Assessing the risk of windthrow for Eldar pine trees in Sorkhe-Hesar forest park

Study of slenderness coefficient and crown ratio of trees in forest parks to determine the stability of the stands against the wind is one of the important issues in the park management. The most important reason is the intensive presence of families and vehicles in these areas, which can cause irreparable damage if trees fall. Therefore, this study was conducted to investigate the sensitivity of Eldar pine trees in different parts of Sorkheh Hesar Forest Park to wind throw. Attempts have also been made to evaluate the suitability of diameter at breast height, as a variable that is simple and inexpensive to measure, to predict the crown ratio and slenderness coefficient as two important indicators for determining the susceptibility of trees to wind throw.

After a field survey, 14 patches were selected for sampling so that sampling sites were well distributed throughout the park. A total of 151 trees were randomly selected and their diameter at breast height, total height, trunk height, slenderness index and canopy ratio were calculated. Independent t-test was used to examine differences between tree lines and patches. Regression analysis was also performed to model the relationship between diameter-slenderness index as well as diameter-crown ratio.

Based on the results, the average basal area of the measured trees was 0.034 m2 and their average volume was 0.208 m3. Considering the variety of planting spaces, basal area and volume of pine stands were calculated 9.45-54.4 m2. ha-1 and 57.8-332.8 m3. ha-1 respectively. The results showed that the average slenderness index was 57.29% and that 0.7% of the trees were in unstable conditions. The average crown ratio was 0.557 and based on this index, 2.6% of trees were unstable. There was a significant difference between the forest patches and the tree lines along the streets in terms of crown length, canopy ratio, trunk height, and slenderness index. The linear model had a significant fitting to the data of diameter-slenderness index as well as diameter-crown ratio, with error percentage of 13 and 11.7% and coefficient of determination of 0.28 and 0.17, respectively.

In general, pine trees in Sorkheh Hesar National Park are in a stable condition and a small percentage of them need biomechanical study of wood tissues in order to determine the maximum tolerable wind speed and take precautions such as pruning. Also, diameter at the breast height in various plantings in terms of density, alone is not enough to predict the slenderness coefficient and the crown ratio of the trees. It is necessary to measure the total height and crown height of the trees to determine the slenderness coefficient and crown ratio. Due to the effect of the area and shape of tree patches in the incidence and severity of wind throw, it is necessary to consider the shape of the spots as a teardrop that is directed towards the prevailing winds in the region to minimize the risk of falling in busy places.