Tests of age-independent competition indices for individual trees in natural hardwood stands

Recent studies have demonstrated that simple indices of competition that incorporate competitor size and inter-tree distances generally perform as well in predicting individual tree growth as more complex approaches of assessing spatial pattern. A major limitation of diameter-distance indices, however, is that their numerical values decrease in a given stand over time even when the stocking level remains constant. In this paper two modifications are proposed which make the index essentially independent of age, thus necessitating only one competition-growth regression for each species on a given site and allowing comparisons between different stands. Tests of several index designs in three evenaged temperate hardwood stands indicated that the correlation between competition and growth is optimal over a wide range of competition radii and that the inclusion of inter-tree distances is of little value despite considerable small-scale variability in the stocking level around individual trees. Highest correlations were obtained when competitors were defined to be only those trees of equal or higher crown class than the subject tree. In these hardwood stands a comparison of the size of a subject tree to that of the competitors was necessary for reasonable correlations with growth. These correlations varied greatly among species even within a single stand and appear to be related to the shade tolerance of the species. For general use the index (ΣD_j)/D_i is recommended, where D_j is the diameter of competitor j and D_i is the diameter of subject tree i. This index can be computed rapidly in the field and does not require mapping of stem positions.