Season-long seed dispersal patterns of the invasive weed Erigeron bonariensis in south-western Spain.
Within-field demography of weeds exhibiting wind-mediated long distance seed movement can be largely governed by extra-field seed-source populations. Thus, for these species, a clear understanding of the temporal and spatial patterns of seed dispersal can benefit the development of effective management options. The spatial distribution of the seeds released from the onset of fruiting, in early summer, to the onset of the rainy season, in early autumn, was studied during 2 years at a Mediterranean-climate locality in Erigeron bonariensis L. (Hairy fleabane), a wind-dispersed invasive weed inhabiting ruderal environments and crop fields. Each year, a small source population was established in an open field in SW Spain and seed traps were arranged in the eight cardinal directions at distances up to 100 m (year 1), or in the NE and NW directions at distances up to 65 m (year 2). Counts of trapped seeds were carried out at 4-6 day intervals and the number of seeds released by the source population was estimated each year in most census dates. Four empirical dispersal models based on either thin-tailed or fat-tailed density kernels were tested using year 1 data for their ability to represent the spatial distribution of seeds. To test for anisotropic dispersal, model parameters were allowed to vary according to the wind pattern in each cardinal direction. Based on information criteria, a model including a fat-tailed, Log-hyperbolic secant kernel showing parameter response to the wind pattern, highlighting striking anisotropic dispersal, was selected and evaluated using year 2 data. Distance percentiles 50 and 80 attained by the seed crop released in year 1 season were modeled at 530 m and 10,498 m, respectively. The opposite quadrants encompassing the dominant downwind (N-NE) and upwind directions (S-SW) received 52.5% and 10.8% of seeds. The year 1 population, consisting of 85 plants, generated a modeled seed rain of at least 10 seeds m-2 up to 200 m downwind. Implications of results for management of this herbicide resistance-prone species are discussed.