Under optimal partitioning theory (OPT), plants preferentially allocate biomass to acquire the resource that most limits growth. Within this framework, higher root mass under low nutrients is often assumed to reflect an allocation response to build more absorptive surface. However, higher root mass also could result from increased storage of total nonstructural carbohydrates (TNC) without an increase in non-storage mass or root surface area. To test the relative contributions of TNC and non-storage to root mass responses to resources, we grew seedlings of seven northern hardwood tree species (black, red, and white oak, sugar and red maple, American beech, and black cherry) in a factorial light x nitrogen (N) level greenhouse experiment. Because root mass is a coarse metric of absorptive surface, we also examined treatment effects on fine root surface area.
Consistent with OPT, total root mass as a proportion of whole plant mass generally was greater in low versus high N. However, changes in root mass were influenced by TNC mass in all seven species and were especially strong in the three oak species. In contrast, non-storage mass contributed to increased total root mass under low N in three of the seven species. Root morphology also responded, with higher fine root surface area (normalized to root mass) under low versus high N in four species. Although biomass partitioning responses to resources were consistent with OPT, our results challenge the implicit assumption that increases in root mass under low nutrient levels primarily reflect allocation shifts to build more root surface area. Rather, root responses to low N included increases in: TNC, non-storage mass and fine root surface area, with increases in TNC being the largest and most consistent of these responses. Increased TNC under low fertility could increase seedling access to soil resources by providing an energy source for: mycorrhizae, decomposers in the rhizosphere, or root uptake of nutrients.