Effects of microtopography and disturbance on fine-root dynamics in wetland forests of low-order stream floodplains

Fine-root mass and net primary productivity (NPP) were assessed in two forested wetlands located in floodplains of low-order streams in southern Alabama. In one experiment, live and dead fine-root (≤5 mm diam) mass in the upper 20 cm of soil were estimated by periodic soil coring for 7 mo. Cores were collected from P-fertilized and unfertilized portions of three microsite types: hummock, flat and hollow. In a second experiment, cores to 50 cm deep were collected from clear-cut and adjacent unharvested (reference) zones for 19 mo. In the first experiment, fine-root mass was significantly affected by microsite but not fertilizer. Roots were least abundant but temporally most variable in hollows, and fine-root NPP (estimated by summing root mass differences between sample dates) was greatest in hollows followed by flats and hummocks. In the clear-cut experiment, total fine-root mass was highly variable in clear-cut and reference sites. The ratio of live- to total-root mass was significantly lower on clear-cut sites on four of nine sample dates. NPP in the upper 50 cm of soil ranged between 1.90 and 4.55 megagrams per ha per year and was consistently, but not significantly, greater on reference than on clear-cut sites. This study suggests that fine-root dynamics in forested wetlands are affected by microtopography and possibly by disturbance to the overstory.