Modeling Diameter Growth of European Beech in Mixtures with Various Tree Species: The Impact of Size-Symmetric and Size-Asymmetric Competition

Mixed forests provide several ecosystem service benefits, and they also often show higher productivity than pure forests. In mixed forests, several interactions among tree species occur, with size-symmetric and size-asymmetric competition being particularly important. We studied diameter growth of European beech in pure stands and in mixtures with oak, maple, pine, spruce, fir, and spruce and fir combined on extremely diverse beech sites in Slovenia, using forest inventory sample plots (n = 26,793, 500 m2 each). For each mixture, we developed models of 10-year individual tree diameter increment (id) using natural splines and incorporating tree, competition, stand, site, and climate variables that were mainly gathered in regular forest inventories. Competition was represented using simple indices: stand basal area (BA) for size-symmetric competition, basal area of overtopping trees (BAL) for size-asymmetric competition, and reduced competition due to harvesting (CUT). The models revealed differences among mixtures and a strong influence of competition. Id was among the lowest in pure stands and substantially higher in mixtures, indicating strong intraspecific competition. Overall, size-symmetric competition was more influential, but size-asymmetric competition appeared important in some mixtures. We recommend growing beech in mixtures with other species and applying a forest management approach that accounts for competition symmetry, which proved crucial in each mixture.