Distribution and density of the root system of macadamia on krasnozem soil and some effects of legume groundcovers on fibrous root density

Abstract

Whole-tree excavations, root-core and minirhizotron studies indicate that the grafted macadamia tree root system is relatively shallow and spreading, with a short taproot and most of the fibrous root system near the soil surface, while ungrafted trees have a longer taproot. The length of fibrous roots diminished with depth and distance from the trunk. This pattern is consistent with other fruit trees, in that the highest density is generally within 1 m of the trunk. Values obtained in core samples in this study were 4.97 (± 0.43) cm/cm³ and 1.67 (± 0.45) cm/cm³ for 0–10 cm and 10–20 cm at 0.5 m from the trunk, and 2.34 and 1.08 cm/cm³, respectively, at 1 m from the trunk at Clunes. These values were similar to those obtained in separate studies in 1991–93, involving assessments at 5 cm depth increments down to 15 cm, where mean root length densities were 2.0–3.5 cm/cm³ and 1.3–1.9 cm/cm³ at 0–5 cm and 5–15 cm depth, respectively, 1.4 m from the trunk. Root length under old trees in bare soil at Dorroughby and Clunes, using minirhizotrons (0.25–0.40 cm/cm²) and soil cores (1.14 and 3.50 cm/cm³, respectively), was similar to that found at other sites in the study area (minirhizotrons 0.28–0.33 cm/cm²; soil cores 1.25–2.80 cm/cm³). There is an apparent lower rate of decrease in root length density with increasing distance from the trunk at 10–20 cm compared with 0–10 cm. New root growth occurred predominantly in autumn, but some new fibrous roots were produced in early winter and spring.

Proteoid roots were found in abundance in soil cores and adjacent to minirhizotron tubes and there were more of them in the root systems of younger trees at Clunes than with older trees at Dorroughby. Proteoid roots were found at a greater depth than previously recorded for other Proteaceae species, and appeared to retain their function in relatively dry conditions for more than a year. Non-proteoid fibrous roots at the minirhizotron surface appeared to be functional for about 1.5 years in relatively dry conditions, before decay after the onset of wet soil conditions.

The effects of 2 newly established perennial legume groundcovers on the root systems of younger and older macadamia trees were studied over 2.5 years. In general, the presence of groundcover either had no effect on the growth of the macadamia roots or increased the root length density at some sampling dates and some depths. At Clunes, where the proteoid root length density was higher than at Dorroughby, the presence of groundcover was associated with higher proteoid root length density than that with bare ground. Arachis pintoi cv. Amarillo generally had a lower root length density than Lotus pedunculatus.