Woody debris decomposition and nutrient dynamics in an old-growth bottomland hardwood forest

Background/Question/Methods

As a result of historical wetland losses, very few old-growth floodplain forests remain in the southeastern United States. Consequently, very little is known about the biogeochemistry of these unique ecosystems. The objective of this study was to assess temporal changes in decomposition and nutrient content of woody debris (WD) within an old-growth, bottomland hardwood forest at Congaree National Park, South Carolina. To accomplish this goal, 20 research plots were established across a toposequence, from the well drained natural river levee to a poorly drained backswamp, perpendicular to the Congaree River. Live wood from a single tree species present across all study plots (red maple, Acer rubrum L.) was harvested in the field, air-dried to a constant weight, and cut into 2.5-3.5 cm diameter, 25 cm long segments. Initial WD dry mass and total nutrient (carbon (C), nitrogen (N), phosphorus (P)) contents were quantified using standard methods. Sample strings, consisting of two WD samples per plot were placed in contact with the soil surface and subsequently collected at  intervals of 0, 1, 2, 3, and 6 months. Collected sticks were dried to a constant weight and re-measured to evaluate changes in mass and nutrient mineralization/immobilization behavior over time.

Results/Conclusions

During the first 3 month interval of the study, mass loss from WD was similar (mean 11%) across all study plots. Average C, N, and P contents also decreased, indicating nutrient mineralization from the wood. During the next 3 month interval (July-October), Red Imported Fire Ants (Solenopsis invicta) infested the majority of the study WD. Following disturbance by fire ants, net decomposition rates increased by 45% compared to the first 3 month interval. C mineralization rates increased slightly (1-2%) with no significant differences among the study plots. An immobilization phase for both N and P also occurred during this time; N increased 99-129% and P increased 95-262% relative to original content. N and P immobilization was significantly greater (p < 0.05) in the lower topographic positions (backswamp plots) compared to higher elevation natural levee plots. These preliminary results suggest that fire ants may have stimulated microbial activity within the wood, leading to immobilization of both N and P. Field sampling will continue for an additional 18 months in order to better understand the ecological significance of WD decomposition and biogeochemistry in old-growth bottomland forests of the southeastern United States.