Palatability of geophyte underground storage organs partially explains their depth in the Cape Floristic Region, geophyte capital of the world.

PREMISE: The soil depth of underground storage organs (USOs) of geophytes involves cost-benefit trade-offs and may relate to USO age, size, and disturbances such as fire and herbivory. Yet determinants of USO depth are poorly studied. We posit that belowground mammalian predation of USOs is a selective force for the positive relationship between USO size and depth.

METHODS

We tested the mammalian predation hypothesis using USO depth, mass, type (bulb, corm, tuber, rhizome) and palatability to human foragers for 1828 geophyte individuals (153 species, 54 genera, 22 families) from the lowlands of the Cape Floristic Region (South Africa) which harbors the world's highest concentration of geophytes.

RESULTS

USOs were found at depths of 0-34 cm, but depth had no consistent relationship with mass. The depth-mass relationship was instead related to USO type: Compared with smaller counterparts, larger bulbs were at shallower depths, and larger corms were deeper. USO palatability affected the depth-mass relationship; palatability was generally associated with deeper burial across USO types: Palatable corms, tubers, and rhizomes mostly had positive depth-mass relationships, and unpalatable types had no relationship. Palatable bulbs had negative relationships that were less significant than for unpalatable bulbs. The phylogenetic signal in the study species was only moderate.

CONCLUSIONS

USO type and palatability explained some variation in their depth-mass relationship in accordance with the mammalian predation hypothesis. USO age and shape, soil type, and predation strategy and intensity should be explored as potential additional correlates of USO depth and size.