Meta-analysis of stand volume variable in Hyrcanian beech forests

Several studies have been conducted to estimate stand volume in Hyrcanian beech forests (pure and mixed beech satnd type), so that different results have been obtained from these studies. Meta-analysis is a quantitative statistical method for combining the results of a set of separate studies that are all about a specific topic. In meta-analysis, by collecting data from many studies, the number of samples increases and therefore the range of changes becomes smaller, as a result, the importance of statistical findings increases. In meta-analysis, if there is homogeneity between the articles used, the model with fixed effects is used, and if there is heterogeneity between different articles, the model with random effects is used to combine different articles. Therefore, in a meta-analysis, a series of researches in Hyrcanian beech forests were examined. In this research, some arguments have been made about the reason for the difference in the satnd volume in different areas of the Hyrcanian forests. This is the first meta-analysis research in the field of stand volume in Hyrcanian beech forests.

Researches related to the topic of stand volume were searched in national and international databases without time limit until the winter of 2022. In this research, after a detailed review of numerous articles, the data (mean, standard deviation and number of samples) of 18 original studies related to the stand volume variable in Hyrcanian beech forests were used. These researches were published in the International Journal of Agricultural and Biosystems Engineering, European Journal of Forest Research, and Natural Areas Journal, Iran's Forest and Poplar Research (seven researches), Wood and Forest Science and Technology Researches (three researches), Iranian Forest Ecology (three studies), Forest and Wood Products (one study) and Forest Research and Development (one study)) were published. In this research, only the estimation of stand volume (not growth of stand volume) was considered. In some cases, the required data, such as standard deviation, were not available in the published articles, so these articles were excluded from the meta-analysis process, or in few cases, they were calculated by accessing the raw data of the authors. These data had different names such as stand volume, forest stand volume, and growth stand volume in forests with different structural characteristics in these articles. Due to the significance of the Cochran Q test (P≤0.05) and values greater than 75% for the I2 statistic, the random effects model was used for meta-analysis. To find the source of heterogeneity between studies, subgroup analysis was performed based on geographical regions (province) and stand composition. The results of a meta-analysis can be shown in the form of an forest plot, which is the most general and common method of displaying effect sizes. In this research, the software Comprehensive meta-analysis ver. 3 was used.

According to the significance of the Cochran Q test and the I2 index, the average stand volume was estimated to be 386.805 sylve/h (95% confidence interval: 334.92-438.68) for Hyrcanian beech forests based on the random effect model. . The lowest and highest amounts were 252.3 and 685 sylve/h respectively in the forests of Mazandaran. Most of the research was done in different places of beech forests in Mazandaran province. The subgroup analysis based on the province (geographical regions) showed that the highest value is based on the random model in the forests of Guilan province with the amount of 424.45 sylve/h (95% confidence interval: 277.57-438.46). The lowest was estimated as 285.63 sylve/h (95% confidence interval: 167.26-404.01) based on the random model in the forests of Golestan province. The meta-analysis of standing volume in the Hyrcanian forests based on tree stand composition showed that the highest value was based on the random model for the pure type of beech at the amount of 394.272 sylve/h (95% confidence interval: 277.510-688-417). The lowest value of 147/38 sylve/h (95% confidence interval: 325.94-444.34) was obtained for mixed beech type. According to the results of this meta-analysis, the volume of stands in the Hyrcanian beech forests in the pure type of beech is 125.9 sylve/h more than in the mixed type of beech.

The average volume of stands in Hyrcanian beech forests was estimated at about 386 sylve/h. The highest amount in the forests of Guilan is about 424 sylve/h. The highest amount is 394 sylve/h for the pure type of beech. So, in management plans to maintain the stability and proper tolerance of the stand, it is recommended to keep the volume of the stand at the average level. Estimating the average value of structural characteristics on a large scale provides quantitative information on the structural condition of forests to adopt a suitable and realistic policy.