Forest Investment Account

Abstract of FIA Project Y051170

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Large woody debris: what is it?

Author(s): Hogan, Daniel L.
Imprint: Victoria, B.C. : B.C. Ministry of Forests, 2005
Subject: Forest Investment Account (FIA), Coarse woody debris, Riparian Areas, British Columbia, Management
Series: Forest Investment Account (FIA) - Forest Science Program

Abstract

Various management approaches have been used in the forest harvesting industry to maintain stream and riparian functions. A specific objective of riparian management prescriptions (amongst several) is to maintain a supply of large woody debris (LWD) from the riparian area to the stream. This usually involves designation of a certain portion of the riparian area as either a riparian management zone (allowing some forest harvesting) or a riparian reserve zone (restricting all forest harvesting). However, the efficacy of these prescriptions for ensuring an adequate supply of large woody debris has not been well documented. In particular, we lack an operational and physically meaningful definition of the term 'LWD'. Despite the common utilization of scaling relations in fluvial geomorphology, wood characteristics have always been based on the absolute size of a log with little consideration to the size of the channel. The objectives of this research are to develop a process-based definition of LWD scaled to the size of a channel. This definition will then be used to establish best management practices for the supply of functional LWD (log length and diameter) from riparian areas adjacent to narrow riffle-pool and step-pool streams (generally, bankfull widths ranging from 1 to 10 m). This includes specification of the width and retention level of a buffer for each major forested biogeoclimatic zone in British Columbia. Development of science-based riparian management prescriptions can both improve stewardship of fish values and forest resources by establishing effective buffer widths, and can optimize production and value from timber resources by not unduly restricting access to timber. Stream channel and riparian area data from 49 watersheds across British Columbia were used to scale woody debris. The results imply that as channel width increases, stable values of debris roughness and the blockage ratio decrease. This suggests that logs at the critical threshold of stability are more geomorphically effective in relatively small channels as both the debris roughness and the blockage ratio increase with decreasing channel size. In biogeoclimatic zones characterized by relatively small trees, the upper limit of functional woody debris would be reached at a smaller channel width relative to a channel in a more productive zone. However, relatively narrow channels in either a productive or nonproductive zone may be less affected by the character and quality of wood recruitment. An analysis of riparian tree recruitment suggests that 95% of the supply of functional woody debris (by volume) can be maintained to a channel by establishing a woody debris reserve ranging from 9 to 16 m, depending on biogeoclimatic zone.


For further information, please contact D.L. Hogan, BC Ministry of Forests - Research Branch (dan.hogan@gems9.gov.bc.ca)

Updated September 08, 2005 

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