A replacement-series evaluation of competition between three Namaqualand ephemeral plant species

Abstract

The objective of the study was to determine whether the species could be arranged in a competitive hierarchy and to establish whether the interactions were symmetrical or asymmetrical. Indices of competitive ability indicated a hierarchy, namely Senecio arenarius > Dimorphotheca sinuata > Ursinia cakilefolia. The stronger competitors (S. arenarius and D. sinuata) were characterized by intraspecific competition having a stronger effect than interspecific competition from U. cakilefolia. The weakest competitor ( U. cakilefolia ) was characterized by interspecific competition from D. sinuata and S. arenarius having a greater effect than intraspecific competition. Total above-ground dry mass per plant of the stronger competitors was enhanced in a mixture, especially when grown with U. cakilefolia. However, U. cakilefolia performed just as well or better in a monoculture than in a mixture. Results indicated that some degree of niche differentiation exists, which may allow coexistence between the stronger competitors ( S. arenarius and D. sinuata ) and the weakest competitor ( U. cakilefolia ). Long-term coexistence between S. arenarius and D. sinuata , with similar competitive abilities, may be brought about by temporal or spatial refuges. The constantly changing conditions in Namaqualand promote coexistence, as no species is able to retain a competitive advantage long enough to exclude the others. In general, biomass allocation patterns showed no specific trend in response to competitive stress.