The dynamics of structure across scale in a primaeval European beech stand

We explored the spatial dynamics of structural complexity in the living tree stratum in a 10-ha stem-mapped portion of an unmanaged nearly monospecific primaeval European beech (Fagus sylvatica L.) stand in Western Ukraine. Development dynamics were assessed through patterns of change in association across scales (from 156.25 m² to 1 ha) among stand basal area (BA), tree density, average and standard deviation (STD) of tree diameters, Gini coefficient (GC), the index of spatial aggregation (R), diameter differentiation index (T) and structural complexity index (SCI). At the smallest scales, STD, GC and T contrasted patches of differing structure (i.e. large between-patch structural differences). As subplot area increased and incorporated more heterogeneity, structural differences between subplots became more subtle and measures of tree-to-tree size variation (STD, T) lost sensitivity whereas it was gained for measures of overall within-patch heterogeneity (GC). At small scales, differences in STD largely explained variation in the SCI (between-plot variability); at intermediate scales, size differences among neighbours (T) explained most of the variability; and at large scales, plot-level differences in BA and its allocation to trees of different sizes (GC; within-plot variability) overrode size differences among nearest neighbours. The characterization of a fine-scale shifting mosaic of patches in different development stages appears to hold for primaeval beech forests in this spatially contiguous area of relatively large extent. The coalescence of small-scale processes into neighbourhoods, and then into patches at larger scales, may be best captured by the change in associations among structural measures across scales because the structural imprint of gap dynamics extends considerably beyond the scale of individual gaps.