PS 4-60 - Spatial dynamics of plant-soil feedbacks in a grassland-shrubland ecotone depend on vital rate

Abstract: Species must move, adapt, or face decline as environmental conditions exceed their physiological thresholds. Substantial movement in plant range limits requires successful dispersal and establishment away from a high-density population core, and brings novel interactions such as feedbacks with soil microbes. The spatial extent of a plant’s influence sets the boundary at which plants can alter unoccupied space or affect the performance of neighbors via soil microbes, thus affecting their spread relative to the scale of dispersal. However, despite observations on the spatial extent of specificity in plant-microbe associations, knowledge of spatial extent is lacking for the feedback process in any system. We conducted a field plant-soil feedback experiment to answer: What is the spatial extent of influence of host-specific belowground microbes on foundational species fitness in a grassland-shrubland ecotone? In the field, we added seeds of black grama and creosotebush along a gradient of distances from established creosotebush or black grama plants. Seedlings experienced microbial treatments that allowed (i) the full complement of belowground effects (including competition from resident plants), (ii) soil biota only, or (iii) abiotic effects only. We measured seedling emergence, survival, and growth to assess how plant-specific effects attenuate with distance.

We found evidence of species-specific, distance-dependent recruitment in germination. Black grama showed 87% higher recruitment near conspecific resident plants compared to near creosotebush resident plants (Chisq=24.84, df=1, P< 0.0001). In addition, black grama recruitment declined with increasing distance away from a black grama resident plant, whereas they did not show distance-dependence in recruitment near creosotebush resident plants (distance:resident interaction Chisq=5.01, df=1, P=0.03). Microbial treatments had no significant effect on germination, which suggests that the distance-dependent self-facilitation in black grama recruitment is likely driven by abiotic mechanisms. In contrast, seedling growth (measured as height at 3 months) did not show any distance-dependence. Black grama seedlings consistently grew taller near resident black gramas, regardless of distance and microbial treatment (resident effect P=0.001). Creosote seedlings also did better near black grama residents, showing classic negative plant-soil feedbacks which only occurred in treatments where microbes were present (treatment P=0.0008; resident P=0.05; treatment:resident interaction P= 0.04). Our study demonstrates that spatial effects of biotic and abiotic drivers of PSF depend on plant species and vital rate response. Future work will combine soil chemistry and microbiome analyses to uncover the underlying spatial drivers of biotic and abiotic plant-soil feedbacks.