Effects of competitive and cooperative interaction among neighboring trees on tree growth in a naturally regenerated even-agedLarix sibirica stand in considering height stratification

This study attempted to explain the variation in growth of individual trees in a naturally regenerated, even-agedLarix sibirica stand using indices that represented the competitive and cooperative interactions among neighboring trees. These interaction indices and DBH were used in stepwise multiple regression procedures to model the growth of individual trees. However, when the data from all trees were used, DBH was the only factor accepted in the growth model. Since DBH can be influenced by the cumulative effect of past interaction and other environmental factors, we stratified the stand into three height strata and repeated the stepwise procedure for each stratum to remove the cumulative effect represented by DBH. Several competition and/or cooperation indices were accepted in growth models of the lower, middle and upper strata. In each stratum, the residual mean square of the growth model was smaller than that of all strata. These facts suggested that height stratification was generally successful in reducing the cumulative effect of past interaction and other factors. The cooperation indices that suggested protection from wind stress by neighboring trees was a significant variable in the growth models of all three strata. This demonstrated that cooperative interaction should be considered in the explanation of variation in tree growth in dry and windy climates such as the present study region. This study was supported with grants from The Nissan Science Foundation, and from the Heiwa Nakajima Foundation.