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.