Variation in explosive seed dispersal: Comparing introduced populations to their native counterparts under competition

Background/Question/Methods

Plants modify architecture and fecundity in response to stress which can compromise seed dispersal; specifically, decreases in height and seed yield can lead to shorter dispersal distances.  Species may exhibit a range of traits required to succeed in its immediate environment, although variation of traits within a species are often ignored when forecasting species spread.  Using a widely-distributed species with varied introduction history, our objectives were to identify 1) whether dispersal traits differ between populations from native and introduced ranges and 2) to determine how intraspecific competition affects fecundity and dispersal.  We collected seeds from 15 populations of a short-lived self-fertilizing winter annual, hairy bittercress (Cardamine hirsuta) from its native (Europe) and introduced ranges (United States and Japan).  Seeds were grown in a common garden to negate maternal effects.  Propagated seeds were sown in greenhouse conditions to test the role of intraspecific variation and intraspecific competition (pot densities of one, five and ten plants) in ballistic seed dispersal.  We exhaustively captured seeds from 235 dispersal events and used parametric statistics and survival analyses to assess differences between treatments and mixed-models to fit each treatment seed dispersal kernels.  

Results/Conclusions

When grown individually, fecundity of plants did not differ by range; however populations from the native range (Malta in Europe) and introduced ranges (Japan, Vermont and Connecticut in the USA) produced more than double that of native French plants.  Maximum heights of Japanese plants were 36% taller than USA and European plants.  Dispersal distances differed among populations:  Japanese plants had greater mean dispersal distances than Maltese plants, and greater maximum dispersal distances than USA plants. When grown in competition, all populations had lower fecundity (<12%) and height (<42%) than their individually-grown counterparts.  Additionally, populations grown in competition and individual Maltese plants had lower fecundity than individually-grown USA plants and lower heights than individually-grown Japanese plants.  Ultimately, mean dispersal of Japanese plants grown at pot densities of one and five was 58% greater than Maltese plants grown under competition.  Maximum dispersal also varied, with Maltese and USA plants grown under competition expelling seeds only 73% as far as individually-grown Maltese plants.  Maltese plants may be adapted to compete at low densities, as it only maintained high fitness and wide dispersal when grown individually.  Japanese plants maintained wide dispersal at low densities, indicating a partial release from negative density-dependence found in native populations.