Models used to represent root systems so that findings can be expanded to larger scales rely, for the most part, more on mathematics rather than biological relationships. Corner's Rules of the size relationships among aboveground structures have been supported by data on more than 70 tree species. Our goal was to determine if Corner's Rules predicted root behavior in ways that could be incorporated in belowground models. Southern magnolia (Magnolia grandiflora) and longleaf pine (Pinus palustris) were chosen because of their very disparate root systems. We scanned 4-5 orders of intact roots from multiple root segments of both species and measured each root segment for length, diameter, magnitude (number of tips) and path length. Concurrently collected root were separated by order, scanned for length, then dried and ash-free weight was recorded. Magnolia roots were larger in diameter and greater in mass at all orders than pine roots. However, within a species, a larger 4th-order root was not associated with larger average diameter 1st-order roots. Thus, Corner's First Rule that plants with thicker axes have thicker ultimate axes was not supported. Corner's Second Rule that systems with lower ramification have thicker ultimate axes was also not supported. Magnolia roots are more highly branched (i.e. have much lower altitudes (longest individual path length) ~16 vs. 65, total path lengths ~300 vs. 1600, and magnitudes ~30 vs. 115) but have larger diameter root tips (~0.8 mm vs. 0.3 mm) and support more total root tip length (110 cm vs. 45 cm) and more mass (~0.7 g vs. 0.12 g) compared to pine roots. Stems are constrained by the weight-bearing needs of their dependent segments and the need for woody tissue. Fine roots are non-woody and do not bear weight, which could explain why our data do not support Corner's Rules.
