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چکیده
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There is a global consensus regarding the positive relationship between biodiversity and ecosystem functioning. However, the strength and nature of this relationship are influenced by the distinct and interactive effects of biotic and abiotic factors. Among these, topographic variables, in conjunction with biotic factors (such as vegetation cover) and their interactions, can significantly affect ecosystem functions and the relationships between various dimensions of biodiversity (taxonomic, functional, phylogenetic, and structural) and ecosystem functioning. Despite the importance of this topic in ecology, particularly in semi-arid Mediterranean forests, various aspects remain insufficiently explored. Therefore, this study aimed to investigate the influence of topographic variables (aspect and elevation) and canopy cover (open vs. closed canopies) on aboveground woody biomass as a proxy for ecosystem functioning, biodiversity and the biodiversity–ecosystem function (BEF) relationship in the semi-arid oak forests of the Zagros Mountains in western Iran. Sampling was conducted across two slope aspects (north- and south-facing) and three elevation classes (1750–1850, 1850–1950, and 1950–2050 m). A total of 60 forest stands (30 with open canopies and 30 with closed canopies) were selected and sampled. The direct and indirect effects of biotic (canopy cover and biodiversity) and abiotic (elevation and aspect) factors on aboveground biomass were assessed using structural equation modeling (SEM). Results revealed that biomass varied as a function of slope aspect and canopy density, being significantly higher in north-facing stands with closed canopies (10.93 ± 0.3) compared to open-canopies stands in south-facing slopes (8.39 ± 0.48). Additionally, taxonomic, phylogenetic, and structural diversities were all greater on north-facing slopes. Tha SEM results further demonstrated that elevation, slope aspect, and canopy cover had significant and direct effects on aboveground biomass, with biomass increasing with elevation in closed-canopies stands. Canopy cover exhibited the strongest direct effect on biomass (β = 0.36, P-value = 0.01). Slope aspect was identified as the most influential physiographic factor, exerting both direct and indirect significant effects on biomass. Among the biodiversity dimensions, structural diversity had the most substantial and significant direct effect on biomass and acted as a key mediating variable linking canopy cover and physiographic factors to biomass. Overall, the findings of this study highlight that ecosystem functioning in semi-arid forests is shaped by the direct and indirect interactive effects of biotic and abiotic factors, underscoring the importance of evaluating the contribution, direction, and magnitude of these drivers in maintaining and shaping ecosystem structure and stability through multiple functional pathways.
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