Like the crown, the root system of various tree species has, more or less, a specific, characteristic shape. This is greatly influenced by the environment, and, in particular, the soil. There is also a certain correlation between the size of the crown and that of the root system. For instance, in the case of the spruce, with its thick, conical crown, down which most rain water runs to the ground, the spreading roots branch mostly in the circle circumscribed by the base of the crown, where the soil has the greatest surface moisture. In the beech and oak, where rain falls between the leaves or is conducted along the branches to their tips, the root system is spread out mainly around the trunk. In species which have a high consumption of water the surface roots extend beyond the dimensions of the circle drawn by the crown — as much as fifteen to twenty-five metres from the trunk of a mature aspen, poplar, ash, etc. The depth to which roots grow also varies markedly according to the species. There are trees with deep roots, which, at least in their youth, have what is known as a taproot penetrating to great depths and only a few branch roots, e.g. the Scots pine, juniper, larch, oak, walnut, ash and the like. Another group includes trees that have shallow roots spreading out in a circle and branching in the upper layers of soil, e.g. the spruce, jack pine, frequently the Douglas fir, birch, beech, hornbeam and others. These trees are not firmly anchored in the soil and are easily uprooted by a strong wind. Then there is a third group with a heart-shaped root system. Their roots grow downward at an angle; examples are the fir, lime tree, maple, etc.
As has already been said, the root system is greatly inlluenced and modified by the environment. In shallow soils over rock, heavy clay soils, or on sites with a high level of underground water, even species whose roots normally penetrate to great depths may have shallow anchorage. Conversely, in humus-rich sandy soils the roots of woody plants which normally spread out may penetrate to greater depths.
Roots anchor the tree in the ground, absorb water and the mineral elements dissolved in it, and serve as storage for reserve food supplies. The structure of the roots is very similar to that of the trunk, only somewhat simpler. Water from the soil is absorbed through the root hairs, which are filamentous outgrowths near the tip of each rootlet. The movement of water in the trunk is governed both by the osmotic function of the roots and by the rate of transpiration in the leaves.
Roots aerate the soil and promote the disintegration of the rock substrata. In this they are aided by the carbon dioxide they eliminate as well as by certain chemical substances which break up mineral particles. The nutriments in the soil are also made available by various bacteria and fungi growing in association with the; roots. Nitrogen-fixing bacteria that increase the nitrogen content of the soil live in nodules attached to the roots of the alder, black locust, honey locust and other woody plants of the pea family (Leguminosac). Most woody plants live in an association, or symbiosis, with fungi whose filamentous growths (hyphac) either encircle the rootlets of the plant (eclomycorrhiza) or penetrate the cells of the root surface (endomycorrhi/.a). The fungus makes the nutriments in the soil more easily accessible to the tree, besides supplying it with other complex compounds. It receives from the tree substances in return, mainly sugars. Chief ectomycorrhizal associates in the case of forest trees are mushrooms or other mushroom-like fungi. It is well known that certain species of mushroom grow in association with certain species of trees, e.g. the species Lrccinum is generally found in the company of the birch, aspen and hornbeam, Boletus beside the pine, oak, beech and spruce, Suillus is a companion of the larch, etc. The symbiosis of mushrooms and woody plants occurs primarily in soils rich in organic substances and raw humus; in soils with insufficient organic matter such a symbiotic association may change to a parasitic one.