Intra- and interspecific interactions of Scots pine and European beech in mixed secondary forests

•We assess the interactions of pine and beech in a mixed secondary forest.•Pine trees exhibited significant repulsion to their conspecifics.•Beech showed strong facilitative interactions with their conspecific neighbours.•The results revealed positive heterospecific interactions between pine and beech.•Beech saplings distribute within a particular spatial scale (0-2 m) around pine.

By the mid successional stages, secondary forests of Scots pine in Europe are dominated by mixed stands of pioneer Scots pine and late-successional European beech. The objective of this study was to explore the interactions of pine and beech with their conspecific and heterospecific neighbours in these forests. To accomplish the objective, pine and beech trees were stem-mapped in forty 500 m2 plots randomly located within 18 mixed stands in Milomlyn Forest District, northern Poland. The interactions within and between the species were analysed through two structurally different univariate and bivariate second-order summary statistics, i.e. pair correlation function g(r) and mark correlation function kmm(r). Field measurements showed that the overstorey was dominated by even-aged pine, whereas uneven-aged beech was the only species in the understorey. Pine trees presented an aggregation, while beech trees exhibited a dispersed structure in all stands. In addition, pine trees showed strong attraction to beech trees at small spatial scales (0-2 m). Negative correlation was found between tree height and diameter at breast height of beech, while there was no correlation between height and diameter of pine trees. We conclude that pine trees exhibit negative intraspecific interactions at small spatial scales that are mostly driven by their competitive interactions. Beech trees show strong positive intraspecific interactions and form clumps within pine canopy cover. The strong positive interspecific interactions of pine and beech are the outcome of their different shade tolerance. Our results help to explain successful coexistence of pine and beech in the study site and highlight detailed tree-tree interactions of the species in mixed stands.