Spatial dynamics and integration within clones of grassland perennials with different growth form

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Abstract
The grassland perennials Bellis perennis and Prunella vulgaris have contrasting genet architectures, `phalanx' (modules tightly aggregated within clones) and `guerrilla' (modules loosely aggregated within clones). Plants of the two species were grown along density gradients in the presence or absence of Lolium perenne. Clonal branching systems were mapped repeatedly and digitized, and changes over time were analysed statistically. Variability in genet size was large in the guerrilla species, due to fast lateral growth and intense competition between genets, but small in the slowly expanding phalanx species. When the experiments were terminated, the distance between modules within genets was similar for the two species in pure stands and in mixtures without Lolium, but in the presence of Lolium, the phalanx species (Bellis) had responded by increased module aggregation and the guerrilla (Prunella) by decreased module aggregation. Genets of the phalanx species showed an integrated response when growing along a density gradient, expanding equally quickly in all directions, whereas in the guerrilla species parts of single genets responded differentially to their immediate environment, expanding more intensely towards lower than towards higher densities of the gradient. The ecological implications of the different growth forms are discussed.