Early growth responses of blue gums on the basaltic plains to ripping, mounding and fertiliser application

Abstract

The effect of ripping, mounding and fertiliser application on growth of E. globulus spp. globulus (Tasmanian blue gum) was assessed on 2 sites on the basaltic plains in south-western Victoria. The sites were gravelly loam duplex soils on a well-drained hillcrest and a clay–loam duplex soil on a valley flat that has impeded drainage in winter. The ground preparation treatments comprised a control (herbicide only), ripping to 65 cm, ripping plus small mounds (20 by 40 cm) and ripping plus large mound (60 by 150 cm). The fertiliser treatments were nil, 125, 250 and 500 g of fertiliser (nitrogen, phosphorus, potassium, sulfur) applied per tree 2 months after planting. The basal phosphorus, sulfur and potassium content of topsoil was 8–11, 11–14 and 140–220 mg/kg, respectively, for both sites. Rainfall in 1996, 1997 and 1998 was 695, 497 and 697 mm, respectively. This paper reports growth of 2 seedlots of blue gums (Yeodene Seed Orchard and Otway Ranges, Lorne) at 10 and 23 months after planting in Sep.–Oct. 1996 at 4 by 2 m spacing. The mean height of trees on the hillcrest site was 1.5 and 4.2 m at these times, respectively. For the valley flat, the mean heights were 0.9 and 3.0 m, respectively. On the hillcrest, ripping significantly increased tree height in the first year, but after 23 months neither height nor stem volume was greater than on unripped lines. Use of small mounds did not affect growth of trees at this site, whereas large mounds increased growth. At 10 months, tree height was greater (P<0.05) on the ripped plus large mounds than on the ripped only treatment, but this effect was lost at 23 months. Stem volume was 15% greater than on the ripped only treatment at 23 months, and 22% greater than the control. Trees from the Otway Ranges seedlot were taller and had 17% greater stem volume than trees from the Yeodene Seed Orchard seedlot at 23 months. For the valley flat, there were significant seedling source × site preparation interactions for height at 10 and 23 months (P<0.05), and for stem diameter and volume at 23 months (P<0.01), with the Otway seedlot performing better on the large mounds. There were no effects of fertiliser application on height, diameter or stem volume at 23 months on either site. The results indicate that planting on large mounds can improve early tree growth on farmland sites on the basaltic plains, but ripping without mounding, or fertiliser application in the first 2 years, is unlikely to improve performance.