Competition among weedy species: diallel experiments

Abstract

Competition among some introduced species sown or volunteering in temperate woodlands of southern New South Wales was studied in three greenhouse experiments in which species were grown alone and in 1:1 binary combinations. In two experiments competition between species was studied at different levels of nutrients. Total yields of most two-species mixtures were not significantly different from average yields in monocultures. For the most part, species yielding highest in monocultures had a competitive advantage in mixtures. There were exceptions; the annual legume, Trifolium subterraneum, was not always the most competitive in mixtures when highest yielding in monocultures. Except in two instances, neither statistically significant, the order of yields in monocultures of four annuals, two grasses and two cruciferous weeds, did not change with levels of phosphorus, calcium, potassium or nitrogen, but competitive advantages in mixtures, notably of the two grasses, were influenced by combinations of levels of these nutrients. Competition among three weedy thistles and a widely sown pasture legume, T. subterraneum, was studied at three levels of calcium and nitrogen. Yields of species in monocultures differed significantly at only three of nine nutrient combinations, but competitive advantages in mixtures differed significantly in all but one nutrient state. The legume was the most competitive species at low calcium, and Carduus pycnocephalus was the most competitive at high calcium; the advantage of both species increased with levels of nitrogen.