The development of symmetry, rigidity and anchorage in the structural root system of conifers

Abstract

The stability of shallowly rooted trees can be strongly influencedby the symmetry of the 'structural' system of woody roots. Root systemsof forest trees are often markedly asymmetric, and many of the factorsaffecting symmetry, including root initiation and the growth of primaryand woody roots, are poorly understood. The internal and environmentalfactors that control the development, with respect to symmetry andrigidity, of shallow structural root systems are reviewed and discussedwith particular reference to Sitka spruce (Picea sitchensis Bong.Carr.). Areas where there is insufficient knowledge are highlighted.A scheme is proposed that represents the root system as a set ofspokes that are variable in number, size and radial distribution.Rigidity can vary between and along each of the spokes. The rootsystem is presented as a zone of competition for assimilates, whereallocation to individual roots depends upon their position and localvariations in conditions. Factors considered include the productionof root primordia of different sizes, effects of soil conditionssuch as the supply of mineral nutrients and water on growth of primaryand woody roots, and the effect of forces caused by wind action ongrowth of the cambium, giving rise to roots which, in cross section,resemble I- or T-beams, and efficiently resist bending.