Intraspecific trait variation in seedlings reveals independence between leaf and root traits but a lack of an independent "collaboration axis" belowground

Plant functional traits, both above- and belowground, help determine resource acquisition strategies. While global trends at the interspecific level suggest independence between leaf and root traits, there are ecological and evolutionary reasons to expect different patterns of variation within species. We investigate whether the same functional dimensions observed across species also exist intraspecifically using tree seedlings. Three functional dimensions represent the resource-acquisition strategies aboveground and belowground, and mycorrhizal collaboration belowground. We ask: (1) Do the same three above- and belowground trait axes exist within species and are they independent from each other, and (2) are traits explained by environmental conditions? We collected root and leaf traits on 131 seedlings from four common, naturally occurring species across eight sites in a temperate, deciduous broadleaf forest in the USA. We measured traits defining the resource-acquisition strategies - specific leaf area, leaf nitrogen, root nitrogen, and root tissue density – and those defining the collaboration axis - specific root length and root diameter. In addition, we measured light conditions for each seedling and soil nitrogen (N) and phosphorus (P) to examine the relationship between traits and abiotic conditions using a novel multivariate regression analysis approach. We found that above- and belowground traits segregated into independent functional axes and that the collaboration axis merges with the belowground resource-acquisition axis. Furthermore, we found limited associations between abiotic factors and traits. Our findings suggest that within species there might be additional constraints to adjust to soil conditions, and therefore impact response to environmental change.