Structural variation in mixed planted forests after forest management abandoning

Forest structure is a key factor in growth, yield and how changes occur in the forest stands, and evaluating the structure of plantations can play an important role in improving afforestation and timely implementation of forestry treatments in each stand. This study aimed to evaluate structural characteristics of plantation forests in the Saib Tabrizi forest park located on Tabriz, after 25 years of planting have passed, and abandon its management.

For this purpose, after initial field surveys, three one-hectare plots were selected and based on quantitative indicators, the structure of each plot-sample was investigated. In each plot, structural characteristics of all trees (species type, diameter at breast height, tree height, canopy height, and canopy diameter) were measured and the distance method (based on nearest neighbor’s) were used to quantify stand structure. In order to study of stand structural some index including nearest neighbors distance index (density of trees), uniform angle index and Clark and Evans index (variety of location), Mingling index (species mixture), diameter and height differentiation indices (dimensional differences of trees), and complex structural diversity index (tree diversity) were used.

Based on the results, the average density of trees was 448 per hectare. In the studied sample plots, the average DBH was equaled to be 12.9 cm, the average height of the trees was 5.1 m and the average basal area was calculated to be 0.013 m2 per hectare. The highest abundance among the species belonged to the species of Pinus nigra with 50.4% and Robinia pseudoacacia 32.4%, respectively. The results of the uniform angle index showed that a cluster pattern in spatial distribution of trees, however Clark and Evans index showed a regular pattern. According to the Mingling index, the studied stands have relatively moderate mixing. The height and diameter differentiation indices showed that the dimensions of the trees had a slight difference, and have a homogeneous structure. According to complex structural diversity index, tree diversity was obtained at the low level.

In conclusion, it can be said that after nearly three decades of planting trees in this park, tree communities have maintained the homogeneity of their structure, and trees have moderate mixture. Therefore, it can be stated that the development of forest structure in the research area is slow. Hence it is recommended to increase the biological sustainability, silvicultural treatments are needed to increase mixture and heterogeneity of structure. The results of this study can provide objective justifications for the importance of consciously creating forestry strategies in the country's forestry as a missing link in the forest management.