G.M. DUNN1, K.F. LOWE2, D.W. TAYLOR1 and T.M. BOWDLER2
1Queensland Department of Primary Industries, Queensland Forest Research Institute, Gympie, and
2Ipswich, Queensland, Australia
An experiment evaluating Albizia lebbeck, Casuarina cunninghamiana and Eucalyptus maculata at 5 densities amongst a pasture containing Chloris gayana cv. Callide (rhodes grass), Chamaecrista rotundifolia cv. Wynn (Wynn cassia), Stylosanthes scabra cv. Seca (stylo), Lotononis bainesii cv. Miles (lotononis) and Trifolium repens cv. Haifa (white clover) was established in south-east Queensland in 1990. The 5 planting densities (78, 182, 343, 771 and 1189 stems/ha) were replicated on lower, mid- and upper slope positions.
Over the first 3 years, C. cunninghamiana (mean height 5.5 m) grew significantly (P < 0.05) faster than E. maculata (4.3 m), which, in turn, grew significantly faster than A. lebbeck (2.6 m). Due to its poor growth and a mortality rate of 12.3%, A. lebbeck was considered poorly suited to the site. The heights of the 3 species were not significantly affected by planting density or position on the slope. Stem diameter at 1.3 m (DBH), measured for 3-year-old C. cunninghamiana only, peaked at 343 stems/ha. The DBHs of mid- and lower slope trees were similar and significantly (P < 0.05) greater, by 17.4%, than those of upper slope trees.
During the third year after establishment, median pasture yield (measured midway between trees at all spacings) was significantly (P < 0.05) increased under the highest 2 density treatments of A. lebbeck (by 17.7% and 39.6%) and E. maculata (by 25.5% and 17.2%) when compared with the other density treatments. The proportion of pasture in each plot killed by the application of herbicide around the base of establishing trees increased with increasing density, from 1.4% (78 stems/ha) to 21% (1189 stems/ha). Thus, although median pasture yield was significantly higher under the highest densities, yields adjusted for these sprayed areas were similar and unaffected by tree density, averaging 7.4 t/ha. At this early stage, pasture yields were also unaffected by tree species or slope position.