Variation of root functional traits indicates flexible below‐ground economic strategies of the riparian tree species Populus fremontii

Abstract Plant‐mycorrhizal type has been suggested as an integrator of plant functional traits, yet most of what is known about these relationships comes from studies of different plant taxa, where the effects of mycorrhizal type cannot be isolated. In addition to affecting carbon‐nutrient exchanges, plants that associate with distinct mycorrhizal types often differ in several traits, with consequences for myriad below‐ground processes. We used two common gardens planted with Populus fremontii , a tree species that can simultaneously associate with both arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) fungi, to examine the degree to which mycorrhizal‐type dominance influences root traits and trait relationships across the root economic space. While P. fremontii formed AM and ECM associations simultaneously, individuals displayed a dominant mycorrhizal type driven primarily by garden location. Trees in the low‐elevation garden, regardless of provenance, were colonized primarily by AM fungi, whereas trees in the high‐elevation garden were colonized primarily by ECM fungi. In root systems at the low‐elevation garden, AM colonization rates were negatively related to specific root length indicating trade‐off with investment in foraging roots. In contrast, root systems at the high‐elevation garden, ECM colonization was negatively related to root tissue density, demonstrating a potential trade‐off between resource acquisition and root growth/defence. All other root economic traits remained similar between mycorrhizal types. While root traits varied little between AM‐ and ECM‐dominated trees (and gardens), their relationships with one another differed in each garden, suggesting unique strategies and trait trade‐offs in a single species. As global change continues to alter environments, species like P. fremontii , which experience a range of abiotic conditions, could signal how other tree species might modify root traits and strategies in response. Read the free Plain Language Summary for this article on the Journal blog.