Indices of interspecific plant competition for Pinus radiata in the central north island of New Zealand

Pinus radinta D. Don was grown on its own and with a range of densities of either buddleia (Buddleja davidii Franchet) or broom (Cytisus scoparius L.) , two important forest weed species, in a field trial at Rotorua, New Zeala nd. Tree growth from the time of planting to age 3 was modelled as a functi on of tree size and a competition modifier. The competition modifier is, in effect, a multiplier that reduces tree growth according to the degree of c ompetition defined by a competition index (CI). A range of CIs, with some s ensitivity to both weed and tree growth and development over time, were ind ividually incorporated into the modifier and evaluated. The "best" CI combi ned measures of weed height relative to tree height, proximity of the weed to the tree, and weed abundance, and was negatively correlated with an inde x of light availability. For a given value of CI, the effect on tree growth was independent of weed species. For diameter growth, the effect of CI was independent of tree age. However, for height growth the negative effect of a given CI value was much higher in year 3 than in years 1 and 2. This sug gest that competition has an immediate effect on diameter but a delayed eff ect on height growth.