Factors influencing litter delivery to streams

• Wind, vegetation and terrain all influence litter transport to streams.
• Litter travel distance increases with increasing wind speed and tree height.
• Alder leaves are transported farther at a given wind speed than Douglas-fir needles.
• 95% of annual litter input originates within 20 m of a stream in gentle terrain.
• Litter input can originate from beyond 25 m in steep riparian zones.

Leaves and needles are an important energy source for streams. However, relatively little is known about factors that determine width of the area contributing litter to streams. We assessed the relative effect of wind speed, wind direction, litter type, tree height and riparian topography on litter delivery to streams. Wind speed effect on litter travel distance was determined by releasing needles (Douglas-fir) and leaves (red alder) from mature and young tree canopies over a range of wind velocities. Short-term litter collections were conducted to determine the effect of wind speed on litter fall. Litter travel distance increased with increasing wind speed and increasing tree height. At low wind speeds leaves traveled further than needles. Litter fall increased with wind speed. These relationships were combined with literature values for seasonal rates of litter production and an annual wind speed and direction record from a riparian area in western Washington to evaluate the relative effect of various factors on litter delivery area width. We found that width of the contributing area for needles was about 35% greater in riparian stands supporting mature conifer trees than at sites with young trees. Increasing riparian area slope from 0° to 45° increased width of contributing area by 71–95%, depending on litter type and stand age. Doubling measured wind speed increased contribution zone width 67–82%. Estimated buffer width required to capture 95% of annual litter input ranged from about 14 m to over 25 m under the conditions which we evaluated. The variety of factors influencing litter delivery area width and the spatial and temporal variation in these factors indicates that the common practice of employing fixed-width buffers to protect stream–riparian interactions cannot be consistently effective.