Woody Root Biomass of 40- to 90- Year-Old Blue Oaks (Quercus douglasii) in Western Sierra Nevada Foothills1

This research examined biomass of blue oak (Quercus douglasii Hook. and Arn.) roots at the University of California Sierra Foothill Field and Range Research and Extension Center. Six blue oak root systems were excavated by trenching around the tree and removing the “root ball” with a backhoe. Before tree removal, soil from two 1-m3 trenches was sieved and roots were collected to estimate root biomass outside the root ball. Root ball mass ranged from 7 to 184 kg, and estimated total root biomass ranged from 12 to 193 kg. Root ball mass correlated with age, diameter at breast height, and bole mass. However, because of the small sample size, these relationships cannot yet be used for predicting belowground biomass. Although little information regarding coarse root biomass of tree species exists, these large structural roots have great value and importance for ecosystem processes. Coarse roots (generally, all roots >2 mm) not only offer physical support for the tree, but also influence the distribution of fine (<2 mm) roots, the roots responsible for water and nutrient uptake. Coarse roots also serve as a carbon and nutrient sink, and as such they represent an important component of biogeochemical studies of forest and woodland ecosystems (Nadelhoffer and Raich 1992, Vogt and others 1986). Historically, most coarse root studies focused on morphology (Henderson and others 1983, Lyford 1980, Stout 1956) or biomass, in relation to stand productivity (Baskerville 1965, Santantonio and others 1977, Westman and Rogers 1977). Morphological studies require careful dissection to expose root systems and often include detailed mapping procedures. Biomass studies, on the other hand, have generally been less precise, and methods included hydraulic excavation (White and others 1971), excavation with large machinery (Honer 1971, Johnstone 1971, Westman and Rogers 1977), and measurement of naturally uprooted trees (Santantonio and others 1977). In some instances, annual coarse root production has been measured as well as total biomass (Deans 1981, Kira and Ogawa 1968). A typical goal of biomass studies has been to obtain allometric relationships between root mass and more easily measured parameters, such as diameter at breast height (DBH) and tree height. With these relationships, the amount of biomass allocated belowground can be determined for an entire stand. For this study, six blue oak (Quercus douglasii H. and A.) trees were excavated in the Sierra Nevada foothills, northeast of Sacramento. Goals of the study included determining biomass distribution radially and with depth, as well as determining allometric relationships that would facilitate whole-stand belowground biomass estimation. This paper reports on allometric relationships between root mass and aboveground parameters (diameter, height, bole mass); biomass distribution data will be reported elsewhere.