Modelling growth responses of juvenile radiata pine (Pinus radiata D. Don) clones subjected to different weed competition levels in Canterbury, New Zealand (2000)
AuthorsKirongo, Balozi Bekutashow all
For a given site, species, genotype and management regime, tree growth is related to the amount, and temporal- and spatial-distribution of foliage, as governed by the prevailing below- and above-ground environment and intercepted photosynthetically active radiation. Canopy foliage budgets, therefore can provide first hand information about how trees cope with adverse environmental conditions and resource deficits in more subtle ways than height and diameter which are not as sensitive to changes in resource availability as leaf area. Process-oriented research, aimed at quantifying needle mortality within canopies and foliage growth dynamics of young radiata pine clones growing under varying competition gradients, therefore has the potential to improve decision tools for foresters charged with establishing tree crops. During this study growth responses of juvenile radiata pine (Pinus radiata D. Don) clones to variable weeding micro-environments during the first 3 years after outplanting were evaluated. The study was established in a dry site on the Canterbury plains of the South Island of New Zealand. A complete randomised block design replicated three times in a split-plot layout was used. Four weeding treatments; 1) complete weeding, 2) 2 m diameter spots, 3) 1 m diameter spots around plants and 4) a control, formed the main plots while seven clones formed the sub-plots. Leaf area and specific leaf area spatial distribution as well as fascicle mortality in time and space were quantified. Annual crown foliage budgets of 3 of the clones were evaluated using regressions developed from limited destructive sampling and image analysis techniques. Changes in mean relative growth rate (RGR) with tree size and age were evaluated by quantifying the morphological and physiological terms of the RGR expansion, to allow for changes in these terms; RGR = ULR * SLA * LWR where, RGR = relative growth rate, ULR = unit leaf rate, SLA = specific leaf area and LWR = leaf weight ratio. Relative growth rate declined with tree age and size for trees in all weeded treatments but increased with age and size for trees in the unweeded control. The decline in RGR was mainly due to reduced ratio of dry matter turn over to leaf area i.e. unit leaf rate. Of tree age and size, size was the major factor influencing the decline in RGR. Of the 3 clones studied in detail, clone 3 had a different above ground carbon allocation strategy to clones 1 and 2, especially as regards leaf weight ratio and leaf area ratio. Weeding influenced tree growth considerably. Significant clonal differences in height and basal-basal area were found. Trees growing in weed-free environments made favourable use of their micro-environments and significant genotype-by weeding interaction was present for height. However, no "rich-kid" effect (some trees growing in weed-free micro-environments performing poorly) was found. Provisional models of height, basal-basal area and tree survival integrating weeding and clonal effects were also developed. One of the significant findings from this study was the strong evidence against the constant RGR model which has been used by many researchers of tree growth analysis. The results further demonstrated that trees growing with weeds were predisposed to perform poorly due to reduced canopy production as well as having higher proportions of older, less efficient foliage. The results from this study lay the framework to explicitly quantify clonal responses to resource deficits arising from management inputs using variables directly involved in canopy production (i.e. leaf area).