Lineage and latitudinal variation in Phragmites australis tolerance to herbivory: implications for invasion success

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Herbivores play a critical role in plant invasions either by facilitating or inhibiting species establishment and spread. However, relatively few studies with invasive plant species have focused on the role of plant tolerance and how it varies geographically to influence invasion success. We conducted a common garden study using two lineages (native and invasive) of the grass Phragmites australis that are prevalent in North American wetlands. Using 31 populations collected across a broad geographic range, we tested five predictions: 1) the invasive lineage is more tolerant to simulated folivory than the native lineage, 2) tolerance to herbivory decreases with increasing latitude of origin of the populations, 3) estimates of tolerance are correlated with putative tolerance traits and plasticity in those traits, 4) a tradeoff exists between tolerance and resistance to herbivory and 5) tolerance has a fitness cost. Response to folivory varied substantially among populations of P. australis, ranging from intolerance to overcompensation. Our model selection procedure deemed lineage to be an important predictor of tolerance but, contrary to our prediction, the native lineage was 19% more tolerant to folivory than the invasive lineage. Tolerance for both lineages exhibited a u-shaped relationship with latitude. A tolerance–resistance tradeoff was evident within the invasive but not the native lineage. Also, tolerance was positively correlated with belowground biomass allocation, leaf silica concentrations, specific leaf area and plasticity in stem density, and negatively correlated with the relative growth rate of the population and plasticity in putative resistance traits. Lastly, although we did not detect costs of tolerance, our results highlight that fast growth rates can maintain high fitness in the presence of herbivory. Herbivory and plant defense strategies for P. australis lineages in North America exhibit complex biogeographic patterns that cause substantial heterogeneity in enemy release and biotic resistance and, consequently, invasion success.

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