Distance-Dependent Competition Measures for Individual Tree Growth on a Taiwania Plantation in the Liuguei Area

Competition among trees within a stand occurs when resource availability is insufficient to meet the total requirements of a tree population for optimal growth. Six distance-dependent competition measures that incorporate tree sizes and distances from neighbors, evaluated over varying competition zones, were used to assess the competition stress among trees. A reduction in the mean square error relative to the no-competition index involved was used to judge the performance of each competition index for 3 growth components (i.e., diameter at breast height (DBH), basal area, and volume). The results showed that except for Martin-Ek, the other competition indices investigated were significantly correlated with periodic growth in the 3 growth components (p ＜ 0.0001). The performance of the competition indices in predicting 5-yr growth indicated that the inclusion of competition indices in the growth-prediction model reduced the mean square error from 14% for volume growth to 17% for DBH growth. Moreover, expanding the search zones in the Hegyi competition index was found to have slightly improved the ability to estimate competition effects.