OLD GROWTH VS

SECOND-GROWTH FORESTS: LASTING IMPACTS OF LAND USE ON SURFACE-HYPORHEIC INTERACTIONS AND NUTRIENT RETENTION IN HEADWATER STREAMS. H.M. Valett, C.L. Crenshaw, and P.F. Wagner. Virginia Polytechnic Institute and State University, Department of Biology, 1002 Derring Hall, Blacksburg, VA 24061-0406

Forest harvesting through the practice of clear cutting has strong impacts on lotic ecosystem functioning. This has been well demonstrated for headwater streams in the Appalachian Mountains, USA where felled trees were dragged down stream beds as a logging practice. While benthic communities may recover in a matter of years, other aspects of the ecosystem may remain altered for many decades. In this study, we compared the structure and function of headwater streams in an old growth forest (n=3) with those (n=3) in an adjacent forested catchment of 75 yr second growth. Chlorophyll concentrations were low (mean = 33.0 and 33.1 mg m^-2) and did not differ between groups (P > 0.05) reflecting the substantial canopy present for all streams. Standing stocks of woody debris determined by the line-transect method, however, were significantly lower (P < 0.001) in second-growth streams. Similarly, stream bed particle size distribution differed between groups (P < 0.05). We used solute injections to assess how impacts from logging (reduction of woody debris, altered sediment composition) may impact stream hydrology and retention of nutrients. Results suggest that surface-hyporheic exchange and biological retention of phosphate-phosphorus may be sensitive indicators of disturbance effects even after 75 years of successional response