Long-term effects of riparian forest harvest on light in Pacific Northwest (USA) streams

Riparian forests exert strong influence on abiotic and biotic processes in adjacent streams by regulating light. Harvesting of riparian forests was once common practice, and consequently, many streamside forests across North America are in varying stages of development as they regenerate, thereby affecting stream light regimes. We used 2 approaches to evaluate the influence of riparian forest harvest and stand recovery on light availability in small mountain streams. We estimated light and canopy cover every 25 m along 11.5 km of a 4th-order stream network dominated by late-successional riparian forests that included 7 streamside harvest units 50 to 60 y old. Estimates of stream light fluxes were lower in harvest units than in up- and downstream sections bordered by old-growth forests even though only 1 stream bank was harvested in 5 of 7 units. Differences in stream light between harvested reaches and adjacent old-growth sections were greater when both banks had been logged. We also conducted a space-for-time analysis based on a literature review of Douglas fir-dominated forests of the US Pacific Northwest. Canopy closure generally occurred within 30 y of harvest and was followed by a period of maximum canopy cover (minimum light) that lasted from 30 to 100 y. Data were limited for stands ranging from 100 to 300 y old, but mean canopy openness and variability in openness along the stream were greater in late-successional forests (dominant canopy trees >300 y old) than in stands that were 30 to 100 y old (18 vs 8.7%), a result consistent with the network analysis. Overall results from our study suggest that streams with mid-successional riparian forests probably are in a period of minimal summer light fluxes.
Key words: stream light, stand development, forest succession, Douglas fir, canopy cover, H. J. Andrews.