Among the traits whose relevance for plant invasions has recently been suggested
are genome size (the amount of nuclear DNA) and ploidy level. So far, research on the
role of genome size in invasiveness has been mostly based on indirect evidence by
comparing species with different genome sizes, but how karyological traits influence
competition at the intraspecific level remains unknown. We addressed these questions
in a common-garden experiment evaluating the outcome of direct intraspecific
competition among 20 populations of Phragmites australis, represented by clones collected
in North America and Europe, and differing in their status (native and invasive),
genome size (small and large), and ploidy levels (tetraploid, hexaploid, or octoploid).
Each clone was planted in competition with one of the others in all possible combinations
with three replicates in 45-L pots. Upon harvest, the identity of 21 shoots
sampled per pot was revealed by flow cytometry and DNA analysis. Differences in
performance were examined using relative proportions of shoots of each clone, ratios
of their aboveground biomass, and relative yield total (RYT). The performance of
the clones in competition primarily depended on the clone status (native vs. invasive).
Measured in terms of shoot number or aboveground biomass, the strongest signal observed
was that North American native clones always lost in competition to the other
two groups. In addition, North American native clones were suppressed by European
natives to a similar degree as by North American invasives. North American invasive
clones had the largest average shoot biomass, but only by a limited, nonsignificant
difference due to genome size. There was no effect of ploidy on competition. Since
the North American invaders of European origin are able to outcompete the native
North American clones, we suggest that their high competitiveness acts as an important
driver in the early stages of their invasion.