Influence of Single-Tree Selection System on Soil Total Nitrogen and its Seasonal Changes under the Mixed Stand of Beech-Hornbeam (Case study: Shast-Kalateh forest, Gorgan)

Nitrogen dynamics is one of the factors that may change under silviculture practices. Regarding the importance of total nitrogen content and its seasonal variation, the present research studies these changes in the mixed beech-hornbeam stands located in district one of Shast-Kalateh forest, Golestan province, Iran.

To study the total nitrogen variation in an average altitude of 850 meters and northeast direction with two-stage and middle-aged structure, 20 samples were randomly collected at the depth of 20 cm in four stands (1 ha) as treatments including non-managed stand and 3 managed stands (selection system) in 10 (Parcell 30), 7 (Parcell 33), and 1 (Parcell 31) years rotation from January 2014 to January 2015. The data and seasonal variation were compared by repeated measure design (p≤0.05).

The results showed that there was a significant differences between managed treatments and control (p < 0.05). The highest (61.37 gr kg-1) amount of total nitrogen was observed in control treatment (Parcell 32) while lowest (41.78 gr kg-1) was in managed treatment (Parcell 31). The total nitrogen content in managed treatment (Parcell 30) (4.80 gr kg-1) was close to the control. Also, seasonal comparison of total nitrogen showed significant differences in all treatments. Among all managed and control treatments, summer (4.31, 3.05, 2.4 and 4. 31 gr kg-1) and winter (5.52, 5.96, 4.55 and 4.77 gr kg-1) had the lowest and highest total nitrogen, respectively.

Single tree selection system creates positive changes in above ground section of forest stand, compared to others. It cans also changes in above and below gound sections of forests and cause to reduce total nitrogen in ecosystem in early years. Decreasing in total nitrogen content is not permanent and due to the self-regulation and self-sustainbality of ecosystem, the amount of nitrogen is driven to equilibrium through time. According to the results, single-tree selection system has lead to a decrease in total nitrogen content, therefore, to help restore ecosystem to balance situation, the rotation period should be calculated with respect to the amount of growth, the capacity to restore stand and the habitat to restore the ecosystem to a perfect balance. Regarding the difference of 0.32 gr kg-1 nitrogen in managed treatment (Parcell 30) with 10 year rotation compared to the control treatment, it is advisable to increase the rotation period 3 to 5 years in selection system to improve stand physiognomy (crown cover, etc.) and edaphic conditions.