Root distribution of Eucalyptus nitens and E. globulus in irrigated and droughted soil

Root development of Eucalyptus globulus Labill. and E. nitens (Deane and Maiden) Maiden grown in Tasmania was compared between treatments that had been continually irrigated since establishment or subjected to cycles of drought stress. Roots in diameter classes 1–2, >2–5, >5–10 and >10 mm were sampled and their frequency recorded in trench walls excavated to bedrock. Length and mass of roots from each diameter class were measured from cores taken from depths 0–10, 10–20 and 20–30 cm. Root length density (RLD) and root frequency (RF) in trench walls were greater in droughted than irrigated soil of both species, in smaller diameter than larger diameter roots and in shallower than deeper soil. E. nitens had greater RLD, root mass density (RMD) and RF than E. globulus. E. nitens had a larger and deeper root system than E. globulus with a greater proportion of RLD, RMD and RF below 10 cm. Reducing the irrigation load during the growth cycle will stimulate additional root development in irrigated or effluent-irrigated plantations, reducing windthrow.