Distance-dependent competition measures for eucalyptus plantations in Portugal

Abstract

Data from permanent plots and spacing trials of Eucalyptus globulus Labill. were used to study distance-dependent competition measures. The data were divided into three subsets representing different stages of stand development and therefore different levels of competition. Different formulations of each type of index were tested. The rules for the selection of competitors as well as the mathematical formulation of each index were considered in the analysis. The linear relationship between the dbh and the distance to which a tree can compete - characteristic of the selection of competitors based on the basal area factor - was not consistent over time. Rules defined as asymptotically restricted non-linear functions of tree size were designed to overcome this problem. The use of a fixed number of competitors was also tested. The evaluation of the prediction ability of each index was based mainly on its performance in multiple linear regression functions for the prediction of the tree basal area annual increment. The results showed the superiority of the indices based on the Richards' function for selecting competitors. This supremacy was more evident when trees in the lower diameter classes were not suppressed. When the asymmetric competition was evident the area potentially available indices showed the best performance.