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DECLINE MODELLING OF OAK TREES UNDER EFFECTS OF PHYSIOGRAPHIC FACTORS IN SEMI-ARID FORESTS OF IRAN
Authors: Mehrdad Mirzaei, Amir Eslam Bonyad, Reza Akhavan, and Ramin Naghdi
Number of views: 392
Recent dust and droughts caused stress to physiological weakness of oak trees, so that insect
outbreaks and pathogens lead to decline of oak trees in the Zagros forests. In addition to evaluate
the effects of physiographic factors (elevation, slope and aspect) on the amount and distribution
of decline, the objective of present study was to make a model of decline of Quercus brantii species
in semi-arid forests of Iran. For this purpose, 509 hectares of Zagros forests, known as Dalab
forests in Ilam province, were selected. Based on systematic-random sampling method, 100
circular plots (2000 m2) were measured in grid inventory of 200×250 m dimensions. In each plot,
physiography factors and percentage of decline were measured. ANOVA and Duncan tests were
used to compare the decline of oak trees in different physiography factors. Then, an individual
tree decline models were developed with a data set from a total of 1370 oak trees. To evaluate
the effects of physiographic factors on the decline of oak trees, a logistic regression model was
used. Omnibus test, log-likelihood, and pseudo r-square coefficients were used to evaluate the
logistic regression model. The results showed that physiography factors had significant effects
on the decline of oak trees. Also, the results showed that with increasing slope and elevation,
the decline of oak trees was increased. Lowest and highest declines belonged to east and south
aspects, respectively. Values of both statistics related to the pseudo determination coefficient are
acceptable (0.395 and 0.584), and these values indicated that the three independent variables
of this study had the power of a relatively high explanation of the variance and changes in the
dependent variable of trees decline. The results of this study can be effective in the oak decline
area conservation.