Roots and Respiration

It’s an almost universally known fact that trees and actually all green plants are the main source of oxygen on earth. Without green plants, giant or microscopic, life on earth as we know it could not exist.  At a young age, we learn that plants take in carbon dioxide and give off the oxygen we breathe. As we age, we learn that plants take in carbon dioxide, water, and nutrients and then utilize chlorophyll and sunlight to break the oxygen free from carbon dioxide and combine it with water (H2O) to create simple and complex carbohydrates.  

Plants, and especially trees, essentially create something from nothing. In a natural environment, plants are able to take free resources and create the sugars and cellulose that feed us, clothe us, and shelter us. This is how most of the world sees trees, but there is a hidden side that many people do not understand or really care about, and that is roots.  Roots are hidden under the soil, often misunderstood, and most of the time completely overlooked by tree owners. It’s the old adage of “out of sight, out of mind.”  Most know that roots provide water and nutrients to trees and act as an anchor, but very few know how roots actually grow and that can lead to many problems.

One of the most surprising things to learn about roots is that they go through aerobic respiration just like animals and need to take in oxygen and give off carbon dioxide to live and grow. Sugars made in the leaves are moved through the tree in a downward flow towards the roots where oxygen and sugars are used to create energy for cell growth. While roots are taking in oxygen, the above-ground portion of the tree is creating much more oxygen than it uses so, as we learned as children, trees and all plants are overall oxygen producers. This surprising fact is made possible by a tiny and very boring-sounding term called “soil porosity.”

If you could take a cross-section of healthy, natural soil, and look at it, it should look like microscopic Swiss cheese or a sponge. The bits of soil, organic matter, and the like should be spaced out by some large and small holes that allow the flow of gas and liquids between pores and into plant roots. These pores allow water infiltration from precipitation and gas transfer and if they are not present, things start to go bad. First and foremost, water infiltration stops so ponding, puddling, and runoff occur, which is an immediate notice to plants and the people living among them. This lack of water flow into the soil can be a nuisance or danger to humans after significant soil disruption, but it is deadly to plants as they no longer have the water that is essential to their growth. This is an acute problem that can quickly bring about the death of a plant, while there is a secondary problem that can take much longer to kill a tree.

While water is the most essential occupier of soil pores, it can still find its way into even heavily damaged soils thanks to its mass that helps it get pulled into the soil via gravity and the surface tension of water that makes it able to flow through even small pores in the soil. The second most essential element for plants’ roots is the exchange and flow of gas in and out of the soil pores. This is where things can go bad without much notice as we don’t see the gas around us and there will never be a build-up of too much gas on the soil as we do with poor water infiltration.  

For this reason, soils can be heavily damaged without much attention being paid as tree roots begin to suffocate and die without any sign. This causes a slow and languishing death for the tree as the roots continue to die back closer and closer to the tree. This can easily be seen above ground several years after the damage first occurs in the dieback of branch tips on the exterior of a tree’s canopy. This above-ground symptom usually shows up in a window of 3 to 5 years after initial soil disturbance. In many cases, the root dieback underground has been progressing for so long that the tree is in a steep decline and will be very difficult to reverse without aggressive remediation. With the knowledge of how tree roots work and the essential role soil porosity plays, it’s now easier to understand the main causes of soil damage and how it can be reversed.

The single largest cause of damage to soils is construction, as soil is compressed and compacted either for a solid base for a foundation or simply the traffic of equipment and feet along a pathway. This compaction is essential to our modern infrastructure. Soils must be compacted to provide a stable base and foundation for our homes, roads, and businesses, so not all compaction is bad. It’s when soil is compacted in the root zone of a tree that problems arise.  This compaction greatly reduces the soil porosity and can be most damaging when the soil is wet. Roots in areas of compaction are often damaged by construction, but it’s more likely that the soil porosity is greatly reduced and root suffocation begins. As is the case with most things in life, prevention is key.

When working around trees, it’s extremely important to recognize where roots run so that they can be avoided as much as possible while still successfully performing the work.  Roots are typically seen as a mirror image of the above ground portion of the tree and this is another significant misconception.  Most roots are found within the top 2 feet of soil and can extend well past the drip line of the tree.  Roots in most cases stay shallow as that is where the best soil fertility occurs and is where gas infiltration and exchange happens in healthy soils.  Many of the small fine feeder roots extend past the drip line of the tree so keeping construction activity far away from the tree is best.  If that is not feasible, the general rule is to create a protection zone with a one-foot radius for every 1 inch of DBH (diameter at breast height).  Any protection around trees should be a physical barrier with no openings to prevent incursions into the protected zones. Orange construction fence is often enough on small projects but larger chain link fence panels should be used on larger projects.  This all occurs when there is proper forethought and planning in regards to the trees, but that rarely happens. 

When soil has been damaged due to some form of significant compaction, all is not lost as this can be remediated with proper care and attention. The most important factor is to recognize damaged soil and/or a tree with root damage as the clock begins ticking the first day the soil is damaged.  Addressing a compacted root zone or damaged roots directly after the damage has occurred can prevent any significant symptoms of decline and will most likely prevent the death of the tree.  After recognizing this damage, the soil must be decompacted utilizing mechanical or pneumatic tools to break up the soil particles and reestablish proper soil porosity.  In areas with limited vegetation, a rototiller can be used to break up soil in preparation for planting as newly planted trees and shrubs will not grow well in heavily compacted soils as their roots cannot penetrate the soil at the edge of the dug hole.  In newly constructed communities, newly planted landscapes often fail due to high soil compaction that is never addressed.  Quite often though, existing plants and their roots must be kept in mind because rototillers will do more harm than good to existing plants.

A simple-looking but highly engineered device called the Air-Spade can be used to blow high volumes of air into the soil to break it up and recreate soil pores all while doing very little damage to existing woody roots.  This device used large volumes of compressed air forced through a specially engineered tip that allows the air to break apart soil particles while only hurting fine root hairs that a plant can easily regrow.  Large portions of compacted root zones should be tilled using this tool and then composted organic matter must be added to act as a long-term protector of porosity.  After the soil is tilled and compost is applied, it is easy for the soil to settle and porosity to reduce soon after the treatment.  The organic matter in the compost will slowly break down, feeding the tree a steady supply of Nitrogen and leaving large pore spaces long after the initial soil settling.  This process can be a bit dirty and is the more expensive remediation option but it works very well to address damaged soils around existing landscapes and trees.

When we look at trees, we see the good they provide us: shade, wood, food, and simple beauty. But we are only seeing half of the tree. The underground portions of a tree are some of the more interesting parts when you think of the job they do.  Without healthy soils, it’s impossible to have healthy roots, and without healthy roots, there cannot be a healthy tree. Keeping roots healthy is easy by limiting soil compaction and keeping a natural nutrient cycle around the tree.  Fixing damaged soils can be much harder but it is possible in most cases and can be a fraction of the cost of having to remove the tree. Keep an eye on your trees and watch to see if they are showing you symptoms of decline.  Dieback at the top and tips of the canopy is quite often a sure sign of root damage and death.