Ground sinking after a tree is removed is a common consequence of altering the underground environment. When a large tree is cut down, its extensive root system remains below the surface as dead organic material. This material, which once provided structural support for the soil, inevitably begins to decompose. As the roots break down and lose mass, they leave behind voids. The overlying soil then collapses slowly into these spaces, resulting in a visible depression.
The Mechanism of Root Decay and Void Creation
The sinking is caused by the microbial decomposition of the dead root material. Once the tree is severed, the roots die and become a food source for soil microorganisms, including fungi and bacteria. These decomposers break down the two main structural polymers of wood: cellulose and lignin.
Cellulose is degraded more quickly by microbes, especially in the early stages of decay. Lignin, a complex biopolymer, is far more resistant and breaks down at a much slower rate. As decomposition consumes the mass of the roots, the physical volume they occupied is lost, creating empty space underground.
Volume loss is complicated by the roots’ role in soil moisture management. A living tree absorbs large amounts of water daily, drying out the surrounding soil. Once the tree is gone, the soil retains more moisture, which affects its density and structure. This exacerbates the settling process, leading to visible subsidence combined with the loss of the root network’s binding structure.
Variables Determining Subsidence Severity
The depth and duration of sinking depend on several environmental and biological factors. The initial size of the tree is a major factor, as a larger tree means a more extensive root system and a greater mass of organic material to decay. The species of tree also plays a role; roots from dense hardwoods like oak contain more lignin and may take a decade or more to decompose fully, leading to a slower, more prolonged period of sinking compared to softer woods like pine.
Soil composition is another determinant of subsidence severity. In sandy or loamy soils, settling occurs relatively evenly as the material compacts easily into the voids. Conversely, in heavy clay soils, which are prone to volume changes based on moisture content, the settling may be more erratic or complicated by unrelated ground movement.
Environmental conditions, particularly moisture and temperature, directly influence the rate of decomposition. Warm and wet climates accelerate microbial activity, meaning settling happens faster and subsidence is noticeable sooner. In cooler or drier environments, the decomposition process is slowed, extending the period of settling over many years.
Practical Steps for Repairing the Sunken Ground
The most effective way to repair the sunken area is to first allow a period for the initial, rapid settling to occur, usually a few months. Once the rate of sinking slows, the depression can be filled using a suitable material that will not continue to decompose. The recommended material is a mixture of high-quality topsoil and fill dirt, often blended with sand to improve drainage and stability.
Avoid using pure organic material like compost or wood chips, as these will continue to break down and cause future sinking. Proper compaction is key to long-term stability. This must be performed by filling the void in shallow layers, known as lifts, with each layer being firmly compacted before the next is added.
Watering the fill material thoroughly aids in settling the soil particles and achieving maximum compaction. Once the final layer is added, the area should be slightly overfilled, creating a minor mound above the current grade. This slight overfill accounts for the residual settling that will occur over the following months, ensuring a level surface is maintained.