HOW COMPETITION INDICES BEHAVE AT DIFFERENT NEIGHBOURHOOD COVERAGES AND FORMULAE COMPOSITIONS IN AN ARAUCARIA FOREST IN SOUTHERN BRAZIL

Abstract

Many tree-level growth models share the usage of competition indices. Although it is common in the forest literature to test several indices in order to define the most suitable for each study, the evaluation of plot sizes or neighbourhood radii is poorly addressed in competition studies. The objective of this work was to assess the correlation of several distance-dependent and distance-independent competition measures and the annual growth in diameter of an Araucaria Forest remnant. Minor modifications in the formulation of some indices were also evaluated. The data come from two measurements spaced over a six-year period, and the variables diameter, location, canopy stratum and crown quality were measured. Both distance-dependent and independent indices were calculated for different plot sizes and competitor selection radii. The performance of plot sizes and neighbourhood radii was assessed via Spearman’s rank correlation with the diameter annual growth, and the indices calculated with the most correlated distances were included in a linear growth model, whose variables were selected based on a stepwise procedure. The results showed that for almost all the indices the correlation increased as the plot size or radius increased. A 50x50 m square plot generated the highest correlation for distance-independent measures. A 20 m radius resulted in the best value of correlation for the distance-dependent indices. Minor modifications in index composition were also beneficial in order to improve performance. The stepwise procedure selected BAL and Hegyi indices to describe the diameter growth of the trees.