What do you do if you have a tree you love (or even just like fairly well) that you want to be preserved through the construction process? We often think of trees as strong and resilient. While their ability to change and adapt is impressive, they can be very seriously harmed - sometimes fatally - by injuries that we may not even notice or that seem like just a minor problem. A lot of the time when we notice a tree has been damaged, but it looks fine in the following months, we assume nothing is wrong. In reality, it often takes three years or more for a tree to die from the chain of events following a serious stress or injury. There are many threats to a tree's health on the job site, and keeping a tree alive and healthy after a building or landscape project requires thinking ahead and developing a tree preservation plan. Today we'll go over the kinds of damage a tree can sustain on the job site, and next week we'll talk about how to prevent damage and what to do if it happens despite your best efforts.
How can a tree be damaged during construction? We'll break it down into two main categories: damage to roots, and damage to shoots. I think we all know what roots are. Shoots are any part of the tree above the ground. The term is really more commonly used when referring to smaller plants, but it rhymes with roots, so we'll stick with it to make this easier to remember.
Let's start with damage to shoots. This is where we see the most obvious kinds of injuries to trees. A large truck might break low-hanging branches, or an excavator could gouge the bark. Open wounds provide entry points for pests and diseases, and if a cut to the bark is extensive enough it can limit a tree's ability to transport food and water.
Is breaking a branch really that much worse pruning a branch off? Yes - dramatically so. First off, the location of the cut matters. A proper pruning cut must be located at a union with another branch if possible, or just above a bud if a branch must be cut in the middle. Not all branch tissue is the same, and cuts made in these two locations will close off properly, while poorly placed cuts will remain open and put the tree at a much greater risk of infection. The other reason a break is worse than a deliberate pruning cut is that a break makes a ragged wound with a much larger surface area than a clean cut. It may not seem like a huge difference from our perspective, but if you think about it on the microscopic level an uneven, zig-zagging break will leave much more surface exposed than a straight cut, and that means several times as many chances for pathogens to get into the tree.
Bark damage can also be much more harmful to a tree than you would expect. In addition to leaving an open wound that can be infected, damage to the trunk also affects a tree's ability to transport water and nutrients. The kinds of cells responsible for getting water to the top of the tree and food to the bottom are all located in a thin layer under the bark of the tree. Most of the wood at the center of a tree does nothing at all aside from providing structural support. If more than one third of the circumference of the tree has damage into this thin layer, the tree will die. By now, you may think trees are starting to sound incredibly vulnerable to minor accidents. But we've only just gotten started on the ways trees can be negatively impacted on the job site.
While less visible, damage to roots is extremely common during construction and can be more harmful than what happens above the ground. In order to fully understand how roots are affected, it helps to have a little background information about where roots grow. Many people's mental image of a tree's root system looks a lot like the above-ground part of the tree mirrored underground. This is not accurate. A tree's roots grow mostly in the top 12" of soil and they extend up to three times the width of the tree canopy. The root zone is sometimes referred to as a root plate because of its flat, broad shape. This means that anything that happens in the top few inches of soil can have a dramatic effect on a tree since that's where most of the roots are. Damage to roots can come as a result of cutting, chemical seepage, compaction, or changes in soil level.
Cutting, as you might guess, occurs when digging in the root zone for any reason. Planting, digging footers and foundations, and laying lines for irrigation, invisible fences, lighting, and whole host of other utilities and amenities all have the potential to cut into a tree's roots. And don't assume that it doesn't matter unless you're cutting big roots - it's the small roots that do all of the work of absorbing water and soil nutrients, so they are just as critically important as the structural roots. Trees can tolerate severed roots to some extent, but once more than a third of their root zone is disrupted the tree may not be able to recover. Some particularly sensitive species can't even survive that much damage.
Chemical seepage can poison a tree starting at the roots, causing serious damage or death. This happens most frequently when containers are rinsed and then dumped in the root zone, or when a container of fuel or another chemical has a leak, but sometimes chemicals are intentionally dumped directly onto soil because people don't realize the harm they can do. Paint, stain, concrete, sealers, cleaning chemicals, machine fluids, and many other chemicals can all soak into the soil to harm the roots. Some of these chemicals remain the soil for a long time and can also hurt other plants long after the project is over.
Compaction in Root Zone
Compaction occurs when heavy equipment is driven over root zones (especially if the soil is wet) or when materials such as topsoil and gravel are stockpiled under a tree. Compacted soils are more difficult for roots to grow through and they have less pore space to hold the water and air roots need. They also tend take longer to accept water, which means not all rain actually reaches to roots. On the other hand, once a compacted soil is finally saturated it can take a long time to drain, and roots can drown. Last year we did an extensive series on soils, and one installment in particular dealt with compaction, so check it out if you want to learn more about the effects of compacted soil on trees and other plants.
Changing Soil Level
The last way in which roots can be damaged is by changing soil levels. This damage is different from the other three because it is done intentionally as a result of design decisions, while the others are all side effects of other processes happening on site. Raising or lowering the soil level in a tree's root zone by as little as 3" can have a devastating effect on a tree. Lowering the soil level is a problem because it removes roots, and raising the soil level can suffocate the roots. One of the reasons why roots stay so near the surface is that it allows easy gas exchange between the soil and the air and because it gives the roots the best access to water from rain and irrigation. Adding just a few inches of soil can negate these benefits and slowly suffocate and dry out your tree.
With all these dangers lurking about the construction site, how can you keep your trees safe? Don't worry - with some early planning and diligence, your trees can be protected and preserved for years to come. Next week we'll go over how to protect a tree, what to do if it is injured, and when to call an arborist for expert advice.
Additional Resources (ordered from least to most extensive)
"Avoiding Tree Damage During Construction" - International Society of Arboriculture
"Construction and Trees: Guidelines for Protection" - Purdue University
"Protecting Trees from Construction Damage: A Homeowner's Guide" - University of Minnesota