The effect of structural diversity on the self-thinning line, yield level, and density-growth relationship in even-aged stands of Norway spruce

Diversity of tree species and structure have the potential to increase various forest ecosystem services such as stress resistance, biodiversity, and productivity. Recent studies indicate the relevance and potential of both diversity of tree species and diversity of stand structure. Here, we analyze the effect of stand structure on the standlevel behavior of even-aged stands of Norway spruce (Picea abies (L.) H. Karst.). Drawing from 11 long-term thinning experiments encompassing 77 plots and 425 surveys, we analyzed the effect of tree size and growth frequency distributions on (i) the self-thinning line, (ii) the Eichhorn rule and yield level, and (iii) the stand density-growth relationship. We revealed an optimum relationship between the inequality of tree size and growth within forest stands and the maximum stand density, standing volume, total yield, and stand densitygrowth relationship. Highest stand density, standing volume, total volume yield, and stand volume growth occurred at moderate structural diversity, in terms of the inequality of tree size and growth. All else being equal, strongly homogeneous, or very heterogeneous stand structures were found to be suboptimal in terms of growth and yield. Our findings emphasize that incorporating information about tree size frequency distributions can greatly improve upon classical models of stand dynamics, even for even-aged spruce monocultures. The profound influence of stand structure on stand growth and yield, even when the classical stand attributes are analogous, warrants further scientific attention; e.g., neglecting differences in stand structure can lead to skewed interpretations of growth reactions to density regulation or species mixing. From a forest management perspective, our results unveil the untapped potential of structural diversification, even for mono-specific and even-aged stands. We elucidate the growth trade-offs when structural diversity is either neglected or overly emphasized.