Niche theory for plant competition in a conditionable environment

Background/Question/Methods

Plants have a significant role in terrestrial ecosystems through both aboveground and belowground interactions. The explosion of research on plant-soil feedback catalogs how plants modify the biotic and abiotic properties of the soil in ways which have fitness consequences for the plants (Van der Putten et. al. 2013). The theory of plant-soil feedbacks has emphasized the role of negative feedbacks and provides predictions for species coexistence and abundance distributions based on the strength of interspecific interactions mediated by the conditioned soil. We extend the theory by modeling beneficial soil conditioning based on the match between the soil condition and each plant’s soil preference. Such a high-level representation of soil condition and plant traits allow us to simplify the complex processes underlying plant-soil feedbacks. As a consequence, we build a niche theory which is similar in spirit to the niche theory of exploitative competition emerging from consumer-resource interactions. Using mathematical models which describe the spatial dynamics of plant abundance and soil condition, we describe which species can coexist based on their soil preference, niche width, and strength of conditioning. We also determine the effect of the abiotic origin (initial soil conditions) and the interspecific difference in dispersal ability.

Results/Conclusions

We find that positive plant-soil feedbacks lead to species clusters. Specifically, plants with similar soil preferences can coexist locally but plants with very different preferences cannot. Contrast this with consumer-resource interactions, where only species with dissimilar resource preference can coexist. We provide a numerical method to recursively find the composition of species clusters. Numerical solutions of the model verify our expectation that a gradient in the abiotic origin can lead to patchy coexistence of different species clusters. We also found that if some species disperse more than others, then patchy coexistence is destabilized and only a sufficiently strong gradient in the abiotic origin can prevent extinctions. Climatic gradients and geological features can act as exogenous drivers which force the soil conditions towards their abiotic origin. We show that as the strength of such exogenous drivers increases, the number of species clusters decreases. Exogenous drivers can also stabilize patchy coexistence by driving the soil condition to be favorable to weak dispersers. Overall, we provide theoretical expectations for spatial patterns in a plant community where plants differ in their soil preference and ability to condition the soil to match their preference.