Abstract:
Sericea lespedeza (Lespedeza cuneata), an invasive plant, displaces native vegetation in North American tallgrass prairie. L. cuneata is allelopathic to some species and able to engage in a mutualistic relationship with N-fixing bacteria. The mutualism between L. cuneata and rhizobia likely improves the nutrient status of L. cuneata, and plants in restored prairie sites with a row-crop agricultural history have higher rhizobia densities that may affect a plant's ability to produce allelopathic compounds. Because of this L. cuneata is a good candidate to test two ecological hypotheses of biological invasions, the novel weapons hypothesis and the mutualism facilitation hypothesis. Therefore, I tested if the novel weapons hypothesis (allelopathy) and mutualism facilitation hypothesis explain invasion of L. cuneata, and I hypothesized that soil from the rhizosphere of L. cuneata from restored prairie sites would be more allelopathic than from native prairie. Soil samples were collected from inside L. cuneata rhizosphere and 1 m away from a L. cuneata at restored and native prairie sites. Soil extracts were used to germinate seed of 12 different species. Tomato (Lycopersicum esculentum var. Rutgers VF) seed was used to test for the potential allelopathic effects of the soil extracts. The remaining species tested were grasses and forbs native to the region where soils were collected from including two congeneric species. Seed germination rate and seedling size were measured to evaluate the strength of allelopathy in soils from each site. The total soil N and C from each site were also measured. Soils from the rhizosphere of L. cuneata in restored prairie sites reduced germination rates of L. esculentum, Indian grass (Sorghastrum nutans), round-headed bush clover (Lespedeza capitata), and slender bush clover (Lespedeza virginica) more than soil from native prairies. Soils from the rhizosphere of L. cuneata in restored prairie sites also reduced the seedling length of L. esculentum, big blue stem (Andropogon gerardii), L. capitata, and L. virginica. L. cuneata did not affected germination and seedling size of forbs except for congeneric legumes, nor was it self-inhibitory. Allelopathy is more pronounced in restored prairie sites, and this may originate from a more robust symbiotic relationship with rhizobia in restored prairie. In the locations tested, L. cuneata has no effect of total soil N. Past row-crop agriculture reduced total soil N by up to half, and L. cuneata presence has not increased it. The invasion of L. cuneata and its production of allelochemicals provides support for the novel weapons hypothesis. Additionally, the different allelopathic responses of L. cuneata between native and restored prairie also provides support for the mutualism facilitation hypothesis because invasion is likely enhanced by mutualists that facilitate production of allelochemicals.
