Soil factors affecting tree stability on certain afforested heathlands

A large proportion of the national annual planting programme is now concentrated on the great areas of upland heaths. The early success of such a plantation depends not only on the quality and fertility of the heath but also on the prevailing climatic conditions. However, after establishment, the crop may be liable to wind damage or early senescence due to a shallow restricted root system. Ironpan formation and indurated subsoil condition has been suggested by Yeatman (1955), Zehetmayr (i960) and others to cause critical impedance to the rooting and is therefore likely to impose limiting factors on both timber yield and tree stability. Recent work by Fraser (1962, 1965), Fraser and Gardiner (1967), and Pyatt (1968b) has established a strong relationship between soil types, rooting and crop stability, particularly for sites suffering from impeded drainage. Little attention has been given to crops on well drained sites, as wind-firmness is considered adeoxuate and obvious problems, such as drainage, do not exist. However, windblow damage does occur. The initial rooting habit of trees is generally considered to be a function of heredity and once the tree is established, changes in the root form begin to take place as a response to variations of the soil environment. The object of this investigation was to study the nature and magnitude of the physical soil factors within the rooting zone, their effect upon the spatial arrangement of the root system and the combined influence on tree stability, to determine: (i) the relationship existing between the physical soil factors within the rooting zone; (ii) the predominant physical soil factor(s) limiting root growth; (iii) the extent to which soil factors affect root form and development; (iv) the affect of variations in root form on tree stability within a given site; (v) differences in stability on sites where root development is mechanically restricted and where development is essentially unrestricted. The study was confined to well-drained soils, at Allerston Forest, Yorkshire, bearing Scots pine (Pinus sylvestris, Linn.) whose root systems were exposed by winching over of each selected tree (a technique developed by Fraser 1962) and the type of development compared with an evaluation of the major physical soil properties prevailing within the root environment.