Roles of disturbance and demographic non-equilibrium in species coexistence, inferred from 25-year dynamics of a late-successional old-growth subalpine forest

We investigated compositional and demographical characteristics of major tree species in an old-growth subalpine forest in central Japan, based on a 25-year study. The stand is minimally disturbed and thus canopy structure is fairly stable. During the census, species dominance ranks in terms of both number and basal area were maintained. But, proportion of population size of major species within community significantly differed, and reflecting limited or no recruitment, declines in population size were found for all species including highly shade-tolerant Abies, suggesting demographic non-equilibrium in this forest community. This is because, according to each species’ regeneration attributes, disturbances fluctuating in space and time are required for persistence of their populations. So, competitive exclusion of other dominants by Abies spp., proposed as a mechanism of succession in several subalpine forests, seems unlikely. In addition to such non-equilibrium process, life-history differences among dominants, expressed as the equilibrium concept, are also greatly involved in species persistence because trade-offs of mortality and abundance/recruitment were observed between species. However, juvenile recruitment is closely related to canopy dynamics and disturbances, so that species persistence and resultant coexistence cannot be fully explained only by such differential demography. Here, it is specifically notable that not only current disturbance regimes but also ancient disturbance histories are both fundamentally of paramount importance for community maintenance and species coexistence. Populations and communities are fluctuating on the template shaped by both ancient and recent disturbances, and hereby, even in the late-successional forest, they are demographically non-equilibrium. Thus, further careful consideration is needed to evaluate complexity of the role and variability of natural disturbances in old-growth forest ecosystems.