Abiotic and biotic factors affecting seed release and dispersal of the invasive thistles Carduus nutans and Carduus acanthoides

Open Access
Marchetto, Katherine Myers
Graduate Program:
Master of Science
Document Type:
Master Thesis
Date of Defense:
October 31, 2008
Committee Members:
  • Katriona Shea, Thesis Advisor
  • population spread rates
  • Rhinocyllus conicus
  • wind dispersal
  • seed release
  • Carduus acanthoides
  • Carduus nutans
  • Particle Image Velocimetry
The effects of abiotic and biotic factors on seed release, seed dispersal, and population spread rates were examined for two invasive thistle species of economic concern, Carduus acanthoides L. and Carduus nutans L. (Asteraceae). Damage to seed structures caused by storage time or shipping, resulting in increases in seed terminal velocities that would lead to underestimates of population spread, were quantified by treating one to five year old C. nutans capitula to non packaged, tightly packaged, or tightly packaged with crushing force treatments. The validity of these treatments in mimicking shipment was verified with measurements of seed terminal velocities before and after shipping from New Zealand. It was concluded that stored capitula up to five years old from demographic studies could be used to calculate unbiased population spread rates with new modeling techniques. Biogeographic studies involving shipment of capitula should ensure proper packaging to avoid crushing forces that would increase measured seed terminal velocities. A wind tunnel study measured seed release of C. acanthoides and C. nutans capitula collected under a range of field precipitation levels either immediately after collection or after one to two days of storage at temperatures of 10°C, 23°C, or 28°C with vapor pressure deficits of 3.4 mb, 9.5mb, and 17.0 mb respectively. Capitula stored at higher temperatures and vapor pressure deficits released more seeds during wind tunnel trials, but the effect of field precipitation was not significant. Seed release from C. acanthoides and C. nutans capitula with a range of natural florivory by a biological control agent, Rhinocyllus conicus, were also examined for seed release in a wind tunnel setting. Terminal velocities of seeds, total seed production, and the number of seeds that remained stuck in capitulum receptacles after mechanical removal of loose seeds were quantified. Empirical results showed significant reductions in seed production and seed release, with increases in seeds stuck in capitulum receptacles and terminal velocities for increasing R. conicus florivory, although estimates of seed terminal velocities for attacked C. acanthoides capitula could not be quantified due to heavy damage. Population spread rates for C. nutans populations in the United States (US) and New Zealand (NZ) were generated using integrodifference models. The results show a decrease in projected population spread rates with increasing R. conicus florivory, but different mechanisms were more important for each location. Relative reductions in population growth rates and population spread rates were also different for the US and NZ. A field study also addressed the effects of surrounding vegetation height, the number and density of conspecifics, and seed release height on wind speeds at focal plants for C. acanthoides and C. nutans. Higher surrounding vegetation and lower capitulum heights lead to significant decreases in wind speeds. Thistle patches with the most thistles and highest thistle density had lower wind speeds at the center of the patch than experienced by an isolated thistle, the more important of these factors being high thistle density. Projected population spread rates for C. acanthoides and C. nutans were calculated for populations growing in extreme conditions of surrounding vegetation height and thistle density to understand how differences in wind speeds might affect population spread rates. The same demographic population projection matrix was used for both species, so that only differences in dispersal capacities of the two species would be evaluated. Population spread rates generated for both species were highest for populations in low surrounding vegetation with low thistle density, followed by low vegetation and high thistle density, high vegetation and low thistle density, and high vegetation and high thistle density. Projected population spread rates for C. nutans were higher than those for C. acanthoides. Finally, the fluid visualization technique, Particle Image Velocimetry (PIV), was explored for use in seed release studies. PIV allows the visualization of velocity vectors from a 2D slice of an air flow. Von Kármán streets, vortices of air circulating at high speeds, were expected to be seen shedding from the wake of C. nutans capitula. Von Kármán streets are regions of high air circulation (vortices) that alternate in intensity over time and are shed downstream from the sides of an object under certain air flow conditions. This vortex shedding is expected by theory and is expected to cause alternating lateral forces proportional to ambient wind speeds on capitula, which could affect seed release. Overall this thesis demonstrates the strong impact that abiotic and biotic factors can have on seed release, and hence on seed dispersal and spread of these two wind-dispersed plants. As both are non-native invaders, this thesis also highlights the need for explicitly stated management objectives, because management recommendations can vary depending on whether control of abundance or spread is most desired.