MINERAL NUTRITION AND TREE ROOT GROWTH

Information on the effects of mineral nutrition on tree root systems is poorly documented but certain generalizations can be made from Sutton's (1969) review. Although different nutrients have varying effects on growth the overall level of nutrition influences the distribution of material between root and shoot in a characteristic way, namely an increase in soil fertility is associated with a reduction in the root/shoot ratio. Soil fertility also has a bearing on the vertical distribution of roots and although shallow rooting occurs more often on infertile sites (Kimmins and Hawkes, 1978) no causal connection has yet been proven. The effect of nutrients on the branching patterns of roots is more readily demonstrated. For example, the primary roots (i.e. roots of primary anatomical structure) of poplars have been shown to proliferate in the vicinity of phosphate (Van der Meiden, 1957). An uneven distribution of mineral nutrients in the soil might therefore be expected to have an effect on root form.

The form of tree root systems is often remarkably asymmetrical under natural conditions (Preston, 1942) and even more so in plantation trees established on ploughing. Uneven root development can originate from the way in which a tree is planted (Schantz-Hansen, 1945; Edwards, Atterson and Howell, 1963; Köestler, Bruckner and Bibelriether, 1968) but the local environment of the root system may eventually override distortions due to planting. On some spaced-furrow ploughed sites the raised turf apparently provides the most favourable conditions for root growth, because, although roots grow from the turf into the undisturbed soil beside it, the woody roots of large diameter tend to be concentrated in the turf itself. There is therefore a tendency for major roots to become aligned with the direction of ploughing, and the asymmetrical root development contributes to instability of the trees in high winds (Yeatman, 1955).

Although woody roots are essential for supporting the tree, surprisingly little is known about the factors that influence their growth in diameter. Exposure to light stimulates radial growth (Fayle, 1968) and mechanical stresses can modify such growth (Büsgen and Münch, 1929; Jacobs, 1954; Wilson, 1975). However, in addition to the effects of light and mechanical