Forest Investment Account (FIA) - Forest Science Program
FIA Project Y082276

    Effect of stand structure and riparian buffer design on wind damage susceptibility and large woody debris recruitment
Project lead: Mitchell, Stephen (University of British Columbia)
Author: Mitchell, Stephen J.
Subject: Forest Investment Account (FIA), British Columbia
Series: Forest Investment Account (FIA) - Forest Science Program
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 (1998) reported a very large point input of sediment from a single windthrow-associated side slope failure but found inputs from other overturned trees to be minor compared to sediment stored within the stream system. Similarly, MacDonald et al. (2003) working in north-central BC found that uprooted trees contributed little sediment. The contribution of windthrow to sidewall failures is controversial and complex and investigation of the mechanism is beyond the scope of this 2 year study. However, we will compare the volume of material associated with overturning and side slope failure and will characterize the conditions in which side slope failure is more frequent. Windthrow forcing of LWD inputs and exposure of sediment sources are problems of physics, geometry and material properties (e.g. mechanics), and modeling these processes is consistent with the approach to windthrow prediction taken in ForestGALES. It is logical to extend ForestGALES to more fully account for windthrow-related outcomes, it is also logical within the scope of this project to extend ForestGALES to account for the effects of best management practices recommended in the Riparian Management Areas Guidebook (BCMOF 1995) for reducing windthrow in riparian buffers. For example, topping or pruning reduces the frontal area of tree crowns and initial field results indicate that topping and pruning reduces the number of stems damaged by approximately 50% over untreated controls within the first year after harvest (Rowan et al. 2003). We now have 5 years of results from 14 study sites to validate this aspect of the model. In a current FSP-funded project (2005-2008), we are working with the BCMOFR Decision Support Group to integrate ForestGALES_BC with TASS 3.0 so that forest managers will be able to design cutblocks and prepare crop plans for a variety of overstory retention scenarios. The riparian management refinements developed in the proposed project will be made available to practitioners through the ForestGALES_BC -TASS interface.
Related projects:  FSP_Y071276
Contact: Mitchell, Stephen, (604) 822-4591,


Executive Summary (21Kb)

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Updated August 16, 2010 

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