Effects of maximum diameter, mycorrhizal type, and seed size on the relative basal area of tree species in an old-growth forest

Old-growth forests play a crucial role as carbon sinks due to their high biomass (basal area) and their high basal area increment. Therefore, understanding the factors that shape the relative basal area (BArel) and the relative increase in basal area (BAfrac_inc) of individual tree species in old-growth forests is essential for forest management. We investigated how and to what extent maximum diameter at breast height (DBHmax) and three ecological traits (i.e. mycorrhizal type, seed mass, and successional status) affect BArel and BAfrac inc of 61 tree species in an old-growth deciduous hardwood forest in Japan. Our analysis was based on 11 years of DBH records (≥5 cm). BArel was greater in tree species with large DBHmax, particularly those associated with ectomycorrhizal (ECM) fungi. In ECM species, conspecific adults distributed in clumps in the forest studied; therefore, in tree species with a high DBHmax, the number of large individuals increased, leading to a higher BArel. BArel was lower for small DBHmax tree species, especially those associated with arbuscular mycorrhizal (AM) fungi because conspecific adults were distributed far from each other, i.e. there were only a few large individuals. Structural equation modeling (SEM) revealed that seed mass also had a significant positive effect on both BArel and BAfrac_inc. However, this effect was much stronger indirectly via DBHmax because of the close relationship between seed mass and DBHmax. Successional status had little effect on BArel or BAfrac_inc, since there were little differences between successional statuses among the dominant species in the study forest. This study showed that the type of mycorrhizal fungi exerts a particularly strong influence on BArel and BAfrac_inc, primarily mediated through DBHmax. These findings suggest that DBHmax and mycorrhizal associations are key indicators for managing old-growth forests to maximize carbon sequestration and uptake.