Foliage profile, size structure and stem diameter–plant height relationship in crowded plant populations

The relationships between vertical foliage profile of an individual plant, competition between individuals, size structure and allocation pattern between stem diameter (D) and plant height (H) were investigated using canopy photosynthesis and two-dimensional continuity equation models including D and H as two independent variables. Broad-leaved type plants (more foliage mass in the upper layer than in the lower layer of the canopy of an individual when grown in isolation) showed curvilinear D–H relationship and bimodal H distribution, and underwent more asymmetric competition than coniferous type plants (more foliage mass in the lower layer than in the upper layer of the canopy of an individual when grown in isolation) under crowded conditions. Coniferous type plants showed almost linear D–H relationship (i.e. simple allometry) and unimodal H distribution, and underwent more symmetric competition than broad-leaved type plants under crowded conditions. However, in both the cases D distributions were unimodal. Allocation patterns between D and H affected these features only a little. These simulation results can explain many actual data already published. The value of η for an individual plant (foliage profile parameter of an individual canopy representing a species-specific branching pattern and canopy morphology when grown in isolation) governs size structure (bimodal or unimodal), the mode of competition, D–H relationship and mean D–mean H trajectory with time under crowded conditions. Therefore, a simple view of the competition-allometry relationship that competition determines allometry should be re-evaluated incorporating the foliage profile of an individual. These theoretical results should also be important when studying species coexistence. The canopy tends to be multi-layered in broad-leaved type plants and mono-layered in coniferous type plants. Therefore, it is hypothesized that species coexistence in the former is mainly by way of separation of vertical space (i.e. niche separation under strongly asymmetric competition) and that species coexistence in the latter is due to nearly symmetric competition in a single canopy layer.