Plant-soil feedbacks and plant community composition in semiarid grasslands

Background/Question/Methods

A longstanding question in ecology is why similar species coexist.  Plant-soil feedbacks may affect plant relative abundance and enable grassland species to coexist.  Such feedbacks appear to affect plant diversity and community structure by either 1) suppressing dominant species, 2) causing the rarity of most plants, or 3) reducing the competitive abilities of all species.  Here I use three plant-soil feedback experiments, each with 16 grassland species and representing separate sites, to differentiate the effects of soil feedbacks on semiarid mixed-grass prairie vegetation.  Then I test whether the direction and degree of soil feedback accounts for variation in relative abundance among species that coexist within each plant community.

Results/Conclusions

Total plant biomass was 12% greater when grown with soil preconditioned by other species than conspecifics, an indication of negative soil feedbacks.  Across the three experiments, 74% of plant species (34 of 46) experienced some degree of negative soil feedbacks.  Separate analyses for species common in two or three of the experiments, however, indicated that 40% (6 of 15) of the grassland species experienced negative soil feedbacks.  The relative abundance of plants in the field was not positively correlated with the soil feedback of plant species for any of the three feedback experiments.  Instead, negative soil feedbacks occurred for the most abundant plant species and other species with intermediate to low abundances.  Thus, less abundant species are not necessarily more likely to have negative soil feedbacks than abundant species.  The negative soil feedbacks reported here may still act as a fundamental driver of species coexistence, but by having negative frequency-dependent effects on many species, including the most dominant.