Distance-dependent competition measures for eucalyptus plantations in Portugal

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 developme nt 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 t he analysis. The linear relationship between the dbh and the distance to wh ich a tree can compete - characteristic of the selection of competitors bas ed on the basal area factor - was not consistent over time. Rules defined a s asymptotically restricted non-linear functions of tree size were designed to overcome this problem. The use of a fixed number of competitors was als o tested. The evaluation of the prediction ability of each index was based mainly on its performance in multiple linear regression functions for the p rediction of the tree basal area annual increment. The results showed the s uperiority of the indices based on the Richards' function for selecting com petitors. This supremacy was more evident when trees in the lower diameter classes were not suppressed. When the asymmetric competition was evident th e area potentially available indices showed the best performance. ((C) Inra /Elsevier, Paris.).