Carbon dioxide regime, species identity and influence of species initial abundance as determinants of change in stand biomass composition in five-species communities: an investigation using a simplex design and RGRD analysis

1 The effect of elevated CO2 on species' performance was investigated in communities composed of five annual weeds that are characteristic of early old field succession in central Europe: Centaurea cyanus L., Matricaria chamomilla L., Silene noctiflora L., Papaver rhoeas L. and Legousia speculum-veneris (L.) Chaix. 2 The experiment was based on a simplex design, repeated at two overall levels of initial stand density, to give a wide range of five-species communities across which the initial composition and species abundance varied systematically. 3 A multivariate method, based on analysing the differences in relative growth rates (RGRD) between pairs of species, was extended for use with more than two species, in order to assess the relative importance of various determinants of change in stand biomass composition. 4 On average, Centaurea (54.6% of final yield) gave the highest yield, followed by Matricaria (22.9%), Silene (16.9%), Legousia (3.1%) and Papaver (2.7%). 5 The major determinants of change in community structure were species identity and CO2 level. Elevated CO2 significantly changed community composition towards the previously more poorly performing species Silene, Legousia and Papaver. 6 Despite strong effects of intra- and interspecific competition on individual species performance, species' initial abundance had relatively little impact on the change in community composition. Most cases where such effects were significant involved Silene: performance of Papaver was poorer in communities with higher initial presence of Silene and higher initial abundances of Centaurea and Matricaria always facilitated performance of Silene. 7 These new methods proved a powerful system for identifying the biotic and abiotic factors determining change in biomass composition in multispecies communities.