Oomycetes do not drive negative plant-soil feedback in a tropical forest in Panama

Background/Question/Methods

Negative interactions between plants and their soil biota have been shown to be important in maintaining plant diversity and determining plant relative abundance in both grassland and forest ecosystems.  Negative plant-soil feedbacks (NPSF) occur when species-specific soil enemies accumulate under established plants, and, in turn, reduce the recruitment of conspecific offspring relative to heterospecific offspring, thereby preventing any one species from dominating the plant community.  In a previous study, we have shown that variation in strengths of NPSF mediated by some aspect of the soil biota is important in determining species relative abundance of tropical trees in Panama.  Here, we present results from a fully-reciprocal shadehouse experiment designed to test whether an important group of soil-borne enemies (oomycetes) are the primary enemies driving NPSF at the seedling stage in this Panamanian forest.  We hypothesized that if oomycetes are sufficiently species-specific with their hosts and drive NPSFs, then the strength of NPSFs should lessen with the application of an oomycete-specific biocide.  Further, oomycete reduction should lead to a breakdown in the relationship between the strength of negative feedback and plant relative abundance.

Results/Conclusions

Contrary to our a prior hypothesis, the addition of the oomycete-specific biocide significantly increased the strength of community-wide NPSFs relative to unmanipulated soil communities (P = 0.010).  Specifically, the strength of NPSF per species increased in all five species examined, and significantly so in three of those species.  However, the effect of biocide application on NPSF was not proportional among species and led to the breakdown in the relationship between the strength of feedback and relative abundance.  In unmanipulated soils, tree species exhibiting stronger negative feedbacks were less common as adults than those species exhibiting weaker negative feedbacks or positive feedbacks (R² = 0.82, P = 0.034).  However, no relationship between strength of feedback and abundance was detected in biocide-treated soils (R² = 0.19, P = 0.458).  Our multiple plant-species approach contradicts single-species studies and suggests that oomycetes are not direct drivers of NPSFs.  Instead, oomycetes appear to be generalists and may compete with the species-specific soil organisms that drive NPSF.