Structural diversity of mosaics of developmental stages in the managed mixed forest of oriental beech (case study: Ramsar 30 watershed)

Background and objectives: Hyrcanian forests are very important in terms of preserving genetic reserves and the existence of some plant species that are unique to this vegetation area. Therefore, further investigation of the characteristics of these diverse forest communities for the purpose of ideal management and multi-purpose planning, with the aim of maintaining diversity in these forests, is always an inevitable necessity. Among these features is the dynamics of the structure, which affects the functioning of the ecosystem and is corrected by forest management and natural disturbances. Understanding its structure and dynamics through predicting the future structure of the forest and its evolution is very important for implementing management goals, reducing forestry costs and increasing the productivity of forest ecosystems. Based on this, this research was carried out with the main purpose of investigating the structural diversity of development stages in the managed eastern mixed beech forests.
Methodology: a 10-hectare area in the managed stands of parcel No. 514, series 5 of the Safaroud forestry plan, which has 42 mosaics of initial, optimal and decay development stages, which are separated based on the indicators and criteria for determining the development stages in forest were identified and selected regardless of its size and shape. The trees inside each mosaic were numbered and measured with full callipering method. The location of all the trees with a diameter at breast height (dbh) greater than 7.5 cm was recorded using the distance and azimuth of the bases relative to each other for each tree. After numbering the trees, characteristics such as dbh and total height of the trees were measured. Then, based on the dbh, the trees were divided into four classes: small (less than 35 cm), medium (35-50 cm), large (50-75 cm) and extra large (more than 75 cm). To quantify the structure of each mosaic in the studied stands, a set of different structural indices was used based on the three characteristics of spatial pattern, mingling and dimensions of tree. After analyzing the data, the structural characteristics of the stands, including the indices of diameter and height differentiation, uniform angle, mean distance to the nearest neighbor, mingling and Clark and Evans index in the mosaics were calculated. Uniform angle and Clarke and Evans indices were used to check the diversity of tree spatial patterns. Also, to check the species admixture, the mingling index was used, and to check the diversity of tree dimensions in each development stage mosaic, the diameter and height differentiation indices were used. The density of trees was also measured using the distance index to the nearest neighbors. Analyzes related to the type of distribution of trees and the type of mixing in the studied stands were done using Crancod software (ver. 1.4).
Results: A total of 3105 trees of beech and other tree species were identified and recorded, and their quantitative characteristics were presented according to the mosaics of development stages. The 10-hectare area had a total of 42 mosaics of different development stages with irregular shapes and variable areas that had been separated. The initial development stage with 18 mosaics had the most and the optimal development stage with 11 mosaics had the least frequency. The area of the mosaics varied from 292 to 5145 square meters. In total, in the investigated stands, the initial stage had the highest surface and the decay stage had the lowest surface in the studied area. The initial and decay development stages accounted for the highest and lowest number of stands dendity with an average of 427 and 212 stems per hectare, respectively. The average basal area was measured as 38.75, 42.26, and 37.63 square meters per hectare in the initial, optimal, and decay development stages, respectively. The mean distance to the nearest neighbor’s index in the mosaics of the initial, optimal and decay development stages, was obtained 4.39, 4.96 and 5.76 meters, respectively. The value of Clark and Evans index was less than one in all three development stages, which indicates that the trees in all mosaics have a cluster distribution pattern. The value of the uniform angle index was calculated as 0.5 and more than 0.5 in all stages, which indicates the cluster distribution pattern of trees. The mean mingling index in the mosaics of initial, optimal and decay development stages, was obtained as 0.31, 0.27 and 0.18, respectively. Also, the mean diameter differentiation index was calculated at 0.43, 0.37 and 0.43 in the initial, optimal and decay stages, respectively. The mean height differentiation index was 0.31, 0.28 and 0.33 in the initial, optimal and decay stages, respectively, which indicates the low to medium height difference of the neighboring trees.
Conclusion: According to the management activities in the studied beech stands, the results of the quantification of the stand structure according to the mosaics of different development stages can be used as a guide and template for the management of other similar forest stands. Also, the characteristics and functions used in this research can be used by forest managers as indicators to measure the interventions made (like tending and marking) as well as to monitor the development stages in these stands over time. The structural diversity of uneven-aged managed forests is different among different development stages. Therefore, in order to understand how management activities, affect the short-term structural dynamics in beech forests, similar researches should be conducted in five to 10-year time series in the investigated stands. Also, to get better results, these studies should be done over time and by monitoring a fixed stand.