Estimates of structural complexity in clonal plant morphology: comparisons of grazed and ungrazed Acaena magellanica rhizomes

The aim of this study is to examine the information given by various indices of rhizome morphology that describe grazed and ungrazed rhizome systems of Acaena magellanica (Rosaceae). Internode lengths, branching probabilities, and branching angles were estimated from grazed and ungrazed rhizomes in the field. These parameter values were then used in computer simulations of rhizome growth, and the structural complexity of the simulated rhizomes were described using size, topology, and fractal dimensions. Grazed rhizomes had shorter internodes, higher probabilities of branching, and more open branching angles than ungrazed rhizomes. This resulted in a more directional growth (herring-bone pattern) in the simulated ungrazed rhizomes, whereas the grazed rhizomes had a more space-filling growth pattern. Most indices, even though they are based on different mathematical and theoretical backgrounds, were highly correlated and thus equally good at describing the structural complexity exhibited by the rhizomes. However, indices have different relationships to theories about function, and we suggest that any study of structural complexity of branching systems should use several different indices of shape depending on the questions asked.