Plant Spatial Arrangement Affects Projected Invasion Speeds of Two Invasive Thistles

The spatial arrangement of plants in a landscape influences wind flow, but the extent that differences in the density of conspecifics and the height of surrounding vegetation influence population spread rates of wind dispersed plants is unknown. Wind speeds were measured at the capitulum level in conspecific arrays of different sizes and densities in high and low surrounding vegetation to determine how these factors affect wind speeds and therefore population spread rates of two invasive thistle species of economic importance, Carduus acanthoides and C. nutans. Only the largest and highest density array reduced wind speeds at a central focal thistle plant. The heights of capitula and surrounding vegetation also had significant effects on wind speed. When population spread rates were projected using integrodifference equations coupling previously published demography data with WALD wind dispersal models, large differences in spread rates resulted from differences in average horizontal wind speeds at capitulum caused by conspecific density and surrounding vegetation height. This result highlights the importance of spatial structure for the calculation of accurate spread rates. The management implication is that if a manager has time to remove a limited number of thistle plants, an isolated thistle growing in low surrounding vegetation should be targeted rather than a similar size thistle in a high density population with high surrounding vegetation, if the objective is to reduce spread rates. Introduction A spatial perspective is important for the study of plant ecology, because plants have a limited capability to move during most life cycle stages (Harper 1977, Bonan 1993, Dieckmann et al. 2000). Plant migration via seeds affects the spatial and genetic structure of populations, metapopulation dynamics, and invasion speeds (Cain et al. 2000, Nathan and Muller-Landau 2000, Levin et al. 2003, Trakhtenbrot et al. 2005, Skarpaas and Shea 2007, Jongejans et al. 2008b). Many studies have addressed the roles of environmental factors on seed dispersal kernels and spread (Nathan et al. 2002b, Soons et al. 2004a, Soons et al. 2004b, Jongejans et al. 2007, Greene et al. 2008, Soons and