Contrasting climatic controls and neighborhood competition shape annual growth in Siberian forests

Climate is a major driver of tree growth in boreal forests, but the extent to which neighborhood competition provides additional explanatory power beyond climate and tree characteristics remains uncertain. We evaluate annual basal area increment (BAI) of Larix gmelinii (Rupr.) Rupr. in a temperature-limited northern taiga forest and Pinus sylvestris L. in a moisture-limited forest-steppe stand using tree-ring data, annually reconstructed neighborhood competition, and linear mixed-effects models. Growth responds positively to summer temperature in the northern taiga and to summer precipitation in the forest-steppe, reflecting contrasting climatic limitations. Reconstructed neighborhood competition provides relatively little additional explanatory power beyond climate and tree characteristics in the main models, but its negative association with BAI becomes stronger with increasing tree age at both sites. At the taiga site, competition also modifies the precipitation-growth relationship, with its negative association with BAI weakening under wetter conditions. In contrast, stem diameter and ontogeny are important sources of variation in BAI. These results indicate that climate and intrinsic tree characteristics explain more variation in annual growth than reconstructed neighborhood competition, while competitive effects remain context dependent and vary with tree age and moisture availability.