Latitudinal gradients in plant-herbivore interactions in an invasive grass Phragmites australis in North America

For plant species, theory and empirical studies suggest that latitudinal clines should evolve with regard to palatability and defense traits. The expectation is that palatability should increase and defense levels should decrease with increasing latitude. However, this theory has never been tested with a species involved in continent wide invasion. We addressed this issue with Phragmites australis. This grass species is native and widespread in Europe and North America, but a European genotype has been spreading rapidly in North America over the past 100 years.

We performed common garden experiments in Rhode Island and Louisiana to quantify defense characteristics (leaf toughness, phenolics concentration), plant palatability to herbivore (colony growth rate of aphid Hyalopterus pruni) and nutrient levels (C and N). We found that traits associated with P. australis-herbivore interactions were strongly dependent on latitude, genotype, and origin (native vs. exotic). Both the native and exotic genotypes showed parallel latitudinal gradients on defense traits and palatability suggesting a genetic basis for these gradients. It shows that the latitudinal clines evolved in exotic genotype in less than 100 years. Our data also suggest that escape from natural enemies may contribute to the success of the exotic genotype in North America.