Size hierarchies of shoots and clones in clonal herb monocultures: do clonal and non-clonal plants compete differently?

Kroon, H. de; Hara, T.; Kwant, R.

Oikos 63(3): 410-419


ISSN/ISBN: 0030-1299
DOI: 10.2307/3544967
Accession: 002493417

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The rhizomatous herbs Brachypodium pinnatum and Carex flacca were grown in monocultures at densities of 5, 17 or 59 clones per 20 x 20 cmsuperscript 2 plot. Competition was studied at shoot (ramet) and clone (the individual plant consisting of all interconnected ramets) levels. Size inequalities (measured as Gini coefficients), size-dependent growth and mortality of both shoots and clones were analysed for 2 years. At the end of both the 1st and the 2nd year of the experiment there were no significant differences in Gini coefficients of shoot height or weight between treatments for either species. In the 2nd year, when the number of shoots and the biomass per plot were extremely high and similar for all treatments, size hierarchies (based on height) of shoots produced in spring decreased during summer and height increment was unrelated to shoot height at the beginning of the growth period. The weight of small shoots may not have increased in proportion to their height, and hence the growth of small shoots may have been suppressed by larger shoots (asymmetric competition). The observation that smaller shoots had a higher mortality risk accords with this conclusion. In the 1st year of the experiment, clone mortality due to establishment failures was density-dependent. Clones in the largest clone density remained an order of magnitude smaller than those in the smallest clone density, indicating that clone competition was more severe in the former treatment. The Gini coefficients of clone size, based on rhizome DW and the number of shoots, did not differ between treatments suggesting that competition between established clones was symmetrical. This was also the case in the 2nd year, as shown by the low clone mortality and the fact that rhizome biomass produced in the 2nd year was linearly related to rhizome biomass produced in the 1st year. Both species gave identical results. It was concluded that competition between ramets within clones was asymmetrical in the species studied, whereas competition between clones is symmetrical, even in extremely dense stands where there is competition for light. This is fundamentally different from competition in non-clonal plants which is typically asymmetrical under such conditions. Reasons for this difference are suggested and some of the implications of the results are discussed.