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Date Submitted

Fall 4-16-2015

Research Mentor and Department

Alexandra Harmon-Threatt




  1. My goal was to explore the influence of drought and soil biota on plant growth via plant-soil feedbacks, and thus to examine potential implications for stable coexistence for similar plants within overlapping species ranges.
  2. I carried out a greenhouse experiment to determine the effect of drought and soil biota on the relative fitness of two milkweed species. The project consisted of harvesting soil samples of each species from the field, further training the soil in the greenhouse to cultivate soil organisms, then growing each milkweed species in trained soil in a fully reciprocal experiment that included watering regiment and soil biota as treatments.
  3. There were strong conspecific negative feedbacks in both species, but these diminished under drought treatments. These results were driven by the growth of below ground biomass and were likely due to an increase in pathogenic soil microbes cultivated on the roots of conspecific plants. However, conspecific soil allowed for more growth in Asclepias syriaca compared to Asclepias sullivantii, suggesting that A. syriaca is less limited by pathogens. Additionally, these negative effects disappeared in treatments were the inoculum was sterilized.
  4. Synthesis. My findings suggest that biotic soil interactions play a significant role in plant growth, which complicates the possibility of coexistence in plants with overlapping ranges beyond resource competition. As drought intensifies under climate change, the effect of drought on soil-feedback mechanisms will complicate future range predictions beyond the physiological stresses of the drought-intensive summers predicted by climate change models, requiring an increase in knowledge of plant-soil interactions so that future ranges can be more accurately predicted.

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