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Lateral root function and root overlap among mycorrhizal and nonmycorrhizal herbs in a Florida shrubland, measured using rubidium as a nutrient analog¹

Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6018 USA

Lateral root spread is an important indicator of the area over which plants forage belowground and has implications for individual plant fitness, belowground competition, and ultimately both population dynamics and community structure. Because fine lateral roots are difficult to excavate and measure, we used rubidium (Rb), an analog for potassium that occurs at very low levels in soils and plants, as a tracer of root activity. We injected Rb into sandy soils of a xeric Florida shrubland and examined uptake by four small herbaceous species that generally do not overlap aboveground. All four species took up Rb at distances of up to 97 cm (with an average lateral root spread of 51 cm), indicating that belowground neighborhoods are larger than aboveground. The arbuscular mycorrhizal species exhibited greater Rb enrichment without any change in lateral spread. The spatial arrangement of plants with elevated levels of Rb relative to Rb patches was consistent with root and mycorrhizal systems that are radially discontinuous and asymmetric. Furthermore, many individuals acquired Rb from the same patches, suggesting overlap in root function and the potential for belowground competition, although small-scale spatial and/or temporal segregation could have occurred.

Key Words: arbuscular mycorrhizae • belowground competition • belowground neighborhoods • Eryngium cuneifolium • Hypericum cumulicola • Paronychia chartacea • Polygonella basiramia • Rb • roots