Dispersal of an Invasive Tumbleweed
Dispersal is of critical importance for species conservation and reducing the spread of invasive species. I investigated the dispersal of the invasive tumbleweed, Centaurea diffusa. Tumbleweed dispersal is composed of three components; the plant stem must break free; wind must be sufficient to move plants; and seed must be retained in the plant with time and distance. Dispersal peaks were variable in space and time. Dispersal events correlated with wind gusts and variability in temperature. However, 18 and 78% of plants tumbled over 2 years at sites near Larkspur and Superior, CO, respectively. Weather did not fully explain the differences in dispersal magnitude. Raising plants from the sites in common garden revealed that Larkspur plants required 57% greater force to break than plants from the Superior site. In addition, plants under dry post-senescence soil conditions required four to six times greater force to break compared to plants under moderate to high soil moisture. Based on wind tunnel measurements, wind velocities necessary to break C diffusa stems ranged from 16 to 37 m/s. I developed a GIS-based model for the movement patterns of this plant based on topography, vegetation and wind patterns. Such modeling could provide information to help reduce the spread of this important invasive weed. However, model accuracy needs to be improved before it will be applicable to management. I conducted field and wind tunnel experiments to estimate seed dispersal with time and distance. The 95 percentile confidence limits for the half-life of seed retention in non-dispersed plants were 3 and 14 weeks suggesting that even late-dispersing plants have strong potential for longer distance dispersal of seeds. Seeds were commonly retained in plants for distances of 200 to 400 m and one still had seed in it after traveling over 1 km. Seed dispersal with distance seems linear, though exponential decay may better represent extreme dispersal events. Both models substantially overpredicted dispersal distances for field data collected during a drought year in the presence of biocontrol insects. This research has substantially increased knowledge of the dispersal of this plant and has highlighted future research needs.
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