J.R. WILSON

CSIRO Tropical Agriculture, St Lucia, Brisbane, Queensland, Australia

Abstract

Four tree species were planted into an old "rundown" green panic (Panicum maximum var. trichoglume) pasture on brigalow clay soil in a subhumid region, south-east Queensland. Studies under shadecloth have shown that the "rundown" condition due to immobilisation of soil nitrogen can be partly reversed by shading the pasture. This study examined pasture productivity and soil moisture status over 8 years, to determine whether re-planting trees into old pasture may benefit grass growth through shade from their canopies increasing soil nitrogen or be a detriment through increased competition by the trees for soil water.
Eucalyptus argophloia (eucalypt), and 3 legume trees, Acacia stenophylla (acacia), Albizia lebbeck (albizia) and Leucaena diversifolia (leucaena), were planted in January 1989 in plantations of 25 trees on a 5 m grid, with 2 replicate plots per tree species, randomised with open pasture control plots. Pasture yield was measured by cutting strips across the plots with a forage harvester towards the end of each growing season. At each harvest, pasture yield in relation to position from a tree was also measured by cutting small quadrats at 1/8, 1/4 and 1/2 the distance along a set diagonal between two adjacent trees. Soil moisture was measured at varying time intervals over the whole experiment using gypsum blocks at 0.1, 0.25, 0.5 and 1.0 m depths at positions 1/4 and 1/2 way between two trees. Tree height was measured in August 1993 and April 1997.
The experiment experienced mostly dry to very dry years, with only the growing season of 1995–96 receiving near average rainfall. Despite this, the eucalypts grew moderately well reaching an average 6.2 m height after 8 years with a final 50% shading of the plot area. The other trees grew very slowly reaching heights of 2.9 m (acacia), 1.8 m (albizia) and 2.6 m (leucaena) and providing shade levels over the plot of only c. 15–20%.
Compared with open pasture, growth of grass under acacia and albizia was not reduced, but in the final year, it was significantly reduced under leucaena by c. 25% and under eucalypts by c. 33%. Judging from both forage harvester and quadrat cuts, the decline in pasture productivity under eucalypts probably started by Year 4 or 5 after tree planting. Herbage N % was higher under trees than in the open pasture with the difference greater at the later harvests. Pasture yield under trees was higher nearer the stem than at a position midway between trees; this occurred despite lower light intensity and lower soil water concentration near the stem. The latter two results strongly suggest that tree canopy shade had a positive influence on availability of soil N. However, soil drying, particularly in the 0.1–0.5 m horizon, occurred faster under trees, especially the eucalypts. It is concluded that, in this sequence of dry years, competition between trees and grass for soil water outweighed any positive influence of tree shade on increasing availability of soil N.