Does invasive Phragmites australis fecundity vary latitudinally?

Background/Question/Methods

The capacity for an invasive plant to spread is determined by its ability to produce propagules. Invasive plants can have high degrees of trait variation across their range, which can include reproductive traits. The invasive wetland grass Phragmites australis has become one of the most prolific wetland invaders into North American wetlands in the past 100 years. Evidence shows that Phragmites spreads mostly through seedling establishment. Phragmites shows latitudinal variation in growth traits, but whether this extends to traits related to reproduction has yet to be determined. Because regional variation in fecundity can have implications in invasion intensity in different areas, we ask does latitudinal variation in Phragmites inflorescence and seed traits exist? We hypothesized that population closer to the latitude of introduction (~ 40ºN) would have higher inflorescence length, spikelet number, seed weight, and germination rates than populations further poleward. To study this, I collected inflorescences from 18 Phragmites populations spanning 44° N-29° N. Inflorescence traits (length, mass, and spikelet number) were taken along with seed traits (average seed mass and germination rates). Nonlinear mixed-effects models were used to test the effect of latitude in explaining the variation in the inflorescence and seed traits.

Results/Conclusions

The effect of latitude was significant in explaining the variation in average seed mass (p-value < 0.0001) and germination rates (p-value = 0.0011), but not inflorescence traits, and we found that plants from the mid-latitudes (35° N -40° N) had the highest average seed mass and germination rates. This partially confirms our hypothesis. These results suggest that the viability of seeds produced in the mid-latitudes are higher than seeds produced from populations further poleward, but that latitude did not have any effect on the overall inflorescence traits that determine overall seed output. Allee effects may be one explanation for these latitudinal patterns as the highest rates of seed viability also correspond to the mid-latitudes. Coincidentally, this is the region Phragmites was first introduced to and has thus built up a high degree of genetic diversity which allows for outcrossing and higher seed set as Phragmites is highly self-incompatible. This effort represents the first steps into understanding variation in Phragmites spread on a regional rather than continental scale. We plan to explore the role of “time since invasion: on Phragmites fecundity and genetic controls on regional variation in fecundity.