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15 Pág. Ecological Modelling, Tree species mixing of oak (Quercus petraea (Matt.) Liebl./Quercus robur L.) and pine (Pinus sylvestris L.) has been shown to have positive effects on ecosystem service provision. From a management perspective, however, it is still uncertain which thinning regime provides the highest possible productivity of mixed oak–pine forests in the long term. Because of a lack of empirical studies dealing with thinning and species mixing effects on oak–pine forests, we simulated forest growth in order to test which thinning type and intensity may provide the highest productivity in the long-term. To achieve this, we simulated the growth of pure and mixed stands of oak and pine for 100 years in 23 triplets located on an ecological gradient across Europe. For this purpose, we applied four different growth simulators and compared their results: the distance-independent single-tree simulator PROGNAUS, the distance-dependent single-tree simulator SILVA, the gap model ForCEEPS, and the process-based simulator 3D-CMCC-FEM. We investigated the effects of species mixing and thinning from the upper (thinning from above) and lower tail (thinning from below) of the diameter distribution by reducing the stand basal area to 50 and 80% of the maximum basal area. We compared simulated results of the relative volume productivity of mixed versus pure stands and of thinned versus unthinned stands to empirical results previously obtained on the same set of triplets. Simulated relative volume productivity ranged between 61 and 156%, although extremes of 10% and of 300% could be observed. We found the relative volume productivity to be influenced by stand age, but not by stand density, except for PROGNAUS. Relative volume productivity did not increase with the site water supply of the triplet location. Highest long-term productivity for oak, pine and oak–pine stands can be expected in consequence of thinning from above, but the effect of thinning intensity differed between simulators. Thinning effects were positively affected by stand density, but not by stand age, except for thinning from above predicted by PROGNAUS. Predicted thinning effects showed good approximation of results from thinning experiments for oak, but not for pine stands. We hypothesize the results might be caused by the insufficient simulator representation of climate and its interaction with other site variables and stand structure. Further work is needed to reduce the revealed limitations of the existing growth models, as we currently see no alternative to such kind of studies and simulators., The authors thank the European Union for funding the project “Mixed species forest management. Lowering risk, increasing resilience (REFORM)” under the framework of Sumforest ERA-NET. The authors from Austria also thank the Austrian Federal Ministry for Sustainability and Tourism for supporting the establishment of the Austrian triplet plots and for covering the work expenses of Markus Engel within the project ”Forstwirtschaft mit Mischwäldern – geringes Risiko, hohe Widerstandskraft – REFORM” under the grant number 101199. All contributors thank their national funding institutions to establish, measure and analyze data from the triplets. Maude Toïgo and Xavier Morin thank François de Coligny and Nicolas Beudez for their help in the development of the ForCEEPS model. Antonio Tomao, Carlo Trotta and Anna Barbati thank Alessio Collalti for his support and suggestions about the simulations with the 3D-CMCC-FEM model.

Old-growth beech (Fagus sylvatica L.) forests in Europe show a structural heterogeneity, which distinguish from managed ones. Recent investigations revealed that facilitative belowground interactions might counteract aboveground competition for light. However, the exact interaction of these mechanisms is unclear. We developed the BEEchPlasticity (BEEP) model to investigate whether the mere focus on the aboveground competition for canopy space with subsequent modeling of the light transmittance through the forest canopy suffices to reproduce the observed structural attributes. The BEEP model is individual-based, and explicitly describes the plasticity of tree crowns in a three-dimensional space through a geometric approach. This conceptual design allows tracing neighborhood effects to the forest stand level. We ran 10 simulation experiments with 1000 time steps on a simulation area of 0.5oha. We analyzed the emerging spatial point patterns, gap dynamics as well as distributions of tree age, tree height, tree crown projection area, and tree diameter. By applying various indices that revealed the emergent horizontal and vertical forest structure, we were able to show a strong mutual link between beech crown plasticity and forest heterogeneity. The crown plasticity enabled the trees to close small gaps, which resulted in a highly dynamic tree regeneration, which in turn led to a small-scale heterogeneous forest structure. We found that, over long periods, crown centroids of canopy trees taller than 20om were more regularly distributed than stem foot points. Absolute crown displacements were greater than reported from beech stands in North-Germany and the Sudetes in the Czech Republic. This indicates a need to restrain the model crown growth in order to recognize tree architecture and stability. We could simulate an increasing forest structural heterogeneity. Starting with a structural homogeneous layer of tree saplings, we showed that forest structures similar to those in unmanaged beech forests took several tree generations or approximately 700 time steps to emerge. We recommend enhancing this model approach with spatial explicit modeling of the leaf-area distribution, which has already been done for single beech trees.