Interference mechanisms of pasture on the growth and fascicle dynamics of 3-year-old radiata pine clones

Abstract

The interference effects of pasture weeds on height, ground-line diameter and foliage growth dynamics of three juvenile radiata pine ( Pinus radiata D. Don) clones were studied in a dry environment on the Canterbury plains of the South Island, New Zealand. Four pasture competition gradients were used: (1) total weeding, (2) 2 m diameter spots, (3) 1 m diameter spots, and (4) a control. Results at the age of 3 years showed that the removal of weeds significantly increased height and ground-line diameter growth, final needle sizes and total tree leaf areas (LAs). Significant clonal effects were observed for the same variables. The weeding-by-clone interaction was significant for height but not ground-line diameter. Trees growing without weeds had earlier needle emergence during the growing season and recorded highest needle elongation. Trees in the weedy plots retained proportionally more older (3-year-old) foliage and added very little new foliage. More moisture was available at soil depths below 10 cm in the completely weed-free plots than in the control plots. In sites which often encounter droughts during the summer growing season (for example, the Canterbury plains), controlling pasture weeds had five beneficial effects: (1) increased moisture availability at lower soil depths, (2) earlier needle emergence and therefore, a longer growing season, (3) longer needles with greater LAs, (4) crowns with increased current season foliage, and (5) crowns with proportionally less older foliage. Judicious control of competing weeds improved current growth, ensuring trees were better prepared to respond optimally in the future. This paper provides an insight to the poor growth response of water stressed trees to future favourable growing conditions, as compared to their counterparts in weed-free plots.