Mountain pine beetle impacts on channel morphology and woody debris in forested landscapes

Abstract

In streams throughout British Columbia, woody debris affects channel geometry, the distribution of channel units, bed texture, bank stability, development of the riparian area, and diversity of aquatic habitat. Understanding changes in the woody debris budget (input, storage, and output) enables critical assessment of the impacts associated with the mountain pine beetle (MPB) on the hydrological and aquatic environment. The objective of this study is to determine watershed-scale impacts of MPB by comparing channel conditions and the woody debris budget in watersheds infested by the MPB with those from similar old-growth forests with pre-infestation channel and riparian data. The use of a woody debris budget directly links large-scale lodgepole pine mortality to stream channel and riparian processes and conditions at the landscape level. Eighteen watersheds in the Sub-Boreal Spruce and Sub-Boreal - Pine Spruce biogeoclimatic zones were considered. The results are used to generate regionally and locally relevant best management practices that will guide operational planning in landscapes impacted by the MPB. The effect of the MPB infestation on channel processes and morphology will largely depend on the response and recovery of woody debris. Although the MPB infestation is now considered at epidemic levels across the landscape, riparian areas surveyed in this report support relatively small volumes of lodgepole pine (if any). In comparison to the volume of wood transferred to the channel during a stand-replacing event, wood transfer to the channel induced by MPB infestation in the next 25 years is likely to be relatively small and within the range of typical conditions throughout the region. In riparian areas with a similar distribution of lodgepole pine in the riparian area, management for the riparian supply of woody debris to small channels in areas infested by MPB may be effectively undertaken using existing regulations and guidelines of the Forest and Range Practices Act, and evaluations undertaken as part of the Forests and Range Evaluation Program.