The carbon and nitrogen budgets of the Calhoun Experimental Forest in South Carolina have been monitored since the loblolly pine spacing experiment was initiated in an old cotton field in 1957. Because coarse woody detritus (CWD) in snags, downed stems, and belowground taproots did not begin to accumulate until the forest approached 30 years of age, previous budgets did not incorporate CWD as a significant carbon or nutrient reservoir. This study quantifies above- and belowground carbon and nitrogen stocks of CWD in the Calhoun forest, and estimates mean residence time (MRT) of four decay classes of CWD. Methods include an exhaustive field inventory, laboratory analyses, and tree ring counts. The objective is to evaluate the importance of CWD as a carbon and nitrogen storage reservoir, and to estimate accumulation and decay rates of CWD—data that are notably missing from current literature of southeastern pine and mixed pine-hardwood stands. This study will provide an improved biogeochemical understanding of C and N cycling through detrital wood, and an accurate estimate of C pool size and MRT that can be used by managers to estimate forest carbon stocks.
Results/Conclusions
Results show that percent carbon content decreases (p<0.001) and percent nitrogen content increases (p<0.001) with decomposition from Class I to Class IV logs, causing a decrease in C:N ratio from 800 to 100. C storage is concentrated in Class II wood, while N storage is distributed across decay classes II-IV. Decay class abundances suggest that many trees decay from Class I to II as standing snags, and from Class II to Class IV within decades on the ground. Both the distribution of decay classes and total CWD stocks are spatially heterogeneous at the 0.1-ha plot scale, with plot estimates ranging from 4 to 23 Mg C ha-1 and 0.01 to 0.08 Mg N ha-1. This is attributed to differences among plots in both standing biomass stocks and onset time of self-thinning, differences exacerbated by ice storms and bark beetle damage. In sum, at age 50 the Calhoun Experimental Forest is storing 12 (+/-3) Mg C ha-1 and 0.04 (+/- 0.01) Mg N ha-1 in aboveground CWD. This represents half of O-horizon C storage, but only 5% of the O-horizon N pool. Thus CWD plays a minor role in N cycling, but is an important C storage pool that requires consideration when quantifying forest carbon budgets or managing forests for carbon storage.
