Depth distribution of roots of Eucalyptus dunnii and Corymbia citriodora subsp. variegata in different soil conditions

Understanding depth distribution of roots may help develop an understanding of plant productivity and the limits to productivity by indicating which parts of the soil profile are being accessed for water and nutrients. The subtropical east coast of Australia provides climatic and soil conditions that produce some of the highest plant productivity rates in the country. This has been recognised by the hardwood plantation industry and over the last decade a substantial estate of plantations has been established with plans for further expansion. However, two of the major species used, Eucalyptus dunnii and Corymbia citriodora subsp. variegata, have had little published research directly related to root depth distribution in the area. We examined root depth distribution in established plantations of E. dunnii and C. citriodora subsp. variegata under three contrasting soil types using the techniques of soil trench profile and coring. The results showed that the fine roots of C. citriodora subsp. variegata are at lower densities in poorly structured subsoils than the roots of E. dunnii. The root densities of both species in the subsoils of a Vertosol soil (with high levels of reactive, shrink–swell clays) were lower than for the other soil types. In native vegetation Vertosols are often colonised by grasses with few, scattered trees from a limited range of species. Our findings show lower levels of root growth in the Vertosols, particularly into the subsoil and this is likely to be the reason that productivity on these, otherwise fertile soils, is restricted.