In the presence of plant-soil feedback (PSF), a plant that makes specific changes to the soil community may either positively or negatively affect the performance of the plant that will subsequently grow in that location. These effects can collectively influence plant species diversity at multiple spatial scales. One conventional prediction is that positive PSF reduces alpha diversity (local diversity) by allowing dominance by early-arriving species, but increases gamma diversity (regional diversity) by promoting community divergence through emergence of alternative stable states each dominated by different species. In contrast, negative PSF increases alpha diversity by allowing local species coexistence, but reduces gamma diversity by promoting community convergence. However, this prediction is based on the assumption that (1) PSF affects only conspecific plants and (2) communities are in a stable state. In reality, (1) PSF is more complex, as both intra- and inter-specific PSF are common and the sign and magnitude of both types of PSF are highly variable, and (2) many communities are kept at a transient state by occasional disturbance. Little is known about how complex PSF affects species diversity over the course of plant succession before communities reach a stable state. Using a simple simulation model of plant community assembly, we investigated how complex PSF might promote community convergence or divergence and influence species diversity during plant succession.
When we included only either positive intra-specific or negative intra-specific PSF in the model, results were consistent with the conventional prediction. When we included both inter- and intra-specific and both positive and negative PSF in the model, PSF promoted community divergence, resulting in reduced alpha diversity, but increased beta and gamma diversity. However, communities diverged not necessarily because alternative stable states emerged, but primarily because PSF lengthened the time to extinction and prolonged the turnover of species during community assembly. In many cases, PSF-induced divergence was transient, though long-lasting, making the conventional concept of alternative stable states inadequate for explaining divergence. We also found that divergence due to lengthened time to extinction and prolonged turnover of species occurred more extensively and for a longer time when immigration rate was lower, indicating that the degree of divergence can be predicted by immigration rate. Taken together, our results highlight the importance of considering (1) complex PSF and (2) transient community dynamics in understanding plant species diversity.