Long-term hydrologic and water quality responses following commercial clearcutting of mixed hardwoods on a Southern Appalachian catchment
Long-term changes (~ 20 years) in water yield, the storm hydrograph, stream inorganic chemistry, and sediment yield were analyzed for a 59 ha mixed hardwood covered catchment (Watershed 7) in the Southern Appalachian Mountains (USA) following clearcutting and cable logging. The first year after cutting, streamflow increased 26 cm or 28 percent above the flow expected if the forest had not been cut. In subsequent years, discharge increases declined at a rate of 5–7 cm per year until the fifth year, when changes in flow returned to baseline values. Later in forest succession, between ages 15 and 18 years, both significant increases and decreases in annual water yield were observed; these discharge dynamics are discussed in relation to vegetation regrowth dynamics. Flow responses predicted from an empirical regional scale model were within 17 percent of experimental values during the first 4 years of regrowth. Intra-annual analysis showed that proportionally larger increases (48 percent) in flow occurred in the low flow months of August–October. Storm hydrograph analysis showed that, on an average, initial flow rate and peakflow rates increased 14–15 percent, and stormflow volume increased 10 percent.
Analyses of stream solute concentrations and catchment nutrient fluxes showed small increases in nutrient losses following clearcutting and logging. Responses were largest the third year after treatment, with annual values of 1.3, 2.4, 2.7, 3.2, 1.4, 0.39, and 2.1 kg ha-1 for NO3-N, K, Na, Ca, Mg, S, and Cl, respectively. Explanations for the retention of nutrients and high ecosystem resistance and resilience are discussed in relation to internal biogeochemical cycles based on long-term process level studies on the catchment. A second, sustained pulse of NO3– to the stream (exceeding post-harvest values) observed later in succession is also discussed in the context of ecosystem processes. Large increases in sediment yield were measured immediately after road construction due to two major storm events. Subsequently, during logging, sediment yield from roads was greatly reduced and insignificant when logging activities were completed. In contrast, cumulative increases in sediment yield were observed downstream over the next 15 years, which illustrate the lag between pulsed sediment inputs to a stream and the routing of sediments through a stream system. The relevance of sedimentation to stream sustainability is discussed in the context of long-term responses in the benthic invertebrate community structure and productivity measured on Watershed 7.