Developmental processes in tree root systems

Abstract

Factors that influence the primary and secondary growth of roots are reviewed in relation to the development of the form of tree root systems. The development of occasional root apices of larger than average diameter is important because they form the main axes that undergo secondary thickening and become permanent members of the woody root system. The formation of these large apices is influenced by injury to, or reduced growth or dormancy of, the subtending main root axis and by proximity to the shoot. The base of the taproot and laterals is seen as a region strongly influenced by shoot activity, resulting in the formation of additional large root apices and enhanced secondary thickening to form the zone of rapid taper in trees, and in the formation of storage organs in some herbaceous species such as radish. The main root axes compete for assimilates and dominance is established between them at an early age. The unequal growth of competing lateral roots is influenced by the local environment of the roots of primary structure. The role of root apices on secondary growth is discussed with reference to work on herbaceous species. In trees the local environment has some direct effects on the root cambium, but such effects appear to be less important than the activity of the roots of primary structure. A hypothesis is developed incorporating the Japanese Pipe Theory for the allocation of assimilates for the secondary growth of tree roots.