Descriptions of tree root morphology inform design of belowground biomass and carbon inventories and sampling for research. We studied root morphology of tanoak (Notholithocarpus densiflorus), an important component in mixed evergreen forests of California and Oregon, USA. Tanoak re-sprouts from belowground lignotubers after disturbances, and stores an unknown amount of carbon in coarse roots underground. We sought to ascribe explanatory nomenclature to roots' morphological features and to identify models describing tanoak root morphology. Twelve tanoak root systems were excavated, dissected, and measured. Roots tapered according to their circumference and location. Larger roots closer to the lignotuber (located at the base of the tree stem) tapered more rapidly per unit of length. Tanoak roots forked frequently. Root cross-sectional area was preserved after forking events (i.e., the sum of cross-sectional areas for smaller roots on one side of the fork correlated with the adjoining large root). Occurrence and quantity of root branches (small roots branching laterally from larger roots) was dependent upon length of the source root segment. Our models of tanoak root morphology are designed to be organized together to estimate biomass of any segment or collection of lateral roots (e.g., roots lost/missed during excavation, or in lieu of destructive sampling), given root diameter at a known distance from the lignotuber. The taper model gives distal-and proximal-end diameters for calculation of volume for segments of root tapering between forks. Frequency of forking and branching can also be predicted. Summing the predicted mass of each lateral root segment, branch, and forked segment would produce an estimate of mass for a contiguous network of lateral roots.

Tanoak (Notholithocarpus densiflorus) is the most common hardwood in northern California forests, yet its capacity for belowground carbon storage is unknown. To study relationships between coarse roots and tree and stand variables, we destructively sampled twelve tanoak root systems in Humboldt County, California. To estimate belowground biomass, we summed measured biomass of the root ball and a subsample of lateral roots along with predicted biomass of unmeasured coarse roots. Tree size was the best linear predictor of belowground biomass and carbon, indicating that a 25-cm diameter tanoak, for example, stored 70 kg of biomass and 34 kg of carbon in its root system. Stand density was also influential: a doubling of stand density index reduced belowground carbon by 22% for the average tanoak. The mean root-toshoot ratio of 0.35 varied between 0.11 and 0.65, with larger tanoak at high stand densities allocating proportionally less biomass belowground than small open-grown tanoak. The findings highlight the importance of accounting for stand density effects, otherwise belowground carbon will be under predicted in low-density stands managed for tree health, vigor, and resistance to drought and wildfire, or overestimated in forests managed at high densities for high carbon sequestration.