Urban Soils and Tree Root Issues

By Jessie McClellan

A couple of years ago, during a notably large storm, a prodigious water oak fell on my neighbor’s’ house. This beautiful old tree could not support the weight of the branches because the root system had been compromised from years of cars rolling, parking on and compacting the soil in the critical root zone. Fortunately, in this case, the family was not in the house and only the house and tree were harmed, but it offers valuable insight into the damage that human activity can cause the trees in our community.

The soil around us is constantly changing. Wind, heat, gravity, and water all affect our soils. These are important positive components in creating an environment for many ecosystems. In an urban environment, one additional component is changing the soil around us: people.

People have a huge impact on the urban soils. Enormous cavities are excavated for underground parking decks, the Georgia red clay is hauled away and piled to be used somewhere else, it is added to other tracts for other projects. The soil is leveled and graded and compacted. It is walked on and parked on, played baseball on and picnicked upon. Human activity is one of the main contributing factor in the changing urban soils.

This changing soilscape directly impacts the growing trees around us. Without careful consideration to how we impact the soils, human activity can have serious negative effects on our canopy.

One negative impact is the creation of a surface crust. This is caused by removal of the natural vegetation and organic layer combined with compaction caused by foot traffic or machinery,. This hard crust doesn’t allow water to infiltrate into the ground, restricts oxygen availability, and be absorbed by the tree roots.

Water drainage and insufficient aeration can have a negative impact on the absorption capabilities of the trees. If the soil is too expansive, such as with clay soils, it can become water-logged which can be equally bad for the root system.

When a new home is built, the ground must first be leveled. During this process, the topsoil is removed and often fill material is brought in from somewhere else. This can create layers in the soil that are different from the natural makeup of the land. These new layers do not have the veins that are found in the residuum soils, which can be damaging to existing trees and can cause problems when planting replacements. The change in soil composition, including the pH and chemical makeup as well as density of the soil can prevent trees from receiving the nutrients they need to thrive.

There are several ways to remediate these effects on our trees. The first is to be aware of compromised areas of your own landscape. Pay attention to where you park, where your children play, or areas that see more foot traffic. Try to park in areas that are beyond the drip line of trees or only on existing parking pads. Divert traffic away from natural areas or create trails that have proper erosion control.

If you notice that the roots have been damaged in your landscape, there are plant health care guidelines that can prolong the life of your trees and repair damaged soilscapes. Contact a certified arborist to help with your tree concerns and you can hopefully prevent your trees from falling during the summer storms.