Effect of some common weed species on Pinus radiata growth at a dry South Island site

Abstract

Reductions in radiata pine (Pinus radiata D.Don) growth caused by a range of common weed species were measured at a site where water availability was known to limit tree growth. Radiata pine seedlings were grown for 3 years on their own or with volunteer herbaceous broadleaves, broom, gorse or browntop. Water and nutrient levels were varied by factorial +/-irrigation and fertiliser treatments. Radiata pine stem volume was reduced in the presence of all four weed types. Herbaceous broadleaf weeds had the greatest effect on tree growth, gorse had the smallest effect, and broom and grass were intermediate. Water supply was thought to have a major effect in determining the influence of weeds on tree growth. INTRODUCTION Due to increasing costs and pressure to minimise herbicide use, it is essential that weed control operations in radiata pine plantations should be applied only at the level required to give optimal gains. Priority should also be given to targeting species that have the greatest impact on crop growth. These objectives can best be achieved by understanding the nature of the interaction between the crop and the weeds, and how this varies over different sites and climates. To this end, a study was designed to investigate the effect on early tree growth of some prominent New Zealand forest weed species.