Relationships between plant–soil feedbacks and functional traits

Plant–soil feedbacks (PSF) and functional traits are two active but not well theoretically integrated areas of research. However, PSF and traits are both affected by life-history evolution, so the two should theoretically be related. We provide a conceptual framework to link plant functional traits to two types of PSF metrics, and hypothesize that individual PSF (plant performance in conspecific vs. heterospecific soil) should be related to the fast–slow trait spectrum, whereas pairwise PSF (the sum of the individual feedbacks for two species growing in each other's soils) should be related to trait dissimilarity. We performed meta-analyses to test these hypotheses by compiling two datasets, one dataset consisting of individual PSF values and plant trait values (specific leaf area, SLA; leaf N concentration, LNC; specific root length, SRL; fine root diameter, FRD; plant height; seed mass), and the second consisting of pairwise PSF values and trait dissimilarity. Our meta-analyses showed that individual PSF values were more negative in faster-growing species with greater SLA, LNC and SRL, supporting the growth–defence trade-off hypothesis. Plant height was positively correlated with individual PSF, perhaps because large, long-lived plants defend against pathogens better than smaller, shorter-lived plants. We also found that larger-seeded species had more positive or less negative PSF, likely reflecting greater tolerance of soil pathogens. The direction of relationships between trait dissimilarity and pairwise PSF varied with trait identity. Dissimilarities in SRL and FRD were negatively correlated with pairwise PSF while height dissimilarity was positively correlated with pairwise PSF. The contrasting relationships may reflect distinct links between trait dissimilarity and niche and fitness differences. Synthesis. Our results demonstrate how an integration of PSF and trait-based approaches can advance plant community ecology.