Long-Term Sediment Accretion in Bottomland Hardwoods Following Timber Harvest Disturbances in the Mobile-Tensaw River Delta, Alabama, USA

Sediment accretion from overbank floodwaters is an important function in bottomland forests, yet few long-term data exist. A long-term research project (1986-2010) investigated effects of disturbances on sediment accretion in a water tupelo (Nyssa aquatica L.)-baldcypress (Taxodium distichum L. Rich.) bottomland in the Mobile-Tensaw River Delta, southern Alabama. Disturbance treatments subsequent to clearcutting were helicopter extraction (HELI), skidder simulation of ground based extraction (SKID), and removal of regeneration with glyphosate herbicide (GLYPH). An adjacent portion of the original stand provided a reference (REF). Periodic measurements of sediment pins (324 pins declined to 252 through attrition) indicate that REF trapped 19.2 cm of sediment between 1986 and 2010. During the same 24 years, HELI, SKID, and GLYPH trapped 31.2, 28.9, and 40.8 cm of sediment, respectively. Annual sediment accretion within the treatments was positively correlated to herbaceous biomass (r = 0.6624) while cumulative sediment accretion was positively correlated with days of overbank flooding (r = 0.8673). Site elevation surveys conducted in 1986 and 2010 matched closely with overall sediment accretion and Cs-137 evaluations of the pre-disturbance period between 1964 and 1986. All methods indicated a long-term sediment accretion rate of 0.7-1.25 cm/yr, which is similar to accretion rates found in other bottomland hardwood systems. This study indicates that riparian forest management practices that increase herbaceous vegetation are of critical importance to sediment accretion rates.