Effect of forest structure on stand productivity in Central European forests depends on developmental stage and tree species diversity

Recently, many studies have found positive biodiversity-productivity relationships in forests. In contrast, different types of correlations have been identified in the analyses of tree diversity-structure-productivity relationships. We suspect that these conflicting conclusions might result from the different developmental stages of the investigated forest stands. We therefore analyzed the development of tree diversity-structure-productivity relationships at the stand level and individual tree level in 192 long-term experimental plots in Central Europe. As a measure of stand productivity, we analyzed stand volume growth (m(3) ha(-1) year(-1)). Tree species diversity was quantified by the Shannon index and structural heterogeneity was represented by the Gini coefficient of basal area. For a more detailed analysis at the tree level using a smaller portion of the dataset, the tree position-dependent indices, diameter differentiation index, and aggregation index were used. Whether the effect of structural heterogeneity on stand productivity was positive or negative depended on the stand development stage. In early developmental stages, high structural heterogeneity lowered productivity. In later developmental stages, however, stand structural heterogeneity had a positive effect on productivity. Our study might provide insights regarding the mechanisms underlying the contradictory findings obtained in recent studies dealing with tree diversity-structure-productivity relationships. This knowledge is vital for the adaptation of forest management to meet future demands on forest ecosystems.