 Because fine-root traits like diameter vary by species, and research examining the function of different root sizes in different species is limited, diameter-based classes of fine roots are mostly arbitrary and complicate cross-species comparisons.For example, two-millimeter-diameter fine roots may occur in trees, but would be very large roots in many annual and perennial species of crops.This classification system assigns an order number to a root based on that root's position in the branching hierarchy of the root system, and is based on the Horton-Strahler scheme for ordering stream tributaries.