AGROFORESTRY RESEARCH – THE JCU NELDER WHEEL
A Nelder wheel was established on JCU’s Townsville campus in August 1999, using Khaya senegalensis (African mahogany) supplied by North Queensland Afforestation Association. The trees are planted along the spokes of the wheel at increasing spacings – allowing study of tree growth at different densities (which relates to plantation management/thinning and yield) and growth of pasture along the shade gradient. The 16 replicated spokes allow a range of other studies to be made (e.g. response to fertiliser treatments, watering regimes, pruning techniques etc.)
Some close spaced blocks were also established for shading experiments, and to provide sacrificial trees for harvesting, to determine relationships between age, height, diameter and biomass and nutrient standing stocks.
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The JCU Nelder wheel in December 2004 - 5 year’s old
The layout of the Nelder wheel -16 spokes with 10 trees along each, spaced at 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5 and 5.5 m spacings.
An initial purpose for these plantings was to provide a shade gradient for a study of shade-tolerance of pasture legumes, undertaken by Heath Addison and supported by RIRDC. Hence the close initial spacings. Introduction of legume cover crops into plantations can reduce erosion, shade the soil in the early establishment phase, increase soil organic matter and soil biological activity, improve soil structure and fertility, and increase the growth rate of trees (Jayasinghe 1991; Lal et al. 1979). The grazing of cattle under tree plantations can result in several benefits, including increased and diversified income, soil stabilisation, and higher plantation crop yield through better weed control and nutrient cycling, including nitrogen accretion (Shelton 1991).
Heath looked at growth of Clitoria ternatea (Butterfly Pea), Chamaecrista rotundifolia (Wynn Cassia) and 4 accessions of Arachis under the blocks, and the first 2 species, plus Centrosema brasilianum (cv. Ooloo, Centro) and Stylosanthes hamata (Caribbean Stylo) in the Wheel. As well as pot trials in the shade-house, he had other field trials at Clare and Babinda.
Heath found that several species still produced more than 60% of their maximum yields under 76% shade. Of 35 cultivars examined, 16 were potentially useful. The most promising species for the wet tropics were Desmodium intortum (Green-leaf Desmodium), Calopogonium mucunoides, Arachis pintoi, D. ovalifolium, D. canum, Centrosema acutifolium, Pueraria phaseoloides, D. heterophyllum, C. pubescens, D. uncinatum and C. macrocarpum. Clitoria ternatea, Arachis stenosperma, Macroptilium lathyroides (Phasey Bean), M. atropurpureum (Siratro) and Centrosema brasilianum were the most successful of the species suited for seasonally dry tropical regions. Chamaecrista rotundifolia was most suited to conditions where slashing was frequent and, as such, may prove useful in plantations that are frequently defoliated (slashing or grazing), or where understorey growth is kept low.
However, many of the species identified as shade tolerant, or being relatively productive under shaded conditions, have a climbing/twining habit that can cause concern in tree plantations, particularly in respect to potential smothering of small trees. When the climbing species are removed the remaining species recommended for the wet tropics are D. intortum, A. pintoi, D. ovalifolium, D. canum, D. heterophyllum and D. uncinatum, while A. stenosperma shows potential for the seasonally dry tropics. Nevertheless climbing species may still be useful beneath older plantations, or where a higher level of pasture management is acceptable in order to help prevent smothering of trees through controlling the frequency and intensity of grazing.
See Jacob Thomson’s project.
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Content created by Bob Congdon
Updated on 4-February-2008