Neighbourhood competition indices (NCI) have been used recently to investigate growth and competitive interactions among adult trees. In this study, we estimed NCI in 8 to 15 year-old stands following clear-cutting with protection of advance-growth, in a mixedwood boreal forest of central Quebec, to improve our understanding of early successional tree dynamics. Such understanding is important to better forecast future composition and structure of forests since clear-cutting with protection of advance-growth creates both biotic and abiotic conditions that differ much from natural disturbances such as fire or large-scale insect epidemics. Twenty-five 450-m2 plots were mapped (position, species and diameter at breast height of all woody stems), and soil and minor vegetation were sampled to determine abiotic and biotic conditions. We used nonmetric multidimensional scalling (MDS) based on soil nutrient concentrations and pH to create a relative fertility gradient. Site scores on axes 1 and 2 of the MDS were used to determine a relative fertility gradient. Target trees of the five most abundant and widespread species were sampled to determine their yearly radial growth. For each species, a set of growth models using different NCI were compared and the best model was chosen using information theory.
Our results show that the best models were those that gave a different competitive weight to each species. Spatially explicit indices generally did not perform better than non-spatial ones. This is likely due to the high density (21 000 stems/ha in average) of trees found in those young stands. Adding soil fertility did not produce better models. Competing trees located to the south of shade intolerant target trees had a stronger competitive effect, but this was not the case for shade tolerant target trees. For these young densely populated early successional forests, no added information was gained from knowing the exact position of competitors, but knowing competitor identity and broad north-south distributions improved our models. Future research should be aimed at determining how mortality differs between species to further improve dynamic growth models.