Phosphorus limitation, soil‐borne pathogens and the coexistence of plant species in hyperdiverse forests and shrublands

Summary Hyperdiverse forests occur in the lowland tropics, whereas the most species‐rich shrublands are found in regions such as south‐western Australia (kwongan) and South Africa (fynbos). Despite large differences, these ecosystems share an important characteristic: their soils are strongly weathered and phosphorus (P) is a key growth‐limiting nutrient. Soil‐borne pathogens are increasingly being recognized as drivers of plant diversity in lowland tropical rainforests, but have received little attention in species‐rich shrublands. We suggest a trade‐off in which the species most proficient at acquiring P have ephemeral roots that are particularly susceptible to soil‐borne pathogens. This could equalize out the differences in competitive ability among co‐occurring species in these ecosystems, thus contributing to coexistence. Moreover, effective protection against soil‐borne pathogens by ectomycorrhizal ( ECM ) fungi might explain the occurrence of monodominant stands of ECM trees and shrubs amongst otherwise species‐rich communities. We identify gaps in our knowledge which need to be filled in order to evaluate a possible link between P limitation, fine root traits, soil‐borne pathogens and local plant species diversity. Such a link may help to explain how numerous plant species can coexist in hyperdiverse rainforests and shrublands, and, conversely, how monodominant stands can develop in these ecosystems. Contents Summary 507 I. Hyperdiverse forests and shrublands 507 II. Evidence for P limitation 509 III. Negative density dependence 510 IV. The importance of soil‐borne pathogens 510 V. A trade‐off between efficient P acquisition and fine root defence? 512 VI. Perspectives 516 VII. Conclusions 518 Acknowledgements 518 References 518