Root Occupation and Root Channels of Jarrah Forest Subsoils

The role of root channels in the occupation of deep soil profiles by jarrah (Eucalyptus marginata Sm.) forest in south-western Australia was explored by coring and excavation. In profiles derived from granite, major descending roots are confined to channels that extend vertically from fissures and conduits in the shallow subsurface caprock layer deep in the clay subsoil. The channels are permanent features of the profile and are occupied by successive generations of trees. The channels have a diameter of 1-300 mm, abundance of 3 5 m-2 (all channels) or 1 m-2 (large channels), and occupy 1% of a horizontal surface at 6 m. Each forest tree has potential access to 100-200 channels. Large roots, fine roots with root hairs, and decaying root remains occur in the channels. Only fine roots (less than Imm diam.) penetrate the clay matrix and occupy the entire profile down to basement at depths of up to 40 m. In profiles derived from dolerite, occupation of the deep clay by large and fine roots is diffuse and root channels are absent. The dissimilar mode of root occupation of the two profile types may be explained by the difference in bulk density. In providing avenues for access by large roots as well as preferential wetting, the root channels may play a dominant role in the water and salt balance of jarrah forests on the granitic profile.