Spatial Analyses for Fine Woody Debris Volume Stock in the Hyrcanian Research Forest of Kheyrood-Kenar.

Background: In the Hyrcanian forests, fine woody debris (FWDs) plays a significant role in providing ecological services. However, their importance is often overlooked in the management and executive sectors. Based on the sustainability indices of natural forests--considering changes in carbon stock and the value of wood products--these seemingly insignificant FWDs are predicted to serve as valuable carbon reserves. Both science and economics recognize their significance. To make informed decisions about forestry operations, especially in post-respiration conditions (the rest law of the Hyrcanian forests), it is essential to accurately assess the accumulation and volume changes of these sinks across different parts of the forest. Optimum management techniques, particularly in forestry, can enhance ecological services. Therefore, our upcoming research aims to analyze spatial statistics related to volume stock changes in FWDs within various sections of the Hyrcanian forests, with a focus on implementing optimal management strategies. Methods: The study was conducted in Khairood-kenar forest using a cluster design at different altitude gradients ranging from 100-1800 m above sea level. To create the cluster design, three circular samples with a radius of 7.32 m were placed in the form of a triangle with azimuth angles of 0, 120, and 240 degrees. The distance between each sample was 36.6 m, and an additional sample of the same area was placed in the center of the design. Each cluster was randomly laid with three replications, resulting in a total of 12 samples across the altitude gradient from sea level. On each transact, the diameter of all dry wells, whose central axis was intersected by the transact line, was measured and recorded using calipers. After examining outliers upon the initial analysis, the volumetric observation variograms were fit using various models based on the distance and spatial correlation of variance. Based on the spatial structure, an optimal model, with the highest coefficient of determination (R2) and the lowest residual sum of squares (RSS), was selected for implementing the ordinary Kriging interpolation method. Based on the fitting of the models in different diameter classes, Kriging, and inverse distance weighting (IDW) methods were used for the interpolation and estimation of response variables that were not measured at different distances. Subsequently, the cross-validation method was used to validate the existing estimates of the FWD volume. The accuracy of both the estimation and the performed zoning was evaluated based on the fit between the estimated variable values and the observation values using R2and root mean square error (RMSE) indices. Results: The total volume of FWDs within the researched forest amounted to 24.3 m ³ /ha. The volume stocks of the first, second, and third diameter classes of the FWDs were 2.14, 6.01, and 16.23 m ³ /ha in the study forest. The variogram modeling revealed that the spatial variations in the volume of FWDs across different diameter classes exhibited an average spatial structure. However, the fit was not optimal when considering the R2 and RSS values. Interestingly, the kriging results were most accurate for the FWD first diameter class compared to the other diameter classes. Despite this accuracy, the overall estimated precision remained poor, with an RMSE of 6.32 and an R2 of 0.16. The result of the variography of the volume inventory of FWDs showed that their first diameter class was isotropic and the other diameter classes were anisotropic. Since the kriging method was not effective for estimating the spatial distribution of the FWDs in the second and third diameter classes, the IDW technique was used instead. The results obtained from the IDW technique for evaluating the spatial distribution of FWDs in the second and third diameter classes revealed no significant accuracy in the estimation. Therefore, it can be concluded from the results that the introduction of the volume of FWDs in the forest floor under the altitude gradient is not recommended for the study area. Conclusion: Based on the results, the volume stock of FWDs for different diameter classes along the altitudinal gradients was independent of autocorrelation and spatial correlation. This means that the FWD volume stock variations in different altitudinal gradients of the study forest are influenced by silviculture, biological, and ecological items with no obvious commonality in this regard. Since the accumulation of the FWD volume in the studied forest is not considered autocorrelated variation, it is expected that the classic tests, such as parametric and non-parametric tests, can be reliably applicable to examine the response variables among the stand types and altitudinal gradients. [ABSTRACT FROM AUTHOR]