after a few years.

Trees that survive into maturity with girdling roots will likely be unstable and subject to windthrow, when trees are uprooted. An arborist can help in selecting quality trees.

Also, look what is overhead, and imagine how the tree is going to impact the vertical space. For example, if there are power lines, make sure to plant a small-maturing tree to minimize future conflicts.

From an environmental perspective, are there trees that help cities reduce air pollution?
Trees are efficient air filters, trapping particulate pollutants in the leaves and bark, and also absorb volatile compounds (air pollutants) through the leaves. They also cool the air, and this reduces the volume of evaporated pollutants. Even a few degrees of reduction through shading can reduce air pollution.

The larger trees are going to give cities the most value over time. Larger trees can hold more particulate matter, and they have longer life spans. Plant the largest tree that can fit in the planting area and also keep in mind what is overhead (i.e. electric lines).

Minimum planting dimensions for large, medium and small maturing trees
(with usable soil depth of 1 foot):
Tree size​Height in feet​Total square foot planting area​Minimum planting strip size in feet​Placement from pavement or wall in feet
​Large​>50​200​7​7
Medium​25-50​100-200​4-7​4
​Small​<25​<1004​​2