Investigation of Oak decline in diameter classes in Sarab-Kazan forests of Ilam

In recent years, a large area of ​​Ilam forests has been affected by oak decline. The purpose of this study was to investigate the oak decline for two inventory periods in Sarab-Kazan forests, west of Sarableh city, Ilam province. The first inventory was done in 2012. Systematic-random sampling with 200×150 meter grid was done in 0.1 ha rectangular sample plots measuring 40 × 25 m (1000 m2) and 80 sample plots were established in 120 hectares. In each sample plot, tree species, DBH, tree health were measured. For this purpose, declining trees were classified into 1: very high (more than 75%), 2: high (50%-75%), 3: medium (25%-50%), 4: low (less than 25%), and 5: healthy. In 2014, the second inventory was carried out in the same sample plots. Mann-Whitney test was used to compare the tree decline in two inventories. The results indicated that tree decline in the very high declining class raised from 3.5% (2012) to 11.2% (2014). There was a significant difference between the first and second inventories according to Mann-Whitney test (P≤0.05). Moreover, the highest decline was observed in class 10 and 15 cm. In contrast, the lowest decline was recorded in 40 and 50 cm. Our findings showed that DBH has a relationship with oak decline. Hence, in order to manage the forest stands with oak decline in Ilam province, it is recommended trees with 7-20 cm DBH were further considered.