Leaf transport and retention in a high gradient stream

Abstract. To a large extent, the invertebrate fauna in wooded streams rely upon detritus inputs from the surrounding vegetation as their main food source. Conifer plantations may alter the timing and nutritional quality of leaf inputs into streams, but detritus retention may also be reduced. While it has often been assumed that conifer needles are retained less than broadleaved species, this has never been tested experimentally. This study describes the results of a leaf release experiment carried out in a small headwater stream, comparing the retention of pine needles and oak leaves (wet and dry) over a range of discharges. In addition, retention was compared before and after placing logs in the stream to examine the effect of woody debris on retention efficiency. Retention efficiency was significantly different among the three leaf types tested with wet oak leaves being most retentive, followed by dry oak leaves with dry pine needles being least retentive. Retention efficiency declined significantly with increasing discharge for all three leaf types. The addition of logs increased the retention of dry oak leaves and pine needles, but not wet oak leaves. The predominant features (stones, logs, eddies and bankside vegetation) that accounted for retention varied with discharge and the leaf type used. The results indicate that while conifer needles may be inherently less retentive than broadleaves, the presence of woody debris within the stream channel increases their retention. However, this small-scale study would need to be expanded to investigate the retention dynamics of conifer needles in plantation streams. Nevertheless, management strategies that allow the formation of woody debris accumulations in plantation streams could be valuable in enhancing detritus retention and consequently could benefit invertebrate communities.

Keywords: pine needles, retention efficiency, woody debris, discharge