By Tim Clancy.
Tree root damage to buildings, walkways, roadways, and other infrastructure components is a common concern for all. Damage repair can be expensive, as are personal injury lawsuits; some argue that in some cities in California, for every dollar spent on repairing pavement, more than double this amount is spent on litigation. Expensive indeed.
Mature Trees Meet Modern Infrastructure
There is currently no way of predicting when, where, or even if, a tree will cause damage because each site and tree is different. Damage may be indirect as roots cause moisture changes by extracting water from the soil, while direct damage by roots is common as roots grow. Soil type influences damage potential—soft, loose soils better accommodate root expansion by deforming to allow roots room to grow. More compact soils are not so accommodating and as roots grow, they can ruin infrastructure.
Young trees increase root length more readily than girth as the roots extend into the soil. As the trees mature, these roots will then increase in girth, like the branches and trunk above the ground, adding annual rings each year. When these roots are just under pavement, we can expect distortions of the pavement at some point. Most trees have surface roots and the factors affecting how these grow are myriad.
Roots tend to grow downward and away from light. They will first grow where conditions are most favorable. That is where there is sufficient water, oxygen, and nutrients. The conditions below paved surfaces often provide an ideal environment for root growth with more oxygen and water available than deeper in the soil. I have seen many cases where roots follow trenches where plumbing or electrical components are located below slabs. These roots exploit a breach in a plumbing pipe and clog it up. It is important to realize that the roots do not break the pipes, but gain entrance through an existing crack or a glue joint that was improperly glued. Roots can damage the integrity of a pipe through exerting pressure on the pipe as the root expands, but this only happens when the roots have no other space to occupy.
Limiting and Avoiding Root Damage
Species selection can be one answer to the problem of future root damage. Some species have many surface roots while others don't. Appropriate-sized planting locations are also a strategy to be utilized. Much severe damage results when a large tree is so close to the pavement that as buttress roots enlarge, they lift pavement. While planting smaller trees can be a good option, we must also realize that we are not going to have the shade we get from larger trees. Another method of addressing the issue of root damage in an urban city setting is to try to time concrete replacement with tree replacement.
In sum, if we want all the benefits that trees can provide, we need to realize that those benefits may come with some costs later on. Weighing the pluses and minuses out, however, replacing a sidewalk doesn't seem like such a bad prospect if I can enjoy twenty years of shade and habitat.