ASSESSING COMPONENTS OF A COMPETITION INDEX TO PREDICT GROWTH IN AN EVEN-AGED PINUS-NIGRA STAND

The relationship between tree growth and competition may depend on som e subjective choices that are commonly left to the researcher. Among t hese are the neighborhood radius, the number of years of growth that a re integrated, and tree age. We have evaluated the importance of these factors when relating growth and competition in a forest stand with c ontrasted densities of the dominant tree species (Pinus nigra) and und erstory shrub species (Adenocarpus decorticans). Previous to this eval uation we performed a randomization test to assess the relationship be tween tree growth and neighbors. By using Daniels index of competition we found that the use of a fixed neighborhood radius underestimated t he importance of tree competition. The coefficient of determination (r (2)) of the relationship between tree growth and Daniels index increas ed asymptotically with the number of years considered. Five years of g rowth gave high r(2) independently of the density of trees and shrubs. The intensity of competition was weakly affected by the characteristi cs of the plot (tree and shrub,densities), and did not change with tim e. In contrast, the potential growth at equal competition - as represe nted by constant ''a'' in the allometric model - changed with time sug gesting a gradual decrease in potential tree growth in the plots with higher tree density, and a gradual increase in those plots with high d ensity of shrubs. These results may reflect tree canopy closure and th e senescence of Adenocarpus decorticans. A method is suggested to sele ct optimum neighborhood radius and growing period for the calculation of competition indices. By applying this method we were able to explai n as much as 79-84% of the variability in tree growth of this stand.