Streamflow and Nutrient Response to Contemporary Forest Harvesting Practices in Needle Branch

The original Alsea Watershed Study water quality monitoring effort showed that Flynn Creek had higher streamwater nitrate concentrations than either Deer Creek or Needle Branch before timber harvesting. The higher nitrate was attributed to the presence of nitrogen fixing red alder (Alnus rubra). Later in Phase I, sub-basin water quality sampling further identified spatial and temporal variability in nitrate concentrations. Landscape elements of slope, vegetation type, soils, and soil wetness can best predict nitrate concentrations. Nitrate concentrations in Needle Branch increased after harvesting in the original study, and were monitored for 2 years after harvest. Part of the original study was to calculate the nitrate nitrogen flux (kg/ha/yr) out of the watersheds. Science at the time had identified maintenance of site productivity as a concern. Water quality results after harvest with contemporary best management practices, shows a remarkable similarity to the nitrate response with no best management practices.  Nitrate concentrations increase with the ‘flushing’ of the soil mantle with fall storms and concentrations decrease over the hydrograph. Total nitrogen was measured, but in nitrate represented 95+% of the total nitrogen.  The post-harvest treatment monitoring period of six years better shows the nitrate decrease as more vegetation occupies the site.