Does Superior Competitive Ability Explain Yellow Starthistle's (Centaurea solstitialis) Successful Invasion of Annual Grasslands in California?

Abstract

Yellow starthistle represents one of the most spectacular examples of biological invasion in the western United States. However, the mechanisms leading to its success have not been clearly elucidated. Although its success has been attributed to superior competitive ability, few competition studies have been performed with yellow starthistle to test this assertion. Yellow starthistle and wild oat (a dominant component of California annual grasslands) were grown in monocultures and mixtures to assess the strength of competitive interactions between them. For either species, intraspecific competition exerted a greater influence over mean plant weight than did interspecific competition. A companion study revealed temporal separation in the phenology of these plants, explaining the weak role of interspecific competition. Additional measurements of growth and soil moisture dynamics in large 270-cm-tall by 50-cm-diam polyvinyl chloride columns also showed a lack of interspecific competition and confirmed that water use patterns differed between these species, indicating niche partitioning. Wild oat reduced soil moisture to 5% but only to a depth of approximately 150 cm. Yellow starthistle depleted soil moisture to less than 5% throughout the column to a depth of at least 270 cm. These patterns were present when wild oat and yellow starthistle were grown individually or together in the columns, indicating that yellow starthistle had a greater impact on soil moisture and to greater depths. Yellow starthistle's invasion of grasslands in California does not appear to be due to superior competitive ability, but may be due to its ability to access deeper soil moisture. These results support the empty niche hypothesis that implies that invasive species are successful in new habitats because they access resources not available to resident species.

Invasive grasses consistently create similar plant–soil feedback types in soils collected from geographically distant locations

“…Thus, PSF is likely unimportant in the invasion dynamics of these species. Cumulatively, this result combined with previous work suggest that the invasion of C. solstitialis is not consistently driven by PSFs and may be more related to species interactions and environmental conditions Spencer et al 2011;Young et al 2011). Previous research has investigated allelopathy for a congener (Centaurea stoebe or Centaurea maculosa) of our study species (Aschehoug et al 2014;Ridenour and Callaway 2001); however, no evidence of allelopathy has been found for C. solstitialis (Qin et al 2007) and we did not investigate any aspects of allelopathy in our study.…”

Impacts of mowing and bud destruction on Centaurea solstitialis growth, flowering, root dynamics and soil moisture

“…), which may have improved its ability to recover following mowing damage. This supports our previous findings (Spencer et al ., ), where we reported that the deep root system for C. solstitialis was a mechanism to avoid intense competition with annual grasses, including Avena fatua L. Results suggest that C. solstitialis is able to compensate for defoliation because it has access to additional soil moisture. In more resource limiting environments, there would likely be a greater negative impact on seed production from the mowing or bud damage treatments.…”

“…We erected 12 PVC columns, 3 m high and 0.46 m in diameter, similar to those described previously (Spencer et al ., ). Columns were filled with c .…”

“…Centaurea solstitialis also produces copious seeds that are readily germinable following dispersal (Benefield et al ., ). These traits better explain its success than the notion of superior competitive ability (Spencer et al ., ).…”

Assessing the biological control of yellow starthistle (Centaurea solstitialis L): prospective analysis of the impact of the rosette weevil (Ceratapion basicorne (Illiger))

“…Using glasshouse studies and climatic niche modelling based on principal components analysis, Dlugosch et al () found that YST in invaded areas has evolved a higher‐fitness life history at the expense of an increased dependence on water, resulting in larger plants and higher reproduction. Spencer et al () showed that intraspecific competition among YST plants exerts a greater influence on mean plant weight than does interspecific competition from exotic annual grasses because: (i) yellow starthistle depletes soil moisture to less than 5% to a depth of 270 cm compared with, for example, wild oats, which reduces soil moisture only to a depth of approximately 150 cm, and (ii) there is temporal separation of their phenology. Because soil moisture can be a limiting factor for YST in arid areas, a biophysical model (Ritchie, ) is used to compute the soil water balance of rainfall and run‐off above the holding capacity, as well as evapo‐transpiration from the plant and the soil surface (Gutierrez et al , ).…”