Coarse woody debris (CWD) plays a variety of important roles in forest ecosystems, from soil and nutrient retention to habitat for many wildlife species. It can also serve as a carbon (C) sink with a residence time ranging from decades to centuries. While aboveground material are commonly included in assessments of CWD, taproots and large coarse roots represent a substantial proportion of dead tree biomass that is rarely measured. However, in frequently burned longleaf pine forests, C mineralization may result from either decomposition or combustion, and the rates of C retention (as CWD or via conversion to highly stable black carbon) and release, and differences in these rates between above- and belowground CWD are not well understood. We determined the amount of belowground CWD potentially available as a C sink for two overstory species common in longleaf pine ecosystems, longleaf pine (Pinus palustris Miller) and slash pine (P. elliottii Englemann), and we established a relationship between amount of belowground CWD and tree diameter at breast height (DBH). We measured DBH on 787 overstory trees damaged in a March 2007 windstorm. Taproot length and mass was measured on a subset of twenty-six stumps (nine slash and seventeen longleaf pines) from trees blown over in the storm. Wood samples were collected to determine stump moisture contents and convert fresh stump weight to dry mass. Linear regression was used to investigate the relationships between DBH and taproot length and between DBH and belowground CWD.
Results/Conclusions
Mean taproot lengths were similar for both species (1.7 m and 1.8 m for slash and longleaf pine, respectively), and taproot length was not correlated with DBH (P=0.1687, R2=0.08). However, belowground CWD was positively correlated with DBH (P=0.0001, R2=0.46), allowing for coarse estimates of belowground CWD to be made based on easily obtainable data. Windstorms and other overstory disturbances can create large amounts of CWD, both above- and belowground. We estimate that 242.0 Mg total or 173 kg/ha of belowground CWD was input to our study site after the windstorm, representing a substantial pool of sequestered C. The quantities of belowground CWD reported in this study highlight the importance of understanding the standing crops and turnover rates of long-term C pools.
