Comparison between two oak stands (healthy and affected by oak decline) in respect to characteristics of competitive environments at Ilam province

This research was carried out to compare two oak (Quercus brantii) stands (healthy and affected by oak decline) in respect to characteristics of competitive environments in Shalam Forest, located at north of Ilam province of Iran. The study area was divided into 20 topographical homogeneous units, using GIS method. Considering the dieback rate at the units, five homogeneous units with different dieback rates were selected. After that, six transects (100 m. length) were allocated at each of the units and five points were laid on them at 25 m. intervals, using the systematic random statistical method. The two closest dead and alive trees to the points were identified. Finally, some characteristics of the focal trees around the two selected trees were measured and recorded, including their distance to the focal trees, species, crown diameter and total height. Competitive condition of the focal alive and dead trees in the stands was computed by Hegyi index. The data were analyzed, using ANOVA and t-test statistical methods. Results showed that there was significant difference between the homogeneous units in respect to the tree's competition rate. According to the t-test results, competition rate of the dry and almost dry trees was greater than that of the healthy or less dried trees. The lowest and the greatest tree competition rates were found in the homogenous units with lowest and greatest crown dieback rate, respectively. The focal dieback trees had lower crown diameter, total height and distance to the neighboring trees in comparison to the focal healthy trees, whereas the neighboring trees to the focal dieback trees had greater crown diameter, total height and density. It might be concluded that tree's competition rate has direct correlation with stand or tree groups density which varies, depends on available soil moisture content and competitive stress at different parts of the stand. The results of this research can highlight importance of tree competition, dependant on stand density as a predisposing factor of oak trees mortality at drought conditions, occurred at recent years and can be useful for rehabilitation management of the oak forests under dieback condition