The impact of competition on the structure of early successional plant species of the Atlantic Coast of South Africa

This paper contrasts interactions between Conicosia capensis, a biennial or short-lived perennial, and the annual, Adenogramma glomerata at two points along an environmental gradient in the strandveld of the west coast of South Africa. Both species are early successional members of a sequence that proceeds to a multispecies community dominated by Euclea and Rhus. Their interaction began 15 months previously when a pipeline was completed in the area. We inferred differences in soil moisture availability from the establishment of C. capensis, which was significantly denser at the base of a runoff slope than on its side, and from the greater amount of open space on the slope. Using these differences to characterize our sites, nearest-neighbour analyses showed competitive interactions between the annual A. glomerata and C. capensis at both sites. In addition, nearest-neighbour analyses showed intraspecific competitive interactions between individuals of A. glomerata at the base of the slope but not on its side. Thus, the impact of competition on the annual is reduced on the slope. Annuals on the side of the slope responded in the second season after disturbance by establishing at twice the density and growing 23 per cent larger there than at the base. It appears from these results that differences in C. capensis density can have large effects on the seed bank of the annual.