Tanoak (<i>Notholithocarpus densiflorus</i>) Coarse Root Morphology: Prediction Models for Volume and Biomass of Individual Roots

Abstract

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.

How do tree- and stand-level factors influence belowground biomass and carbon storage in tanoak (Notholithocarpus densiflorus)?

“…Sampling across different sites with different soils and climate is also advisable. While Namm and Berrill (2016) Bijak et al 2013). Nevertheless, we expect our allometric models for tanoak to give better predictions than any single R:S ratio for the species or forest type (Hunziker et al 2014).…”

“…Drying duration varied by root size, but all samples were dried until a consistent mass was attained. Namm and Berrill (2016) provide additional detail regarding the destructive field sampling procedure and the sample processing protocols.…”

“…The subsample of roots extending beyond the excavated area were used to develop models predicting root biomass and C for the remainder of the root system. Models describing root morphology (Namm and Berrill 2016) were developed and organized together to reconstruct individual root size and length, and predict biomass and C not sampled during the excavation. For this root reconstruction model, the end diameter of severed roots within the excavated area and their path lengths from the lignotuber were used for initial input.…”

“…A random number within a Poisson distribution defined the number of branches resulting from each root section. The length of the root segment was the parameter used to determine the quantity of branches (Namm and Berrill 2016). Branches were appended to their parent root section, (halfway along parent section length), and were programmed with the same fork and branch protocols as the other root segments.…”

“…The constant proportional bark volume and the increase in root wood percent C with path length from the lignotuber (Namm and Berrill 2012) were accounted for in calculations of simulated root biomass and C estimates. Carbon content of roots within the excavated area was estimated by applying the average C content for both the lignotuber and the roots adjacent to the edge of the excavated area to their total dry mass.…”