There is now a large body of evidence that suggests that negative frequency-dependent processes are important to the maintenance of species richness in both temperate and tropical plant communities; albeit, mechanisms underlying such processes are poorly understood.  In temperate grasslands, negative plant-soil feedback mediated by species-specific soil fungi have been implicated as being important to plant distribution and diversity because such fungi limit the recruitment of conspecific seedlings around parent plants, thereby making space for the recruitment of other plant species.  In tropical forests, analyses of long-term census data consistently reveal strong patterns of negative frequency dependence; however, the contribution of plant-soil feedback to the generation of these patterns is unknown because empirical approaches designed to access the specificity of interactions formed between trees and soil biota are lacking.  Accordingly, this study utilizes the framework suggested by Bever et al. (J. Ecol. 1997) to access the strength and direction of plant-soil feedback generated among six species of shade-tolerant trees common to the Barro Colorado Nature Monument in Panama.  Specifically, seedlings of each of the six species were grown in association with soil biota collected either in close proximity to conspecific adults or in close proximity to heterospecific adults of the remaining five species. 

Results/Conclusions

I found strong evidence for negative plant-soil feedback.  With species included as a random factor, seedlings grown in association with conspecific soil biota grew poorer overall than those grown with heterospecific biota (home vs. away contrast: df = 1; F = 18.22; P < 0.001).  Specifically, relative growth rates of four of the six species were significantly suppressed when grown in association with conspecific soil biota.  Furthermore, contrasts examining pair-wise species interactions suggested that this system was characterized by negative feedback.  These results provide one of the first lines of evidence that tropical trees are associated with species-specific soil biota that drive plant-soil feedback and that such feedback may serve as a mechanism that contributes to patterns of negative frequency-dependence detected in tropical forests around the globe.