Quantification of species-level root-placement patterns and relative root distributions
Species root-placement patterns were categorized into seven types based on their occurrences in the 0-30 cm soil zone (Fig. 1). These included roots that occurred only in the 0-10 cm soil zone (type 1), 10-20 cm soil zone (type 2), 20-30 cm soil zone (type 3), throughout the 0-20 cm soil zone (type 4), throughout the 0-30 cm soil zone (type 5), throughout the 10-30 cm soil zone (type 6) and roots that occurred both in the 0-10 and 20-30 cm soil zone (type 7). For a soil cube, individual species’ root-placement pattern was assigned with one of the seven possible rooting types (Fig. 1). Relative root distributions were determined as the relative root abundance in the 0-10, 10-20 and 20-30 cm soil zones, with zones without roots assigned a score of 0%. Root abundance was indicated by species fine root length because root length is likely a better indicator of foraging effort than root mass. As individual root segments could not be accurately traced to specific parental trees, we focused on species-level relative root distribution for the 109 species with occurrences ≥ 5 cubes. Each 30-cm deep soil cube represents a root neighbourhood with three soil zones.