Background/Question/Methods Invasive spread is greatly influenced by spatial structure. For instance, the position and physical characteristics of landscape elements may affect the action of dispersal vectors, such as animals, water, or wind. Field experiments were conducted on the effects of plant array size and density on wind speed at inflorescences for two wind dispersed invasive thistles, Carduus acanthoides and C. nutans, growing in different conspecific and interspecific environments. These data were used to calculate projected population spread rates using integrodifference equations coupling demography and dispersal models. To advance our understanding of how air movement interacts with inflorescences to influence seed release, we also explored the use of a fluid dynamics technique, Particle Image Velocimetry (PIV).

Results/Conclusions Wind speeds were significantly lower for low capitulum heights, for the largest and most dense conspecific thistle plant arrays, and for plants in high surrounding vegetation. There was a significant interaction between thistle species and surrounding vegetation height. Large differences in projected population spread rates arose for the extremes of thistle array size and density, and surrounding vegetation height. This result highlights the importance of spatial structure for the calculation of accurate spread rates, and shows that species may spread at different rates depending on landscape attributes. In addition, the use of PIV to visualize the 2D air movement around C. nutans capitula led to new insights into seed release by wind. For example, the change in wind speed, and therefore air pressure, generated by air flow over the top of capitula could lead to the production of seed lift, thereby increasing seed release and dispersal.