A MODEL OF COMPETITION INCORPORATING PLASTICITY THROUGH MODULAR FOLIAGE AND CROWN DEVELOPMENT

The model of competition for light presented here uses modular autonom y to incorporate plasticity in plant growth under competition. Once pl ants are characterized as composed of modules, then model structure fo r competition changes in a fundamental way. Interations between the pl ant module and its local resource environment must be modeled rather t han the traditionally viewed interactions between whole plants and the ir neighbors. We assume that a plant module interacts with its local r esource environment regardless of whether this environment was altered by a neighbor or by the same plant. Two spatial processes are conside red: resource acquisition and growth. The spatial pattern of resource acquisition by a module determines a growth and allocation pattern, e. g., the elongation of branches into a gap. The spatial structure of a module and its connection to the whole tree then determines the patter n of resource distribution and resource acquisition of the next time s tep. Plasticity of plant growth is incorporated by variation in both t he efficiency of resource capture of modules and patterns of resource allocation for individuals of different canopy positions and results i n individuals in the community having different spatial structures. Th e model simulates the three-dimensional development of tree crown stru cture over time. It is applied to the 30-yr development of a dense, sp atially aggregated stand of Abies amabilis beginning with an initial p attern of seedlings. The importance of incorporation of plasticity is apparent when the model output is compared to observed height distribu tion and crown structure data. Simulations indicate that asymmetrical crown development, one form of plasticity, is advantageous to stand pr oductivity and becomes more advantageous as the degree of spatial aggr egation in the initial spacing of trees increases.