EFFECT OF ANNUAL WEEDS ON WATER AND NITROGEN AVAILABILITY TO PINUS-RADIATA TREES IN A YOUNG PLANTATION

The importance of competition between trees and annual weeds for nitrogen (N) and water was evaluated in a Pinus radiata D. Don (radiata pine) plantation between two and three years of age growing on a sandy podzol in the southeast of South Australia. Increasing the width of the weed-control strip spanning the tree row had little effect on tree water status, but increased the uptake of N by trees. Application of N fertilizer alleviated N deficiency and tree growth suppression induced by weeds, suggesting that when N supply is high (e.g., on fertile sites) intense weed control is unnecessary in plantations beyond two years of age. On the other hand, competition for N by weeds would seriously aggravate N deficiency in young pine trees grown on low-N soils. When N fertilizer was applied, weeds rapidly absorbed mineral N from the soil, and within three months, the N content of weeds was 10.2 g N m-2 greater than in the unfertilized control. This represents 68% of the amount of N applied in fertilizer. Weeds therefore increased the uptake of fertilizer N by plant biomass, and thereby improved N retention on site. Tree growth increased with N fertilizer at all levels of weed control and responses to weed control and N fertilizer were additive. The growth of weeds increased the soil organic carbon status and the rate of N mineralization. The presence of weeds reduced the incidence and severity of stem deformation, which was exacerbated by N fertilizer application and complete weed control. In general, strip weed control may be a better option than complete weed control in managing weeds in young P. radiata plantations.