Interactions between influences of genotype and grass competition on growth and wood stiffness of juvenile radiata pine in a summer-dry environment

Clones of radiata pine (Pinus radiata D. Don) were established in two experiments on two contrasting pasture sites in Canterbury, New Zealand, with four levels of competition from grass as main plots within each experiment. Control was extended over the first 5 years at one site, whereas grass at the other site was controlled by a single herbicidal spray immediately after planting. At the latter site, 1 m diameter spots cleared with herbicide were compared with the same size spot created using a woven mat. Grass competition added up to 3 years to a rotation, and the gain from control was correlated with spot size. Competition control was most critical during the first year after planting. Trees grown within spots cleared by mats grew more slowly than those in weed-free spots cleared with herbicide. Green dynamic modulus of elasticity was related to genotype and increased with both slenderness and higher levels of pruning. Significant interactions between genotype and level of competition were observed for growth, indicating that choice of genotype can be a useful way of coping with pasture competition if managers wish to reduce the use of herbicides.