ORIENTATION OF THE LATERAL ROOTS OF TREES .1. UPWARD GROWTH OF SURFACE ROOTS AND DEFLECTION NEAR THE SOIL SURFACE

The main lateral roots of some tree species grow close beneath the soil surface, even on irregular or sloping ground. Experiments were carried out to explore the origin and behaviour of such surface roots. Pinus contorta Douglas ex Loudon and Picea sitchensis (Bong.) Carr. plants were grown under growthroom and glasshouse conditions. Lateral roots were excavated and vertical and horizontal co-ordinates measured. The liminal angles (0-degrees = vertically downwards) of first-order lateral roots of seedlings of both species decreased linearly with depth of origin on the taproot. Some of the roots of highest origin had liminal angles > 90-degrees (ie they grew upwards). However, these roots deflected downwards on approaching the soil surface. If additional soil was added soon after germination the upper lateral roots grew further in an upward direction and then deflected beneath the elevated surface. Two-year-old Picea sitchensis transplants were planted centrally in large containers of soil and an elevated ridge of soil was made 160 mm to one side of the plants. Vigorous upwardly growing roots developed from the damaged and bent root systems that had been produced in the forest nursery. The upwardly oriented roots deflected horizontally when their tips arrived below the flat peat surface, but when they extended beneath the ridge they turned upwards within it. As they approached the top of the ridge they again deflected downwards. It is concluded that the direction of growth of surface roots is the result of two opposing influences. Certain of the main lateral roots are plagiogravitropic and grow obliquely upwards but as these roots approach the soil surface they respond to some signal from the environment that causes downward deflection. The nature of the signal was not determined but it was shown that downward deflection occurred whether or not the soil was exposed to light. The biological importance of this form of surface root orientation is discussed. The growth behaviour described enables the main lateral roots to explore the nutrient-rich surface layers of the soil while avoiding the injury to the root tip that would result from exposure.