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 potential for further expansion. Two of the major species used, Eucalyptus dunnii and Corymbia citriodora subsp. variegata, have hadlittle published research directly related to the prediction of their growth in the area. This study developed growth models for both species to aid selection of suitable sites for establishment and it also investigated the root architecture of both species.
The model developed for E. dunnii was developed from correlation of data on growth with over seventy site and soil variables from across thirty-three plots with an average age of six years. The model provides an estimate of site productivity, in the form of site index at age 10 based on three simple site variables (rainfall, water storage capacity of the soil and altitude). The model for predicting growth of E. dunnii was relatively well correlated (R2 = 0.619) with site index at age 10.
In a similar manner a growth model for C. citriodora subsp. variegata across twenty four plots achieved good correlation (R2 = 0.710) between site index at age 10 with four site variables, two that related to water availability and two related to daily mean temperature. The patterns of the two species were quite different, confirming anecdotal observations of rapid early growth of E. dunnii compared to C. citriodora subsp. variegata but also showing the convergence of growth rates over time.
Extrapolation from the two models showed that potential gains of up to 18% in height growth could be made by applying the two models at each site to determine the optimal species for the site. The models were also used to examine alternative rainfall scenarios and quantified the effects of above and below average rainfall.
Root architecture was examined in established plantations of E. dunnii and C. citriodora subsp. variegata under three contrasting soil types. Results showed little support for the hypothesis that the fine roots of C. citriodora subsp. variegata penetrate better into 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.
Pot trials found contrasting root growth strategies between four species; E. dunnii, E. pilularis, C. citriodora subsp. variegata, and Grevillea robusta. In particular, E. dunnii and C. citriodora subsp. variegata had significantly lower root:shoot ratios thanG. robusta. E. dunnii and C. citriodora subsp. variegata had higher root:shoot ratios than E. pilularis, the difference being significantly different for C. citriodora subsp. variegata.
The younger C. citriodora subsp. variegata, in the pot trials, had higher root growth than E. dunnii but lower relative levels of fine roots, a difference not observed in older plantation trees. The change in root distribution over time provides an explanation of the rapid early growth of E. dunnii and the subsequent equalisation in growth by C. citriodora subsp. variegata as the trees mature.