Seed release from invasive thistles.
Jongejans, Eelke*,1, Pedatella, Nick1, Shea, Katriona1, Skarpaas, Olav2, Auhl, Richard1, Isard, Scott1, 1 The Pennsylvania State University, State College, PA, USA2 NINA, Oslo, Norway
ABSTRACT- For accurate predictions of the spread of biological invaders, good data and models are needed on both the demography and dispersal of the invasive species. Here we report on an important but relatively unstudied aspect of the dispersal of two noxious thistles (Carduus nutans and Carduus acanthoides): the abscission of seeds from flower heads. The aim was to investigate under what weather conditions seeds release and disperse, since differential seed release has been hypothesized to importantly determine dispersal distances. The relative effects of wind speed and turbulence, flower head wetness, temperature and development stage were studied in a full-factorial design wind tunnel experiment. More seeds were released from heads that had just been wetted than from heads that had been wet for a longer time, but seed release was highest from dry heads, especially when the heads were more developed. Turbulent air flows caused more seeds to release in both species, but only in C. acanthoides did seed release increase with turbulence level. This experiment was combined with detailed field observations over a 5 week time period in which the local weather conditions and the seed release from 50 thistles was monitored. All of this information on the conditions under which seeds start to disperse is used in spread models to investigate if our predictions of spread rates improve and approach observed and historically-deduced rates of invasions.
Key words: seed release and dispersal, biological control, invaded grasslands
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