Evaluation of the inter-species competition in the pure stands plantation using the competition index

Many competitive models have been developed from simple to multidimensional in order to estimate the competition of trees in forest stands. All of them are measured to show the interaction between trees, and the purpose of these indices is to evaluate the competitive tension according to the interactions between trees. These indices are generally based on the morphological characteristics of trees or a function of the distance of the neighborhood of trees. The individual competition of trees evaluate using competition indices is based on mathematical techniques, which indicate the degree of competition that the tree is affected by its neighbors. This result is often assumed to be the negative effects of the neighborhood on the growth of trees to absorb limiting resources.

Distance-independent competition indices are usually considered as tree size ratios (such as dbh, height, basal area, or canopy cover) to show the competitiveness of that species. In distance-dependent indices, the dimensions of neighboring trees and their distance from each other are used. In order to calculate the competitive indices including distance-independent competition index (IDI), distance-dependent competition index by diameter (DDIdbh), and distance-dependent competition index by height (DDIh), the parameters of diameter and the total height of all trees were measured in sample plots (one-hectare area) in four stands of poplar plantation in Gilan province. The distance of the trees was also measured. Then, in order to calculate the wood density and use its value in the distance-independent competition index, 15 trees were randomly selected in different diameter classes in each plot and were cut to perform the wood density analysis, and the process of calculating the density was carried out.

The results showed that the stands had significant competitive conditions in terms of both types of indices. All stands had a significant difference (p ≥ 0.05) in terms of the independent distance index. However, the comparison of the distance-dependent index for tree diameter showed that stand 3 has a significant difference from other stands, and 1, 3, and 4 stands also had significant differences (p ≥ 0.05). The comparison of the distance competition index for tree height also showed that stands 1 and 2 had no difference but showed a significant difference with stands 3 and 4 (p ≥ 0.05).

Calculating all types of competition indices can be useful in silvicultural programs such as thinning. The results of such studies can provide a clear understanding of the spatial structure of trees.