Woody debris input and function in old-growth and clear-felled headwater streams

Woody debris plays an important role in forested streams by transferring energy and nutrients from the forest to the stream, providing habitat for stream biota, and contributing to channel structure. Alterations to the quantity of woody debris input to a stream channel, such as through clearfell, burn and sow (CBS) forestry, may therefore have implications for a stream’s morphology and biological functioning. We surveyed five old-growth (OG) and five CBS-affected (<7 years since logging) headwater streams flowing through wet eucalypt forest in southern Tasmania to determine if CBS forestry influenced the quantity and functional role of woody debris. CBS-affected streams had a significantly greater abundance and volume of woody debris situated within and above the stream channel than OG streams. This was a direct result of CBS operations which inputs large quantities of logging slash to the stream channel. Woody debris in CBS-affected streams was significantly less decayed than OG streams but had an increased functional role through colonization by bacterial biofilms and algae or storage of organic matter. This episodic input of woody debris following CBS forestry may strongly influence stream functioning for decades to come, due to the reduced woody debris recruitment from the surrounding forest. If the goal of forest management was to emulate a natural stream ecosystem with continuous woody debris input, then forest managers should consider a more continuous supply of woody debris recruitment into headwater streams throughout a harvesting cycle.

► Woody debris was surveyed in old-growth and clear-felled headwater streams. ► Clear-felled streams had significantly more woody debris than old-growth streams. ► Woody debris in clear-felled streams had an increased functional role. ► This episodic input of wood may influence stream functioning for many decades. ► Foresters should consider a more continuous supply of woody debris recruitment.