Decomposing small-scale structural complexity in even-and uneven-sized Norway spruce-dominated forests in southern Finland

Management targeting increased structural complexity requires not only targets but also mechanisms by which to create those target structures. We identified the most influential structural attributes and spatially explicit metrics for small-scale structural complexity using multiple linear regression at the beginning, end and over a 15-year period in two Norway spruce-dominated stands in southern Finland. Both stands had plots that exhibited an even-sized (ES) structure created through low thinnings and plots that exhibited an uneven-sized (UES) structure perpetuated through single-tree selection harvests. For both structure types, best models for structural complexity used a combination of non-spatial structural and/or compositional attributes and spatially explicit metrics that included the variation in tree size, tree size differentiation, tree density, stand basal area and spatial aggregation. Structural complexity in the ES structure type was mostly a function of tree size differentiation, whereas stand basal area and tree size differentiation were the most influential in the UES structure type. A conceptual model is presented to illustrate how spatial heterogeneity relates differently to small-scale structural complexity, which could be enhanced in ES structure types by increasing variation in size differentiation and in UES structure types by increasing variation in tree abundance.