Interspecific variation of tree root architecture in a temperate agroforestry system characterized using ground-penetrating radar

Root system architecture regulates belowground access to soil resources. Variation in root architecture is important in agroforestry systems given management objectives to optimize resource acquisition between trees and crops. However, the distributions of live tree roots in agroforestry systems remain understudied due to methodological constraints. In this study, we used ground-penetrating radar (GPR) to describe variation in whole-plant root architecture among tree species at the same agricultural site, with a specific focus on vertical coarse and fine root distributions within the zone of competition with neighbouring crops. Using GPR, we detected coarse roots of five trees species (Quercus rubra L., Juglans nigra L., Populus deltoides x nigra DN177, Picea abies (L.) Karst, and Thuja occidentalis L.) at a tree-based intercropping system in southern Ontario, Canada. A subset of soil profiles were assessed for GPR accuracy. A cumulative root distribution function was used to estimate the rooting depth (D-95) of coarse roots. We also measured tree coarse root distributions and fine root density distributions 2 m into the crop rows, in the zone of competition. GPR accurately detected approximately 58 % of coarse roots for each study tree. Coarse root architecture varied among species, with differences in D-95 and rooting patterns. Fine root length density distribution also varied among species, but was consistently high at 0.10 and 0.20 m depths regardless of species. Our results suggest differential tree suitability for minimizing belowground competition with crops. Additionally, we illustrate the viability of GPR to characterize vertical profiles of live tree root systems, which is critical for improving our understanding of whole-plant functional traits and belowground plant interactions.