Tree roots vary in size from fine non-woody roots to large woody roots reaching 12 inches or more in diameter. Trees have evolved root systems in response to the ecological, soil and climatic conditions found in their native area, which have become part of their genetic inheritance. Tree roots exhibit as much variability as the tree’s above ground morphological characteristics. Fundamentally, the root system provides support for the trunk and the crown of branches and leaves, but it is also, through a complicated plumbing system, the tree’s primary source of water and soil nutrients essential for healthy growth. As a general rule, a tree’s root system extends out from the trunk to a distance matching the spread of the canopy. It is not apparent, but the magnitude of a tree’s root system exists in a complex balance with the extent of the tree’s leaves and branches. If a significant extent of the tree root system is destroyed, a corresponding portion of the branches and leaves will die back. Severe pruning of branches and leaves will likewise cause a proportional die back in the root network. This close relationship is the reason why when a large tree is transplanted, the loss of roots from digging it out of the soil will be balanced by pruning of leaves and branches. In this manner, the newly-transplanted tree will start out with a degree of functional equilibrium.
Major tree roots and their branches are typically woody and develop in a radial pattern from the trunk. Roots grow in response to the presence of the resources needed to sustain the tree. If the soil is compacted or a rock layer is present, the roots will avoid such obstacles and change their growth direction. Some trees possess a large prominent tap root which extends deep into the soil in order to reach subsurface water sources. Other trees may lack a tap root but instead have an extensive network of small-diameter, near-surface roots to capture surface water. Extending out from the larger roots is a system of fine non-woody feeder roots, which have multiple tips and are the main site for the absorption of moisture and minerals. Trees grow best when light clay-loan soils allow abundant root growth. A substantial portion of the root system of all trees is found in the top few inches of the soil.
Extracting needed minerals from the soil is a complex process and trees do not accomplish it alone. Trees have developed a mutually-beneficial relationship with certain specialized types of mycorrhizal soil fungi. These fungi colonize tree roots and increase their area of nutrient absorption. The fungi release enzymes into the soil that help dissolve organic nitrogen, phosphorus and other nutrients making them available to the tree. The benefit to the mycorrhiza is that the host tree provides conditions for fungal growth and reproduction. If the soil lacks these mycorrhiza, tree growth is retarded. This condition may be remedied by inoculation of the soil with commercially-available mycorrhiza. Gardeners should be aware of the normal root growth patterns of their trees. Tree root health can be assured by allowing sufficient space for normal root development, avoiding soil compaction beneath the canopy to prevent oxygen deprivation of the roots and conducting only limited root pruning.