Effect of Neighbouring Trees on Eucalypt Growth in a Semi-Arid Woodland in Australia

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Abstract
(1) The effects of neighbouring trees on the growth of Eucalyptus populnea in a native semi-arid woodland in Queensland, Australia, were investigated using three different measures of interference: (i) nearest-neighbour distance (NN): (ii) Weiner''s neighbourhood interference (W); (iii) a competitive-interference measure (CI) which included the size of the influenced individual. (2) The relationship between relative girth increment and the interference measures was obscured by the range of tree sizes and it was necessary to treat the data in three separate size classes. (3) The performance of the three different measures of interference in acting as predictors of relative girth increment varied across the size classes. All three measures gave a usable result for the large size class (> 60 cm gbh), both W and CI performed well for the small size class (0-30 cm gbh), but only CI gave a stable result for the middle size class (30-60 cm gbh). (4) Use of a vector in the model for plant influences allowed the generation of an influence surface, which was then used to derive CI values. This surface could also be used to calculate values for relative-recruitment potential at any point within the system. (5) The competitive-interference model used leaf area as a measure of plant size which is related to resource demand. This measure reflects important functional variables such as water uptake, litter input, root volume and shading, and can applied to different growth forms. It should then be possible to generate influence surfaces which combine trees, shrubs and grasses and to investigate interactions between these different growth forms.