How soil compaction affects trees

Tree root systems are shallow and widespread, extending far beyond the edge of the canopy. Many delicate and tiny feeder roots grow near the ground surface. It is a myth that trees have deep taproots that anchor them into the ground. In fact, few mature trees possess a significant taproot.

Surface soils often are compacted on construction sites as a consequence of equipment moving over the soil. The most severe damage occurs in the top 4 to 6 inches of soil (Craul 1994), but soil structure can be affected deeper as well. The degree of compaction depends on several factors: amount and type of pressure and vibration applied, presence and depth of surface organic litter, soil texture and structure, and the soil moisture level (Adams and Froehlich 1981). Compaction is maximized on soils with a wide range in particle size (e.g., loamy), with weak structure, and with a moisture level at field capacity on which a heavy, vibrating weight is applied in a small area. The greatest increase in soil density occurs during the first few equipment passes over the soil (Adams and Froehlich 1981).

Compacted soils affect tree growth by restricting root activity and development. When macropores that allow air and water movement are collapsed, a larger proportion of micropores are left through which air and water move slowly. The result of the increased density is slow water infiltration, reduced drainage, poor aeration, and increased potential for erosion. The tree response is reduced root activity, increased resistance to root penetration, less mycorrhizal activity, and increased susceptibility to root diseases (Harris 1992). Furthermore, dense soils present a physical impediment to root elongation.

Matheny, N. & Clark, J.R., Trees and Development: A Technical Guide to Preservation of Trees During Land Development, 1998, International Society of Arboriculture, Champaign, IL.