Integrating multiscale ecological indicators to assess ecosystem functioning in virgin Hyrcanian beech forests

Virgin oriental beech ( Fagus orientalis L.) forests of the Hyrcanian region in northern Iran are among the last remnants of temperate old-growth ecosystems worldwide, yet their ecological benchmarks remain poorly defined. We conducted an integrated, multiscale assessment of three virgin 1-ha pure oriental beech stands located in the Hyrcanian forests of Guilan Province, with 60 randomly distributed plots capturing a suite of 62 structural and functional indicators. These encompassed forest structure, herbaceous diversity, humus chemistry, and soil physical, chemical, and biological properties, together with fine root traits. Results revealed exceptionally high basal area, abundant large-diameter trees, and broad crowns that collectively typify structurally mature, late-successional stands. The understory was dominated by shade-tolerant, Euro-Siberian taxa, highlighting long-term ecological continuity with minimal disturbance. Humus layers stored substantial carbon and nitrogen, while soils showed moderate fertility and evidence of constrained phosphorus availability, indicated by low microbial P and enzymatic activity. Fine root traits suggested conservative strategies optimized for stable nutrient acquisition under mature closed-canopy conditions. Multivariate analyses identified two dominant nutrient pathways: (i) a tightly coupled humus–microbial–soil C–N continuum that stabilizes long-term fertility, and (ii) a partially decoupled P cycle driven by enzymatic and fine root processes. This study provides the first quantitative, multiscale ecological indicator ranges for virgin oriental beech forests, establishing a comprehensive benchmark for these globally rare ecosystems. The integration of structural, biological, and chemical indicators provides a functional reference framework for detecting nutrient imbalance, soil degradation, or canopy disturbance in managed beech forests. These reference values support forest managers in identifying early signs of structural or functional degradation, designing long-term monitoring programs, and evaluating restoration success. For policy makers and silviculturists, these benchmarks provide measurable targets for adaptive management, biodiversity conservation, and ecosystem-based planning under changing climatic and anthropogenic pressures. More broadly, these results emphasize the importance of preserving intact forest systems to better understand how ecosystem structure and nutrient dynamics co-evolve under minimal human disturbance, with direct relevance for temperate beech forest management worldwide.