Seed mass, dispersal and establishment in Carduus thistles

19 The negative correlation between dispersal and establishment appears to be well documented 20 in the plant literature; smaller seeds tend to disperse better but germinate less well, and 21 produce smaller seedlings. However, because dispersal capacity is often quantified using 22 proxies, such as the settling velocity of wind-dispersed seeds, little is known about the exact 23 shape of this negative correlation, and how it is modified by other plant traits and 24 environmental conditions. We studied the dispersal-establishment correlation in two wind25 dispersed thistles (Carduus nutans and Carduus acanthoides). We applied a mechanistic wind 26 dispersal model (WALD) to seeds released under a range of environmental conditions, and 27 tested germination and seedling growth under standardized conditions in a greenhouse. 28 Dispersal distance and establishment (germination and seedling growth) were not 29 significantly correlated, although in both species, smaller seeds dispersed farther, and showed 30 lower germination and lower seedling growth rates. This apparent paradox can partly be 31 explained by the significant influence of other factors such as release height and environment 32 (wind and vegetation), which explained more variation in dispersal than did terminal velocity. 33 Another potential explanation is the variation in seed traits: germination is strongly positively 34 related to seed mass, weakly positively related to plume loading, but not significantly related 35 to terminal velocity. This weakening of the correlation with germination is due to additional 36 layers of trait (co)variability: for instance, seed mass and pappus size are positively correlated, 37 and thus big seeds partially compensate for the negative effect of seed mass with larger pappi. 38 Our mechanistic approach can thus lead to a better understanding of both potentially opposing 39 selection pressures on traits like seed mass, and diluting effects of other seed, plant and 40 environmental factors. 41