Population models built from observational data typically describe competition phenomenologically. While the phenomenological approach captures the trajectory of natural populations, it may not be useful in understanding the underlying mechanisms. In a sagebrush steppe ecosystem, models from decades of population observations show that four dominant plant species are in stable coexistence maintained by niche differences present during early life stages. But what are these stabilizing mechanisms? We conducted field and greenhouse experiments to investigate the contributions of belowground microbes and direct root competition to stable coexistence in this ecosystem. In the field, seeds and seedlings of all four species were planted reciprocally near mature plants of all four species and in bare soil. These neighbor treatments were crossed with three ‘mechanism’ treatments: all niches (direct transplant), root exclusion (transplant in live soil within a cylinder), and microbial and root exclusion (transplant in sterilized soil within a cylinder). We investigated the effects of neighbor and mechanism treatments on seedling emergence, survival, and growth in two repeated experiment years. The field experiments were augmented with greenhouse experiments focusing on soil microbial effects in year 1, and seedling competition effects in year 2.
Results/Conclusions
From the field experiments, we found that belowground microbes were more detrimental to seedling growth than root competition, a result that was consistent across two growing seasons. However, these effects did not extend to seedling emergence or survival, nor were effects on seedling growth neighbor-specific. In other words, the effects of belowground microbes on seedlings of a given species did not depend on the species identity of mature neighbor plants that acted as microbial donors. Interestingly, even seedlings in the control condition did not show the strong intraspecific effects evident from the observational data. Across seedling response variables, we found that the projected relative abundance of the four plant species became more strongly asymmetric in the absence of soil microbes: dominant species increased in relative abundance, while subordinate species decreased. Our results suggest that in this sagebrush steppe ecosystem, soil microbes play a strong role in plant growth, but that plant-soil feedbacks may be a weak stabilizing mechanism for coexistence. In addition, plant-plant interactions inferred from observational data may overestimate intraspecific interaction strengths relative to interspecific interactions.