Individual tree growth for coexisting Norway spruce and Scots pine as function of competition of neighbouring tree species

Trees in mixed forest can experience a competition reduction due to niche complementarity (spatial and temporal partitioning of resources) and/or facilitation between species. These positive interactions can take place at belowground (size symmetric) and/or aboveground levels (size asymmetric), among intra or inter-specific competitors. Here we aim to further understand the interactions between Norway spruce and Scots pine to detect possible causes of competition reduction in mixed stands composed by both species along a wide ecological gradient throughout Europe. Five different competition index and structures (size-symmetry, size-asymmetry, combined, and splitting the competition indices into intra- and inter-specific components) where included in basal area growth models fitted for each species using data from REFORM triplets. The results showed that there is a slightly negative effect of Scots pine on Norway spruce basal area growth, while Scots pine clearly was benefitted from the admixture with Norway spruce. Size-asymmetric is stronger than size-symmetric competition in these mixtures, highlighting the importance of light in competition. In addition, tree size and site conditions also modulated species competition along Europe. In general, competitive reduction in mixed forest involved greater advantages for Scots pine growth, although Norway spruce could also be beneficiated increasing resilience follow abiotic disturbancesMiškų ir ekologijos fakultetasVytauto Didžiojo universiteta