1.1 | Plant-soil feedbacks and woody plant encroachment
Plant-soil feedback could favor an encroaching species if it benefits the encroacher (intraspecific positive feedback) or inhibits competitors (interspecific negative feedback) or both (Bever et al. 1997; Aldorfová et al. 2020). A typical experimental approach to determine if the soil microbial community is driving plant-soil feedbacks is to compare plant growth in soils with live microbial communities with soils that have had their microbial communities sterilized with heat or fungicides (Kulmatiski and Kardol 2008). Greenhouse feedback-experiments typically have a training phase, where soil is conditioned by the growth of a species of interest and a phytometer phase, where plants are grown in the training soil to evaluate whether a feedback affects their growth. A positive feedback occurs when the fitness of subsequent conspecific or heterospecific plants benefit from growing in soil altered (conditioned) by a given species. Conversely, a negative feedback describes a reduction in fitness when growing in conditioned soil (Kulmatiski et al.2008a). Plant-soil feedback is a well-documented mechanism that can favor the fitness of range-expanding and invasive species in plant communities (Kulmatiski and Kardol 2008; Aldorfová et al. 2020).
We conducted a fully-crossed greenhouse experiment between redcedar and four common North American prairie grasses (Andropogon gerardi ,Schizachyrium scoparium , Bromus inermis , Pascopyrum smithii ) to evaluate if redcedar creates plant-soil feedback with any of those species and to determine the strength and direction of that feedback. If plant-soil feedbacks are a mechanism that help redcedars encroach into prairies, we hypothesize that we would observe the following outcomes: (a) redcedar would have neutral or positive conspecific feedbacks; (b) grass growth in redcedar soils would be reduced when compared to growth in intraspecific soils; (c) grass growth in live redcedar soil would be reduced when compared to sterile redcedar soil.