Does allelopathy explain the invasiveness of Campuloclinium macrocephalum (pompom weed) in the South African grassland biome?

Abstract

Campuloclinium macrocephalum is an Asteraceous alien weed that invades roadside vegetation and grassland in South Africa. The role of allelopathy and competition in its invasiveness was investigated using Eragrostis curvula (weeping lovegrass, an indigenous grass), E. tef and Lactuca sativa (lettuce) as test species. Trials were conducted in Petri-dishes, pots and in the field. Root and shoot extracts of adult C. macrocephalum plants did not inhibit seed germination in any test species. The greatest effect was radicle stunting produced by leaf extracts at 10 and 25% w/v. Eragrostis curvula was less tolerant of the extracts than E. tef. Allelopathic effects could however not be confirmed in pot trials evaluating the interference potential of the weed or weed residue effects against E. curvula. E. curvula growth and biomass was not affected by plant densities of one or five C. macrocephalum per pot, whereas C. macrocephalum suffered a 17% mortality and density-dependant trade-offs of size and biomass for survival. Under field conditions C. macrocephalum had a broader ecological niche than E. curvula, invading hygrophilous and undisturbed grasslands not amenable for E. curvula establishment, this included well drained disturbed soils on which the latter proliferated. Evidence of competitive exclusion of E. curvula by C. macrocephalum or vice versa was not detected. The coexistence of both species irrespective of relative density suggested these species have different resource requirements. Allelopathy was not an adequate causal mechanism to explain invasiveness in Campuloclinium macrocephalum. A more traditional hypothesis such as the absence of natural enemies, at this stage, better justifies the weed's invasion success.

Allelopathic effect of the invasive Acacia dealbata Link (Fabaceae) on two native plant species in south-central Chile

“…native trees or herbs), different types of biological assays have habitually been used. Apparently, the most successful are based on the use of aqueous extracts from the donor plants (Wardle et al 1996, Lin et al 2004, Mutlu & Atici 2009, Zhu et al 2009, Goodall et al 2010, Chon & Nelson 2013 to simulate field conditions (Scognamiglio et al 2013), or even by placing the host seeds in direct contact with the plant structures of the donor species in an aqueous environment (Aguilera et al 2015a).…”

Are environmental factors important facilitators of pompom weed (Campuloclinium macrocephalum) invasion in South African rangelands?

“…The impact that pompom weed has on species composition is subtle rather than one that completely transforms the communities it invades. For example, E. curvula and pompom weed were the most dominant species across sites (Appendix), yet in a separate study both species were found to be excellent competitors that coexisted rather than being competitively exclusive under controlled conditions and in the field (Goodall et al 2010). Three theories could contribute to this phenomenon.…”

“…1) (Henderson 2001), the infestation in Gauteng Province is sufficiently disjunct for the aim of discriminating between factors that facilitate invasion from those that influence vegetation dynamics, and secondly, to determine the impact that pompom weed density has on plant community composition in an isolated megainfestation. The Gauteng infestation, being the original point of introduction in the 1960s (Goodall et al 2010), is considerably larger and more concentrated than in other provinces. By the 1970s it was already widespread around the city of Pretoria with populations ranging from 30,000 to [100,000 plants per site (pers.…”

“…At present South Africa is the only country where pompom weed has become naturalised. It is not an allelopathic species (Goodall et al 2010) and hence the novel weapons hypothesis (Callaway and Ridenour 2004) does not apply in its invasion strategy. The aims of the study were to: (1) conduct vegetation assessments to provide a reliable estimate of proportional species composition in relation to environmental gradients, taking into account the variation in pompom weed densities; (2) compare factors facilitating pompom weed invasion with those driving vegetation pattern, and (3) compare vegetation condition against invasion susceptibility.…”

Genetic Variation for Weed Competition and Allelopathy in Rapeseed (Brassica napus L.)

“…Morphological traits related to the interception of radiation by leaves which determine competitiveness for light, including leaf size, number and leaf area index, stem elongation, upward leaf movement [21][22][23][24] and leaf layer density [25], have not been studied in canola. These traits are associated with shade avoidance, enabling plants to photosynthesise and grow to improve their competitiveness [21,22].…”