Interactions between plants and soil microbes can strongly influence plant competitive outcomes. While past studies mostly focused on how soil microbes drive negative frequency-dependent in plant community dynamics (i.e., stabilizing effects), recent studies have highlighted the role of microbially mediated fitness differences. However, factors predicting the relative importance of these two processes remain understudied. One overlooked facet of plant-soil microbe interactions is which aspect of the plant demography is impacted by soil microbes. Moreover, while models typically assume an instantaneous build-up of microbial effects following the establishment of plant individuals, the conditioning of microbial communities is instead a gradual process.
Here, we developed a novel two-species patch occupancy model that explicitly considers different soil stages (i.e., unconditioned vs. conditioned), and allows soil microbes to affect different plant demographic vital rates. Specifically, soil microbes impact either the recruitment rate, the mortality rate, or the fecundity of plants. We simulated the dynamics of competing plants and their host-specific pathogens. We studied how the plant’s ability to condition soil microbes affects the plant competitive outcome, and how the predicted stabilizing and fitness effects of soil microbes vary depending on the plant’s demographic response to soil microbes.
Results/Conclusions
We showed that a plant’s ability to condition soil microbes determines how strongly the plant experiences its microbial effects, therefore affecting their competitive outcome. When soil microbes decreased conspecific recruitment rates (e.g., pathogens in the Janzen-Connell hypothesis), the rate at which soil microbes were conditioned by the plant always promoted plant coexistence. On the other hand, when soil microbes increased the mortality rate or decreased the fecundity of their host plant, the soil conditioning rate could determine the competitive hierarchy such that the plant with a slower conditioning rate became the superior competitor. When we allowed microbial dispersal in our model, such that microbial effects were no longer constrained to the focal individual, the strong fitness effects of soil microbes were ameliorated and coexistence was again promoted. Our result suggests that microbially mediated fitness effects occur when soil microbes directly influence plant mortality and fecundity, and when their effects act at a spatially local scale. We argue that only by considering which demographic vital rates are affected by soil microbes can we better understand the pathways through which soil microbes affect plant community composition.