Important role of a few large-diameter tree species in basal area and its increase in an old-growth deciduous broadleaf forest in Japan

Abstract

Because of their large diameter and biomass, old-growth forests provide valuable wood and high levels of ecosystem services (e.g., carbon sinks). Most of the aboveground biomass or basal area (BA) in old-growth forests is provided by small numbers of large-diameter trees. If the diversity of these trees is lower in temperate forests than in tropical forests, then the BA of temperate forests should be dominated by small numbers of large-diameter species. To explore how and to what extent BA and increase in BA (ΔBA) are determined by species with large diameters in temperate old-growth forests, we examined changes in the size structure and demographic parameters of large-diameter tree species based on 11 years of diameter at breast height (DBH, ≥ 5 cm) records for a 6-ha plot in an old-growth deciduous broadleaf forest in Japan. To estimate the extent to which difference in the successional status affect future ΔBA, we compared demographic parameters between them. Individuals with DBH ≥ 50 cm represented only 4.2% of all trees; however, their relative BA (rBA, ratio of BA of individual trees to the BA of entire forest) and relative increase in BA (rΔBA, ratio of increase in BA of individual trees to increase in BA of entire forest) were 36.4% and 25.1%, respectively, indicating that both BA and ΔBA were dominated by a small number of large trees. The rBA and rΔBA of the four species with the highest maximum DBH (DBHmax) were 71.1% and 61.4%, respectively, indicating that BA and ΔBA were also dominated by a very small number of species with exceptionally high DBHmax. Even among individuals with DBH ≥ 50 cm, the top four species in terms of DBHmax represented 90.0% and 80.6% of the total BA and ΔBA, respectively, confirming that both BA and ΔBA are highly dependent on species with high DBHmax values irrespective of the successional status. Most of the top 10 species in terms of DBHmax also showed increasing diameter growth rates and survival rates with increases in DBH class. Although the species with the second-highest DBHmax value had relatively low BA and ΔBA, BA gain exceeded BA loss for 11 years, thereby increasing the overall BA of the forest. These findings suggest that small numbers of large-diameter tree species play an important role in the sustainable increase in biomass among temperate old-growth forests in Japan. However, late successional species have higher rates of gain in BA, lower mortality, and higher recruitment rates than mid successional and early succesional species, indicating that their populations are increasing strongly. It is expected that the late successional species will eventually become more dominant than the mid and early successional species, unless there occurs some large-scale disturbance.