Decline in relative growth rate of 3 juvenile radiata pine clones subjected to varying competition levels in Canterbury, New Zealand

Abstract

Relative growth rate (RGR) is the ratio of growth to size at the beginning of the growth period. The assumption that RGR is constant, has been the subject of debate for a long time. In this study, the changes in mean RGR (RGR) of 3 clones of Pinus radiata D. Don, grown under varying weed competition levels (complete weeding, weeding of 2 m diameter circles, weeding of 1 m diameter circles, and no weeding), were quantified for 3 consecutive years. RGR was calculated each winter, using the morphological and physiological characteristics of RGR change which quantified as follows: (Formula Presented) RGR = dW \LA Lw K(J K = --x -x --x --dT LA Lw W ULR SLA LWR where: LA = leaf area, Lw = leaf weight, W = dry weight of the plant, SLA = specific leaf area, i.e. the ratio of area and leaf weight, LWR = leaf weight ratio, i.e. the ratio of total leaf weight of the plant and the total dry weight of the plant, T = time (age), ULR = unit leaf rate. The results showed that the unweeded control treatment had the lowest absolute RGR. RGR declined with time and tree size in all the weeded treatments, but increased in the control. The theory that RGR is constant (RGR = k model) is shown to be false, as it varies with both time and tree size. Changes in RGR during the period studied were best described by an exponential decay function: RGR = a + b × exp(-cW). Clonal differences were also demonstrated: clone 3 showed a lower RGR than the other two clones. This was due to the increased LWR and LAR, leaf area ratio (SLA x LWR) of this clone. The main reason for mean RGR decline were: (1) reduced ULR with age and size of the plants, (2) changes in allometry and (3) physiological aging which might have accentuated self shading effects. Tree size influenced RGR changes more than age. This study aims to dispel the notion of a constant RGR, and at the same time explains the reasons for the decline o this parameter in young trees.