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University of Canterbury (2003)

Modelling the influence of weed competition on growth of juvenile Pinus radiata at a dryland site

Watt, Michael

Titre : Modelling the influence of weed competition on growth of juvenile Pinus radiata at a dryland site

Auteur : Watt, Michael

Université de soutenance : University of Canterbury

Grade : Doctor of Philosophy 2003

Résumé partiel
The reduction in growth of commercially grown conifers by weeds over the juvenile phase is an important management issue. This thesis investigates the mechanisms of interaction between juvenile Pinus radiata and the woody weed broom (Cytisus scoparius L.) at a dryland site, and uses this information to develop processbased models of tree growth, sensitive to competition from weeds. All the results presented in this thesis were taken from an experiment established on a drought prone site, located on the Canterbury Plains, New Zealand. Within the experiment a comprehensive and detailed set of measurements were taken from trees growing weedfree and with broom competition, from the beginning of the second year to the end of the third year after planting. The presence of broom severely retarded the growth of trees over the two year measurement period. Treatment differences in basal area, which became significant one month after measurements started, increased from six-fold at the end of the first year to 12-fold after two years. At the end of the second year, trees growing without broom were two-fold taller, and had a crown diameter four-fold greater than trees in plots with broom. Above-ground biomass was the most sensitive indicator of competition. Compared to trees in weed-free plots, above-ground biomass of trees in plots with broom was reduced by 25-fold after two years. During the first year, the strong link between predawn needle water potential, ѱe, and fractional biomass growth for trees in plots with broom (r²=0.91) suggests growth losses in this treatment were almost entirely attributable to seasonal water deficit. Although ѱe was also strongly related to fractional biomass growth, fb, over the second year (r²= 0.71), values of fb across a comparable range of ѱe, were on average 15% lower than during the first year. Inclusion of a simple shading index explained this difference, suggesting broom restricted tree growth during the second year through competition for both light and water. Nitrogen fixation by broom, and the extent of fixed nitrogen transfer to Pinus radiata was assessed from measurements of the natural abundance of the ¹⁵N isotope, ¹⁵N. Values of ¹⁵N were highest in Pinus radiata growing without broom, intermediate in Pinus radiata growing with broom, and lowest in field grown broom. Broom was an effective fixer, deriving 81 % of nitrogen in above-ground tissues from the atmosphere, which was equivalent to 111 kg N ha⁻¹ yr⁻¹.

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