Soil heterogeneity generated by plant-soil feedbacks has implications for species coexistence

Background/Question/Methods

Environmental heterogeneity is predicted to promote species coexistence, but manipulations of the spatial heterogeneity of soil resources have in some cases increased or decreased diversity. Soil biotic communities are spatially variable and respond to plant community composition, often feeding back to affect plant population growth. This plant-soil feedback (PSF) may promote coexistence, depending on the relative PSF effects on conspecific vs. heterospecific plants. To determine the effects of heterogeneity generated by PSFs on recruitment dynamics, we measured recruitment of three congeneric pairs of introduced perennial plants grown as monocultures in experimental soil environments. Field soils collected from conspecifics and congeners were alternated in patches or mixed together to produce heterogeneous and homogeneous soils, respectively. We quantified the effects of feedback-driven soil heterogeneity, the relative contributions of species’ vital rates (i.e. germination and mortality), and their interactions on recruitment parameters. We also determined how soil heterogeneity and feedbacks from soils of different origin directly affected vital rates. Finally, we calculated net pairwise interaction coefficients to predict whether PSFs could mediate coexistence between congeneric plants.

Results/Conclusions

We found that Solanum dulcamara recruited later into heterogeneous than homogeneous soils, particularly when total germination was low. Soil heterogeneity significantly affected germination and mortality, and the direction and magnitude of the effect differed by species. Contrasts between soil patches of different origin suggest that mixing soils of different origin had non-additive effects on vital rates (e.g., Rumex crispus mortality was higher in homogeneous than in conspecific or congener soil). Net pairwise interaction coefficients predict that soil feedbacks might mediate stable coexistence of Rumex spp., may produce unstable dynamics in Plantago spp., and are unlikely to promote coexistence in Solanum spp.The relative contribution of vital rates to recruitment dynamics appears to depend on species-specific responses to soil heterogeneity. Non-additive effects of mixing soils of different origin may represent an interaction between the soils’ abiotic and/or biotic properties. Finally, net pairwise feedback coefficients predict PSFs may promote coexistence of certain genera, which was not evident from the individual PSF responses that are most commonly measured. Thus, soil heterogeneity generated by PSFs can have species-specific effects on vital rates that, in turn, affect recruitment dynamics and may ultimately mediate coexistence.