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Abstract: Tree size distributions of old-growth forests are a fundamental tool for both scientific analysis and conservation management. In old-growth forests the diameter distribution shape may depend on spatial scale, but theoretical models often show fundamental similarities that suggest general underlying mechanisms controlling regeneration, mortality, and growth under different disturbance regimes. In this study we investigated the horizontal structure of two old-growth beech (Fagus sylvatica L.) forests in central Italy using a series of spatial scales and analytical approaches. Individual plots (each one 0.13ha in size) were progressively aggregated into larger groups to simulate an increase in sampled area. A third-order polynomial was fit to the diameter distributions of the aggregated areas, and the curves were ranked by the AIC method. The rotated sigmoid (RS) shape was the most common descriptor of tree size distributions, and became dominant once aggregated spatial scales exceeded 0.6–0.8ha. The same result was obtained when a single plot was made progressively larger. Given the relatively small scale (starting from 0.1ha) at which the RS model emerged, old-growth beech forests of the Italian Apennines show a complex, yet highly resilient structure, recognizable even in small (<1ha) patches. Most likely, the rotated sigmoid structure is a function of endogenous structural processes (e.g. self-thinning) in this shade-tolerant species in combination with intermediate-to-fine-scale disturbance (which especially impacts the largest trees) and resulting gap dynamics. [Copyright &y& Elsevier]