The influence of weed competition for light and water on growth and dry matter partitioning of young Pinus radiata, at a dryland site

Above-ground biomass growth and allocation was studied in juvenile Pinus radiata D. Don trees growing with and without competition from the woody weed broom (Cytisus scoparius L.), at a dryland site, over a 2-year period. The relative significance of broom competition for light and water on seasonal growth of P. radiata was quantified using a growth modelling technique which effectively removed the confounding influences of season and initial tree size from analyses. Allometric regression analysis was used to separate size dependant changes in allocation from functional adjustments made in response to treatment. At the end of the second year, trees growing without broom had an above-ground biomass 25-fold greater than that of trees growing with broom. The presence of broom induced consistently more negative values of predawn needle water potential (ψ(e)) in P. radiata over the 2-year period. Treatment differences were most pronounced during the first year when ψe reached a minimum of -4 MPa in plots with broom, compared to a minimum of -1 MPa in plots without broom. Broom leaf area index increased markedly over the 2-year period, from average values of 0.76 m2 m-2 in year 1 to 2.26 m2 m-2 during year 2. During the first year, the strong link between ψ(e) and fractional biomass growth for trees in plots with broom (r2=0.91) suggests growth losses were almost entirely attributable to water deficit. Although ψ(e) was strongly related to fractional biomass growth over the second year (r2=0.71), values across a comparable range of ψe, were on average 15% lower. 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. Compared with P. radiata in weed-free plots, trees growing with broom allocated dry matter preferentially to stems (+8%) at the expense of foliage (-4%), roots (-2%) and branches (-2%). These findings on allocation confirm derivations based on pipe model theory and indicate that the root to shoot ratio in juvenile P. radiata remains relatively stable even under severe water deficit.