Fine root respiration in Quercus rubra (L.) aligns with the economics trade-offs in bi-dimensional root trait space

Plant economic theory argues that growth strategies maximize either the rate or longevity of return per resource investment in a unidimensional trade-off. Belowground trade-offs may not mimic those aboveground due to soil resource heterogeneity, different physical constraints imposed by the shape of roots compared with leaves and fungal symbioses, and often multiple dimensions of variation are found. Root respiration represents a substantial carbon flux out of forest ecosystems, but its placement in these trade-offs is unclear, and its incorporation into carbon cycle models is limited by available data. Most research on root traits has focused on interspecific variability, but here, we investigated whether trade-offs among one species' populations align with those between species by sampling Quercus rubra (L.) populations along a Midwest, USA latitudinal gradient. Across populations, we assessed whether fine root traits follow uni- or multidimensional trade-offs and how these axes relate to root respiration. Respiration rates, morphological traits and root nitrogen were measured on excised fine roots at 14 sites, spanning a wide variety of environmental conditions, and then analyzed for trade-off axes. We uncovered substantial root trait variation among Q. rubra populations that aligned with two distinct trade-offs, one between branching intensity (BI) and average diameter and a second with root tissue density on one end and specific root length, root nitrogen concentration and root specific respiration (RSR) on the other. Reliance on ectomycorrhizal fungi, which colonize root tips, may be a possible explanation for the first axis, with higher BI representing more collaboration. Along the latter axis, RSR increased with root nitrogen concentration and decreased with root tissue density. These results support a similar bidimensional trait space between Q. rubra populations to that between species, with an economics trade-off that might be a useful predictor of the fine root respiration carbon flux.