Plant spatial arrangement affects projected invasion speeds of two invasive thistles

Abstract

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. 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 The influences that differences in wind speed caused by different vegetation elements (e.g. conspecifics or other surrounding vegetation) may have on population spread rates of wind dispersed invasive species are unknown. Here, we examine the effects of differences in conspecific density and surrounding vegetation height together on wind speeds, and simulate population spread of two wind dispersed species from varying source environments. Understanding the effects of these factors on invasive spread has the potential to inform management decisions. For instance, optimal management strategies depend not only on plant demography under different growth conditions, but also on expected spread rates (Menz et al. 1980 Prevention of seed escape. Both study species are invasive weeds, so efforts were taken to prevent seeds from escaping. C. nutans capitula were tightly wrapped in fine pollen bag material. C. acanthoides produces smaller, more numerous capitula, so adhesive spray was used to prevent seed release.Statistical Analysis. Wind speed data were analyzed using linear mixed effects models in R (R Development Core Team 2009). Species, surrounding vegetation height, thistle patch array, and measurement height were used as explanatory variables. Weather station wind speeds were used as a covariate to correct for differences in ambient wind sp...

Covariation in abscission force and terminal velocity of windborne sibling seeds alters long‐distance dispersal projections

“…While wind speeds may be predictably affected by surrounding vegetation (Marchetto et al . ), and regular diurnal variation in wind speeds may exist (Skarpaas, Auhl & Shea ), micro‐variation in wind conditions on the scale of seconds or minutes in the field are largely unpredictable (Borger et al . ).…”

Quantification of plant dispersal ability within and beyond a calcareous grassland

“…; Bullock & Moy ), and may therefore differ between different grassland habitats (Marchetto et al. ). We conclude that under field conditions and in similar communities, release height is the most important factor for dispersal and should be taken into account when assessing diaspore dispersal of calcareous grassland species into the adjacent landscape.…”

“…In dense vegetation, as in the studied community, release height might therefore become more important for intermediate distance dispersal (1-40 m) than terminal velocity of the diaspore. Clearly, the dispersal potential of species that release their diaspores below the vegetation height depends on vegetation density and the resulting wind profile (Verkaar et al 1983;Bullock & Moy 2004), and may therefore differ between different grassland habitats (Marchetto et al 2010). We conclude that under field conditions and in similar communities, release height is the most important factor for dispersal and should be taken into account when assessing diaspore dispersal of calcareous grassland species into the adjacent landscape.…”

Water loss from flower heads predicts seed release in two invasive thistles

“…Xerochastic dispersers may also take advantage of increased wind speeds and unstable atmospheric conditions caused by differential surface heating rates in the late morning and afternoon that can increase seed transport (Lowry and Lowry 1989;Greene et al 2008;Dauer et al 2009). Large projected population spread rates for Carduus acanthoides and C. nutans can result when only afternoon wind speeds are used (Marchetto et al 2010a), when VPD is high and seed release is more likely.…”