Crown architecture and species coexistence in plant communities

The relationships between crown architecture and species coexistence were studied using the diffusion model and the canopy photosynthesis model for multi-species plant communities. The present paper deals with two species having different crown shapes [conic-canopy plant (CCP) and spheroidal-canopy plant (SCP)], for various initial mean sizes at the establishment stage and physiological parameter values (photosynthetic rate, etc.). Recruitment processes were not incorporated into the model, and thus simulations were made for the effects on the pattern of species coexistence of either sapling competition starting from different sapling banks or competition in single- cohort stands with little continual establishment of species until a stand- replacement disturbance. The following predictions were derived: (1) SCPs can establish later/slowly in the lower canopy layer even if they are overtopped by a CCP which established first/rapidly; (2) if SCPs established first/rapidly and occupy the upper canopy layer, a CCP can rarely establish later/slowly in the lower canopy layer; (3) smallest-sized CCPs can persist well in the lowermost canopy layer overtopped by a SCP, suggesting a waiting strategy of CCP's saplings in the understorey of a crowded stand; (4) even if CCPs established first/rapidly and occupy the upper canopy layer, an SCP can establish later/slowly in the lower canopy layer. Therefore, the species diversity of SCPs which established first/rapidly and occupy the upper canopy layer limits the number of CCP species which can establish later/slowly. In contrast, the species diversity of CCPs which established first/rapidly and occupy the upper canopy layer does not affect the number of SCP species which can establish later/slowly. The combination of initial sizes of a CCP and an SCP at the establishment stage (i.e. establishment timing) affects the segregation of vertical positions in the canopy between the two species with different crown shape, and not only species-specific physiological traits but also crown architecture greatly affects the coexistence pattern between species with different crown architectures. The theoretical predictions obtained here can explain coexistence patterns found in single-cohort conifer-hardwood boreal and sub-boreal forests, pointing to the significance of crown architecture for species coexistence.