Editor’s Choice: Phylogenetic dependence of plant–soil feedback promotes rare species in a subtropical forest

The editor’s choice for our June issue is “Phylogenetic dependence of plant–soil feedback promotes rare species in a subtropical forest” by Jiang et al. Here, Associate Editor Pierre Mariotte explains the importance of this research.

Plant-soil feedback (PSF) is the process by which plants influence biotic and abiotic soil properties, which in turn, differentially affect the success of conspecific versus heterospecific plant recruitment. Previous studies have shown that rare species suffer stronger negative PSF than common species, thus explaining their lower abundance in plant communities. However, how can rare species still coexist and persist if they are strongly self-limited? Jiang et al. suggest that phylogenetic conservationism might explain the persistence of rare species. Indeed, phylogenetic conservationism in PSF favours plant recruitment under phylogenetically distant heterospecific trees where they have a lower chance of encountering host-specialized soil enemies, thus offering the chance for rare species to recruit under phylogenetically distant trees. Among the three potential scenarios (Figure 1), only one would allow the persistence of rare species, which is when the strength of heterospecific negative PSF is stronger for common than rare species.

^{Figure 1 – Conceptual model of variation in the strength of soil feedback from conspecifics and heterospecifics between rare and common species. The x-axis indicates conspecific versus heterospecific soils and y-axis indicates the strength of soil feedback, with a larger value (+) indicating a weaker negative PSF or a stronger positive PSF, while (−) a stronger negative PSF. (a) Rare species shows no change in conspecific and heterospecific soil feedback strength, while common species has weaker heterospecific effect (positive slope), leading to recruitment advantage in heterospecific soils, thus preventing colonization of rare species. (b) Common species shows no change in soil feedback, while rare species has weaker negative feedback in heterospecific soils, leading to recruitment advantage under heterospecifics but the strength is not strong enough to overwhelm the common species. (c) Rare species has stronger conspecific soil feedback but weaker heterospecific feedback than common species, leading to species coexistence. Note negative slopes are unlikely to occur as predicted by the natural enemy hypothesis of invasive species and thus are not considered here.}

In their study, Jiang et al. aimed at elucidating the role of PSF in the persistence of rare species in subtropical forests by using shade-house and field experiments, in which they specifically tested (1) if rare species really experience stronger negative PSF than common species, as previously shown, (2) if there is a distinct phylogenetic dependence in PSF between rare and common species and (3) if it results in a different strength of heterospecific PSF. They used pairs of rare and common species from the genus Quercus and Ormosia and tested their performance in soils conditioned by themselves or by other species.

***^{Figure 2 – Variation in the strength of conspecific plant–soil feedback (a) and heterofamilial feedback (b) between the rare and common sister species pairs in the field experiment.}***

Both the shade-house and field experiments confirm that rare species suffer strong conspecific PSF, but weak heterospecific PSF, whereas common species experience little negative PSF, but strong heterospecific PSF (i.e., Figure 2, and as expected under the hypothesis of Figure 1c). This study by Jiang et al. highlights the importance of asymmetric heterospecific PSF with stronger heterospecific negative feedback for common rather than for rare species. Altogether, the results of this study demonstrate that PSF not only promote the recruitment of common species (i.e., in the vicinity of the common species) but also the recruitment of rare species (i.e., in the vicinity of phylogenetically distant species), thus facilitating the rare-common species coexistence in plant communities.