Data from: The importance of forest structure to biodiversity-productivity relationships.

While various relationships between productivity and biodiversity are found in forests, the processes underlying these relationships remain unclear and theory struggles to coherently explain them. In this work, we analyse diversity-productivity-relationships through an examination of forest structure (described by basal area and tree height heterogeneity). We use a new modelling approach, called ‘Forest Factory’, which generates various forest stands and calculates their annual productivity (above wood increment). Analysing ~300,000 forest stands, we find that mean forest productivity does not increase with species diversity. Instead forest structure emerges as the key variable. Similar patterns can be observed by analysing 5,054 forest plots of the German National Forest Inventory. Furthermore, we group the forest stands into nine forest structure classes, in which we find increasing, decreasing, invariant and even bell-shaped relationships between productivity and diversity. In addition, we introduce a new index, called optimality, which describes the ratio of realized to the maximal possible productivity (by shuffling species identities). The optimality and forest structure indices explain the obtained productivity values quite well (R² between 0.7 and 0.95), whereby the influence of these attributes varies within the nine forest structure classes.