Quantifying the Coarse Root Biomass of Intensively Managed Loblolly Pine Plantations.

Abstract

Nearly all of the C accumulation during a typical forest rotation is in plant biomass and the forest floor. Most biomass studies focus on aboveground C accumulation, and there is little information about biomass-C accumulation belowground. In older, loblolly pine forests, the majority of root biomass is in coarse roots, and coarse roots persist longer after harvest than aboveground biomass and fine roots. The main objective of this research was to assess the belowground carbon accumulation in coarse roots of a managed loblolly pine (Pinus taeda L.) plantation, which was subjected to different levels of management intensities. Additional objectives included determining the depth of excavation required to sample a majority of the coarse roots, quantifying coarse roots that were not associated with the taproots of either hardwoods or planted pines, developing an inter-specific hardwood regression relating diameter at breast height to coarse root biomass, and estimating total coarse root biomass per hectare. Estimates of total belowground biomass ranged from 56.4 to 62.4 Mt ha-1and were not affected by treatment. Pine and hardwood taproot biomass was affected by treatment, with vegetation control and disking significantly increasing pine taproot biomass and decreasing hardwood taproot biomass. Pine coarse roots not associated with the taproot were unaffected by treatment, but hardwood coarse roots not associated with the taproot were significantly reduced with vegetation control. Necromass was substantially lower than between-tree biomass, indicating that the decomposition of coarse root biomass from the previous stand is fairly rapid for coarse roots not associated with the taproot. Total aboveground biomass was significantly affected by vegetation control, with the lowest production on CHNO plots (180.2 Mt ha-1) and the highest production on plots receiving complete vegetation control, DIVC (247.3 Mt ha-1). Coarse root biomass ranged from 19 to 24% of total biomass. Silvicultural practices that increased aboveground pine productivity by reducing hardwoods did not increase total coarse root biomass C. Additionally, there is no evidence that coarse roots provide long-term C storage because they decompose rather quickly after harvest and during subsequent rotations.