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Riparian management areas (?buffers?) are intended to minimize impacts of forest management activities on water quality, aquatic ecosystems and riparian community diversity (BCMOF 1995). On average 15% of cutblock boundary segments in wind exposed areas of coastal BC are partially windthrown following harvesting, and riparian buffers are particularly susceptible (e.g. Rollerson and McGourlick 2001). Designing effective riparian prescriptions requires that the probability of windthrow and the probable impacts of windthrow be estimated. We have made significant progress in recent years in developing empirical models to characterize windthrow probability on cutblock edges and within partial cuts (Lanquaye-Opoku and Mitchell 2005, Scott and Mitchell, 2005). With my students and collaborators, I am working with the UK Forestry Commission to adapt their mechanistic windthrow risk model ForestGALES for BC conditions. The twin approach of obtaining large datasets and fitting empirical models, and then using empirical results to parameterize and validate mechanistic models, makes for efficient use of sampling data, creates a stimulating research environment, and enables us to rapidly develop locally-validated tools for managers. Prediction of windthrow impacts in riparian buffers can also be addressed using empirical and process modelling approaches. Potential impacts of windthrow in riparian buffers include loss of overstory, introduction of large woody debris (LWD) into streams, pulse introduction of foliage and fine branch material, loss of bank stability and exposure of sediment sources (Lewis 1998, MacDonald et al. 2003). There have been a number of investigations into windthrow frequency in riparian buffers in coastal forests (e.g. Steinblums et al. 1984, Rollerson and McGourlick 2001) but less attention has been paid to windthrow impacts. Over the two year term of this project, we will focus on post-harvest windthrow as a pulse source of LWD inputs and as mechanism for sediment exposure. The central research questions are: what factors affect the quantity and condition of LWD that enters the stream channel in the years after a windthrow event; what factors affect the volume of sediment exposed; and, how do management actions affect the volume of windthrow and sediment exposure. The proposed work addresses a component of LWD recruitment modeling that is underdeveloped. As noted by Wai (2005) who is developing AQUAWOOD for streams in the BC Interior, LWD recruitment models such as OSU STREAMWISE (Meleason et al. 2003), CWD (Bragg et al. 2000), and Riparian-in-a-Box (Kennard et al. 1998), model inputs of dead trees as a result of natural stand development within the riparian buffer. Bragg (2000) simulated the effect of ?catastrophic? disturbances such as wildfire or bark beetle, but this still assumes trees die standing. Other models (e.g. RAIS - Welty et al. 2002) enable to user to introduce windthrow of living trees, but by user-input rather than with a probabilistic windthrow model. Furthermore, the processes between tree fall and recruitment into the stream channel are not well characterized in these models. In our work in the riparian buffers experiment at the UBC Malcolm Knapp Research Forest, we have observed that larger windthrown trees are still suspended above the channel 7 years after the windthrow event. Some of the uprooted redcedar are still alive. Therefore, it will be necessary to characterize the relationship between bank configuration, tree characteristics and time-to-recruitment into the channel to more accurately represent the effect of a post-harvest pulse of windthrow activity. The role of windthrow in riparian buffers as a source of sediment, appears to depend on the degree to which windthrow is associated with side-slope instability. Lewis (1998) reported that windthrow-associated side slope failures were a major source of sediment delivery in northern California. In Washington State, Grizzel and Wolff ( ...
Mitchell, Stephen J.. 2008. Effect of stand structure and riparian buffer design on wind damage susceptibility and large woody debris recruitment. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2008MR150
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), British, Columbia
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