Measuring and modeling the crown ratio index for wild Pistachio (Pistacia atlantica Desf.) in Baghe-Shadi protected forest, Yazd province

The crown ratio is one of the most important attributes of trees for modeling growth and evaluating the competitive environment in forest stands. This research was carried out to determine the crown ratio for wild pistachio. Continuous transect sampling was carried out with the counting limit of 5-cm diameter at breast height. In each transect, the total height of the tree, the height of the crown, the diameter at breast height (DBH) and the elevation were measured. To study the type and severity of the relationship between elevation and DBH with crown ratio, correlation analysis was conducted. For modeling using linear regression analysis techniques, the curve estimation (including simple linear, second, third, linearized logistic, exponential, inverse and power) and nonlinear models (logistic, Richard, Weibull, and exponential) were used. The results showed that the linear model fitted to DBH data in order to predict crown ratio was statistically significant but the R-square was not noticeable (R2 = 0.33). Nonlinear models explained a maximum of 34% of variations in crown ratio with high precision. Elevation from the sea level was not a suitable predictor for the crown ratio. In this study, due to the low stand density, lack of effective competition between trees and the uniformity of the competitive environment, the crown ratio was less affected by independent variables such as DBH. Accordingly, it seems that in the low density forests of the Iran-Touranian region, the development of powerful models to predict the crown ratio will have little success.