INTERSPECIFIC COMPETITION BETWEEN PINUS RADIATA AND SOME COMMON WEED SPECIES— FIRST-YEAR RESULTS

A trial designed to quantify the reduction in Pinus radiata D.Don seedling growth caused by competition from a range of important weed species was established at Rotorua, a moist North Island site, and at Rangiora, a South Island site with low summer rainfall. At both sites, P. radiata seedlings were grown on their own and with either herbaceous broadleaves (a volunteer mixture of species from which grasses were excluded), Cytisus scoparius L. (broom), or Ulex europaeus L. (gorse). Trees were also grown with Buddleja davidii Franchet (buddleia), Holcus lanatus L. (Yorkshire fog) plus Lolium multiflorum L. (Italian ryegrass), and Cortaderia selloana (Schult) Asch. et Graeb. (pampas) at Rotorua and with Agrostis capillaris L. (browntop) at Rangiora. Resource (nutrient and water) levels were varied by factorial ± irrigation and fertiliser treatments. At Rotorua, P. radiata stem volume after 10 months was greatest in weed-free, gorse, broom, and Yorkshire fog plots and least in herbaceous broadleaf and buddleia plots, with pampas intermediate. At this time, there was no strong evidence of interspecific competition for water or nutrients. At Rangiora, trees growing with gras