Density had both positive and negative effects on shoot growth of pleurocarpous chalk grassland mosses. In an attempt to understand the mechanisms involved, a simulation model of pleurocarp shoot growth was built that included interaction for light and water along an explicit vertical dimension. The model allowed coexistence of at least four 'species' differing in shoot angle and branch density. Density and mass/area ratio of side branches with determinate growth strongly affected biomass gain. From an experiment on the effects of light quantity and quality on branching patterns of Ctenidium molluscum and Calliergonella cuspidata we hope to derive rules to allow incorporation of plasticity in branching patterns into the model.

Key words: branching, bryophytes, competition, facilitation, simulation model