The ecology and evolution of reed canarygrass

Background/Question/Methods

Phalaris arundinacea (Reed canarygrass, Phalaris from here) is one of the most aggressive plant species currently invading North American wetland ecosystems. A growing body of evidence suggests that this invasion process is governed by a combination of ecological and evolutionary processes. In this context, it has been observed that invasive genotypes’ plant tissue has a higher C:N ratio than that of native genotypes when grown under the same conditions. Because the recalcitrance of litter generally increases with C:N ratio, invasive genotypes may stimulate litter accumulation. Recently, we suggested that this evolutionary development could induce an ecological feedback process: invasive genotypes may stimulate litter accumulation, which may improve their own growing conditions. This idea, however, has not yet been tested empirically. The aim of this study was to empirically test whether the invasion of Phalaris could be driven by the hypothesized feedback between invasive genotype abundance and litter accumulation. We performed a biogeographically comparative field survey of litter accumulation by Phalaris in native (Czech Republic, Europe) and invasive (Vermont, USA) habitats. Also, we compared the response of native and invasive genotypes to different levels of litter in a greenhouse experiment.

Results/Conclusions

The field survey revealed that litter accumulation in Phalaris-dominated plots was significantly higher in invasive habitats than in native habitats. This result could not be explained by differences in productivity between habitats. The greenhouse experiment revealed that invasive genotypes grew faster with increasing litter density: root biomass, shoot biomass and root:shoot ratio all increased with higher levels of litter application. Surprisingly, native genotypes did not show a significant growth response to litter. Looking more closely at the invasive genotypes, we found that only the genotypes with an inherently high C:N ratio responded positively to litter. Our results corroborated the hypothesis that the invasion of Phalaris in North American wetlands may be driven by a positive feedback between litter accumulation and Phalaris growth. These experiments also present a surprising twist to the hypothesis, in that both elements of the feedback process (litter accumulation and increased Phalaris growth) may be due to evolutionary changes in Phalaris. Our study illustrates the need to consider both evolutionary and ecological processes in exotic plant invasions, because these processes may amplify each other and thereby exacerbate the invasion process.