Hierarchical structures in competitive plant communities

To explore how a stable size hierarchical structure may emerge in plant com munities, a simple competition model for plant individuals with two types o f competition mode, asymmetric and symmetric, is studied numerically and th eoretically. The model incorporates size-ratio dependent interactions betwe en individuals, which reflects the sensitivity of resource gain to differen ces in individual biomass at the seedling stage. From numerical simulations , the stability of the multi-layered size structure is determined by the re lationships between the number of population, the degree of competitive asy mmetry and the potential maximum relative growth rate (i.e. seedling's rela tive growth rate) as follows: (1) As the number of population and/or the de gree of competitive asymmetry increases [decreases], multi-layered [mono-la yered] size structure becomes stable; (2) As the potential maximum relative growth rate increases, the domain of stable mono-layered [multilayered] si ze structure becomes larger [smaller]. It is further shown that, when the p otential maximum relative growth rate is fixed, (3) multi-layered size stru cture is more likely to be stable under asymmetric competition than under s ymmetric competition; (4) mono-layered size structure is more likely to be stable under symmetric competition than under asymmetric competition. When the hierarchical structures are self-organized by the dynamic instability o f the homogeneous state due to nonlinearity of competition, it is proved by theoretical analysis that these states are always locally stable. This is also suggestive of the mechanisms of species coexistence in plant communiti es.