Trees offer a number of important benefits to the landscape, which is why many people want to try to save them during construction projects. A mature shade tree can have a cooling effect on the surrounding landscape in the summer and act as a windbreak in the winter months, reducing heat loss from homes.
Trees can also prevent runoff and limit erosion, especially on sloped areas. Above all, trees are beautiful and make people feel good. It’s easy to become attached to large trees that have been part of the landscape for decades, and wanting to protect and save them during construction is a natural reaction. Unfortunately, this is a difficult task that requires careful consideration and planning in order to be successful.
Assess Tree Health
Before putting any effort into trying to save a landscape tree, you need to determine whether it is worth saving. It costs far more to remove a tree that is next to a structure than from an open lot. In many cases it might be more cost effective to cut down an old tree that is adjacent to a construction project and replace it with a new, younger tree. For a tree to be a good candidate for preservation it needs to be healthy and vigorous without any significant structural defects. It also needs to be located far enough away from the building site for the majority of the root system to remain unharmed. Consider having an arborist come to assess the tree and help you make a plan for how to save it.
Some tree species are more tolerant of nearby construction than others. Certain trees are very sensitive to root disturbance and are not typically good candidates to try to save. This list includes: oak, white pine, sugar maple, beech, hemlock, black walnut and tulip tree. Other species are quite tolerant, such as: river birch, black gum, crabapple, honey locust, larch, red maple, sycamore, Norway spruce and white spruce. This is hardly a complete list, and an arborist will be able to give a better estimate of a tree’s chances of survival.
Obvious injuries like broken branches and torn bark usually aren’t nearly as serious as damage to tree roots. Trees often die several years after construction, or else have thinning crowns that correlate with root injury. Thus, protecting the tree’s root system is the most important thing that can be done, and it starts with understanding where tree roots are located and how they function.
Though artists often draw tree roots extending deep into the soil, that’s not an accurate picture of how roots actually grow. Almost all tree roots can be found in the top 6-24 inches of soil. Just one large taproot extends deeper into the soil, providing the tree with stability. The rest of the roots extend outward to take up water and nutrients, going out at least as far as the tree is tall, possibly even twice that distance.
Roots are damaged when they are cut or with compaction or by changing the grade. Cutting roots usually occurs on just one side while excavating or digging trenches for utilities, which can make trees unstable and prone to blow-down in storms. Compaction of the soil around the roots can be just as devastating. It is well-known that plants “breathe in” carbon dioxide and release oxygen, but that only occurs in the green, above-ground tissues. Roots beneath the ground need to take in oxygen. When the soil is compacted by continuous foot traffic or heavy machinery, there are few pore spaces remaining that can hold oxygen. Along these same lines, water does not percolate very well into compacted soils, keeping roots from getting the moisture they need. Additionally, changing the grade of the land around a tree can also cause issues by smothering roots with too much soil or exposing them.
How to Save Trees
Once you have assessed the health of a tree, and considered the ways it might be damaged during construction, you can go about making a plan for how to save it. First and foremost it is important to protect as much of the root zone as possible. A healthy tree may survive if at least 60-70 percent of its root zone is unaffected. A good practice is to avoid doing any work within the tree’s dripline — the area between the trunk and the reach of its furthest-most branches. Look to follow these guidelines to give trees the best chance of survival.
- Install fencing around the root zone to keep vehicles and workers away. The fencing should extend at least to the dripline – the further the better. A more accurate way of determining where to put the fence is to measure the diameter of the tree 4.5 feet off of the ground (diameter at breast height, DBH). For every inch of trunk diameter, extend protection by a foot. For example, if a tree is 10 inches in diameter, fencing should be located at least 10 feet away from the trunk.
- Do not drive heavy machinery over the root zone.
- Minimize foot traffic in the area and, if unavoidable, build boardwalks or lay down sheets of plywood to limit compaction.
- Do not store building materials over the roots.
- Do not change the grade of the site near the tree. Avoid exposing tree roots within the dripline and do not cover roots with extra soil.
- Use permeable pavers in walkways or patios that allow rainwater to percolate into the soil beneath.
- Do not wash equipment or dump chemicals over the roots.
- Make sure road runoff isn’t going to be directed to the tree with grading changes. Exposure to road salt can kill or further stress trees.
- Water during and after construction whenever there is less than one inch of rainfall in a week.
- Aerate and fertilize if necessary.
- Apply mulch over the roots to conserve soil moisture.
- Keep a close eye on the canopy for signs of dieback. When roots are injured there is typically a corresponding loss of branches aboveground. This can occur several years after the initial injury. Have an arborist prune out dead and weak branches to remove hazardous conditions.
Ultimately, saving mature trees during construction is not easy, but it is possible with care. Large trees can’t be replaced, so sometimes they are worth the effort.
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